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10
Vitamin B 6
JAMES E. LEKLEM
Oregon State University, Corvallis, Oregon

I.

INTRODUCTION AND HISTORY

Vitamin B 6 is unique among the water-soluble vitamins with respect to the numerous
functions it serves and its metabolism and chemistry. Within the past few years the attention this vitamin has received has increased dramatically (1–8). Lay publications (9) attest
to the interest in vitamin B 6.
This chapter will provide an overview of vitamin B 6 as it relates to human nutrition.
Both qualitative and quantitative information will be provided in an attempt to indicate
the importance of this vitamin within the context of health and disease in humans. As a
nutritionist, my perspective no doubt is biased by these nutritional elements of this vitamin.
The exhaustive literature on the intriguing chemistry of the vitamin will not be dealt with
in any detail, except as related to the function of vitamin B 6 as a coenzyme. To the extent
that literature is available, reference will be made to research in humans, with animal or
other experimental work included as necessary.
As we leave the twentieth century behind, there may be a tendency to lose the sense
of excitement of discovery that Gyorgy and colleagues experienced when they began to
unravel the mystery of vitamin B complex. Some of the major highlights of the early
years of vitamin B 6 research are presented in Table 1. Paul Gyorgy was first to use the
term vitamin B 6 (10). The term was used to distinguish this factor from other hypothetical
growth factors B 3 , B 4 , B 5 (and Y). Some 4 years later (1938), in what is a fine example
of cooperation and friendship, Gyorgy (11) and Lepkovsky (12) reported the isolation of
pure crystalline vitamin B 6 . Three other groups also reported the isolation of vitamin B 6

bran and yeast). This early research into vitamin B 6 then provided the ground work for
research into the requirement for vitamin B 6 for humans and the functions of this vitamin.
Identification of the other major forms of the vitamin B 6 group, pyridoxamine and
pyridoxal, occurred primarily through the use of microorganisms (19,20). In the process
of developing an assay for pyridoxine, Snell and co-workers observed that natural materials were more active in supporting the growth of certain microorganisms than predicted
by their pyridoxine content as assayed with yeast (20). Subsequently, this group observed
enhanced growth-promoting activity in the urine of vitamin B 6 deficient animals fed pyridoxine (20). Treatment of pyridoxine with ammonia also produced a substance with
growth activity (21). These findings subsequently led to the synthesis of pyridoxal and
pyridoxamine (22,23). The availability of these three forms of vitamin B 6 reduced further
research into this intriguing vitamin possible.
II. CHEMISTRY
Since Gyorgy first coined the term vitamin B 6 (10), there has been confusion in the terminology of the multiple forms of the vitamin. ‘‘Vitamin B’’ is the recommended term for
the generic descriptor for all 3-hydroxy-2-methylpyridine derivatives (24). Figure 1 depicts the various forms of vitamin B 6 , including the phosphorylated forms. Pyridoxine
(once referred to as pyridoxal) is the alcohol form and should not be used as a generic

Fig. 1 Structure of B 6 vitamers.


Physical Properties of B 6 Vitamers

Vitamin B 6

Table 2

Percent stability compared to solution in dark (24). 8 h, 15 h ϭ length of time exposed to light.
From Storvick et al. (25); pH 7.0.
c
pH 3.4, 0.01 N acetic acid.
d
pH 10.5, 0.1 N NH 4 OH, lactone of 4-PA.

form a Schiff base and thus act as a catalyst in a variety of enzyme reactions. These
features have been detailed by Leussing (31) and include the 2-methyl group, which brings
the pK a of the proton of the ring pyridine closer to the biological range; the phenoxide
oxygen (position 3), which aids in expulsion of a nucleophile at the 4-position; the 5phosphate group, which functions as an anchor for the coenzyme and prevents hemiacetal
formation and the drain of electrons from the ring; and the protonated pyridine nitrogen
that is para to the aldehyde group aids in delocalizing the negative charge and helps regulate the pK a of the 3-hydroxyl group. A recent publication has extensively reviewed the
chemistry of pyridoxal-5′ phosphate (4).
PLP has been reported to be a coenzyme for over 100 enzymatic reactions (32). Of
these, nearly half involve transamination-type reactions. Transamination reactions are but

Fig. 2 Schiff base formation between pyridoxal-5′-phosphate and an amino acid.


Vitamin B 6

343

Table 3 Enzyme Reactions Catalyzed by Pyridoxal-5′-Phosphate
Type of reaction
Reactions involving α carbon
Transamination
Racemization
Decarboxylation
Oxidative deamination
Loss of the side chain
Reactions involving β carbon
Replacement (exchange)
Elimination
Reaction involving γ carbon
Replacement (exchange)

the methods mentioned above, adequate extraction of the forms of vitamin B 6 is critical.
TCA and perchloric acid are effective extractants.
Methods for the determination of the glycosylated form of vitamin B 6 PN-glucoside
(PNG), in foods are available (35,39). Both microbiological-based (40) and HPLC (35)
procedures have been utilized. All procedures for B 6 vitamers should be conducted under
yellow lights to minimize photodegredation.
IV. OCCURRENCE IN FOODS
To appreciate the role of vitamin B 6 in human nutrition, one must first have knowledge
of the various forms and quantities found in foods. A microbiological method for determining the vitamin B 6 content of foodstuffs was developed by Atkin in 1943 (32). While this


344

Leklem

Table 4 Vitamin B 6 Content of Selected Foods and Percentages of the Three Forms
Food
Vegetables
Beans lima, frozen
Cabbage, raw
Carrots, raw
Peas, green, raw
Potatoes, raw
Tomatoes, raw
Spinach, raw
Broccoli, raw
Cauliflower, raw
Corn, sweet
Fruits
Apples, Red Delicious

Bread, white
Bread, whole wheat

Vitamin B 6a
(mg/100 g)

Pyridoxineb
(%)

Pyridoxalb
(%)

Pyridoxamineb
(%)

0.150
0.160
0.150
0.160
0.250
0.100
0.280
0.195
0.210
0.161

45
61
75
47

0.070
0.169
0.420
0.510
0.060
0.019
0.240
0.034

61
58
81
56
61
59
61
83
—

31
20
11
29
10
26
30
11
—

8

18
10
18
17
14
12

0.100
0.183
0.545
0.730

52
71
29
31

28
12
68
65

20
17
3
4

0.224
0.550
0.170

13
21
39
38
—
—

11
16
24
49
51
—
—


Vitamin B 6

345

Table 4 Continued
Food
Meat/poultry/fish
Beef, raw
Chicken breast
Pork, ham, canned
Flounder fillet
Salmon, canned
Sardine, Pacific canned, oil
Tuna, canned

16
7
8
7
2
13
19
—

53
74
8
71
9
58
69
—

31
19
84
22
89
29
12
—

0.040
0.010
0.080

in certain foods. A glycosylated form of pyridoxine has been identified in rice bran (44)
and subsequently quantitated in several foods (40). The glycosylated form isolated from
rice bran has been identified as 5′-O-β-d-pyridoxine (44) (Fig. 3). Suzuki et al. have shown
that the 5′-glucoside can be formed in germinating seeds of wheat, barley, and rice cultured
on a pyridoxine solution (45). In addition, a small amount of 4′-glucoside was also detected

Fig. 3 Structure of 5′-O-(β-d-glucopyranosyl)pyridoxine.


346

Leklem

Table 5 Vitamin B 6 and Glycosylated Vitamin B 6 Content of
Selected Foods

Food
Vegetables
Carrots, canned
Carrots, raw
Cauliflower, frozen
Broccoli, frozen
Spinach, frozen
Cabbage, raw
Sprouts, alfalfa
Potatoes, cooked
Potatoes, dried
Beets, canned
Yams, canned
Beans/legumes


Vitamin B 6
(mg/100 g)

Glycosylated
vitamin B 6
(mg/100g)

0.064
0.170
0.084
0.119
0.208
0.140
0.250
0.394
0.884
0.018
0.067

0.055
0.087
0.069
0.078
0.104
0.065
0.105
0.165
0.286
0.005

0.351
0.997
0.086

0.038
0.026
0.046
0.355
-0-

0.165
0.043
0.097
0.046
0.313
0.271
0.079
0.009
0.206
0.443
0.230

0.078
0.016
0.045
0.019
0.010
0.024
0.017
0.002

0.237
3.515
0.076
0.098
3.635

0.326
0.087
0.055
0.153
0.015
0.007
0.382

n.d., none detected.
Sources: Data taken from Ref. 40 and Leklem and Hardin, unpublished.

in wheat and rice germinated seeds, but not in soybean seeds. Also of interest is an asyet-unidentified conjugate of vitamin B 6 reported by Tadera and co-workers (46). This
conjugate released free vitamin B 6 (measured as pyridoxine) only when the food was
treated with alkali and then β-glucosidase. Tadera et al. have also identified another derivative of the 5′-glucoside of pyridoxine in seedlings of podded peas (47). This derivative
was identified as 5′-O(6-O-malonyl-β-d-glucopyranosyl)pyridoxine. The role of these
conjugates in plants is unknown. Table 5 lists the total vitamin B 6 content of pyridoxine
5′-glucoside content of various foods. There is no generalization that can be made at this
time as to a given class of foods having high or low amounts of pyridoxine-5′-glucoside.
The effect of the 5′-glucoside of vitamin B 6 nutrition will be addressed in the section on
bioavailability and absorption.
Food processing and storage may influence the vitamin B 6 content of food (48–57)
and result in production of compounds normally not present. Losses of 10–50% have
been reported for a wide variety of foods. Heat sterilization of commercial milk was found
to result in conversion of pyridoxal to pyridoxamine (49). Storage of heat-treated milk

to a very limited extent. The phosphorylated forms disappear from the intestine via hydrolysis by alkaline phosphatase (67,70), and a significant part of this takes place intraluminally. Prior intake of vitamin B 6 in rats over a wide range (0.75–100 mg PN-HCl per kg
diet) was found to have no affect on in vitro absorption of varying levels of PN-HCl
(71). This study provides further support for passive absorption of B 6 vitamers. However,
Middelton has questioned the concept of a nonsaturable process (72). Using an in vivo
perfused intestinal segment model, he found there was a gradient of decreasing rates of
uptake from the proximal to the distal end of the intestine and that there was a saturable
component of uptake, especially in the duodenum.
The various forms of vitamin B 6 that are absorbed into the rat intestinal cell (intracellular) can be converted to other forms (i.e., PL to PLP, PN to PLP, and PM to PLP), but
that which is ultimately transported to other organs via the circulation system primarily
reflects the nonphosphorylated form originally absorbed (69,70). A similar pattern of uptake and metabolism has been observed in mice (73); however, in mice given PN, pyridoxal was the major form detected in the circulation. Portal blood was not examined. The
liver was likely the primary organ that further metabolized the PN absorbed and released
PL to the circulation.
Bioavailability of a nutrient from a given food is important to an organism in that
it is the amount of a nutrient that is both absorbed and available to cells. The word available is key here in that the vitamin may not be needed by the cell and simply excreted
or metabolized to a nonutilizable form, such as 4-pyridoxic acid in the case of vitamin B 6 .
Methods used to evaluate the bioavailability of nutrients such as vitamin B 6 include
balance studies in which input and output are determined. Included in these studies is the
use of stable isotopic techniques (74). A second approach is to measure an in vivo response, such as growth, after a state of deficiency has been created. The third type of
study is the examination of blood levels of the nutrient or a metabolite of the nutrient
over a specified period of time after a food is fed. The concentration of a metabolite, such
as PLP, is then compared with concentrations after ingestion of graded amounts of the
crystalline form of vitamin B 6. Gregory and Ink (74) and Leklem (75) have reviewed
vitamin B 6 bioavailability.
One of the early studies that suggested a reduced availability of vitamin B 6 involved
feeding canned combat rations that had been stored at elevated temperatures (75). Feeding
diets containing 1.9 mg of total vitamin B 6 resulted in a marginal deficiency based on
urinary excretion of tryptophan metabolites. Some 18 years later, Nelson et al. observed
that the vitamin B 6 in orange juice was incompletely absorbed by humans (77). These



whole wheat bread, and peanut butter (83). Compared to the vitamin B 6 in tuna, the vitamin
B 6 in whole wheat bread and peanut butter was 75% and 63% as available, respectively.
The level of glycosylated vitamin B 6 in these foods was inversely correlated with vitamin
B, bioavailability as based on urinary vitamin B, and 4-pyridoxic acid(84). We have observed an inverse relationship between vitamin B 6 bioavailability as based on urinary 4pyridoxic acid excretion and the glycosylated vitamin B 6 content of six foods (85). These
foods and their respective availabilities were as follows: walnuts (78%), bananas (79%),
tomato juice (25%), spinach (22%), orange juice (9%), and carrots (0%). While the glycosylated vitamin B 6 content of foods appears to be a significant contributor to bioavailability, the presence of other forms of vitamin B 6 and/or binding of specific forms of vitamin
B 6 to other components in a food may also contribute to availability. The question of the
extent to which vitamin B 6 bioavailability affects vitamin B 6 status (and thus requirement)
has been studied in women (86). When diets containing 9% of the vitamin B 6 as PNG
were compared with diets containing 27% PNG it was observed that vitamin B 6 status
was decreased. The decreased bioavailability was consistent with that observed in humans
by Gregory et al. (87) who estimated that the bioavailability of PNG may be as low as
58% of the bioavailability of free pyridoxine.
VI. INTERORGAN METABOLISM
Extensive work by Lumeng and Li and co-workers in rats (88) and dogs (89) has shown
that the liver is the primary organ responsible for metabolism of vitamin B 6 and supplies


350

Leklem

the active form of vitamin B 6 , PLP, to the circulation and other tissues. The primary
interconversion of the B 6 vitamers is depicted in Fig. 4. The three nonphosphorylated
forms are converted to their respective phosphorylated forms by a kinase enzyme (pyridoxine kinase EC 2.7.1.35). Both ATP and zinc are involved in this conversion, with ATP
serving as a source of the phosphate group. The two phosphorylated forms, pyridoxamine5′-phosphate and pyridoxine-5′-phosphate, are converted to PLP via a flavin mononucleoticle (FMN)–requiring oxidase (90). A review of the interrelation between riboflavin and
vitamin B 6 is available (91).
Dephosphorylation of the 5′-phosphate compounds occurs by action of a phosphatase. This phosphatase is considered to be alkaline phosphatase (92) and is thought to be
enzyme-bound in the liver (93). PL arising from dephosphorylation or that taken up from
the circulation can be converted to 4-pyridoxic acid by either an NAD-dependent dehydrogenase or an FAD-dependent aldehyde oxidase. As discussed below, in humans only aldehyde oxidase (pyridoxal oxidase) activity has been detected in the liver (94). The conversion of pyridoxal to 4-pyridoxic acid is an irreversible reaction. Thus, 4-pyridoxic acid

be readily converted to 4-pyridoxic acid. Such a mechanism may prevent large amounts
of the highly reactive PLP from accumulating.
The PLP that is formed in liver (and other tissues) can bind via a Schiff base reaction
with proteins. The binding of PLP to proteins may be the predominant factor influencing
tissue levels of PLP (93). This binding of PLP to proteins is thought to result in metabolic
trapping of PLP (vitamin B 6 in cells (88,93). PLP synthesized in liver cells is released
and found bound to albumin. Whether the PLP is bound to albumin prior to release from
the liver or released unbound and subsequently binding to albumin has not been determined.
The binding of PLP to albumin in the circulation serves to protect it from hydrolysis
and allows for the delivery of PLP to other tissues (104). This delivery process of PLP
to other tissues is thought to involve hydrolysis of PLP and subsequent uptake of PL into
the cell (92). Hydrolysis occurs by action of phosphatases bound to cellular membranes.
Other forms of vitamin B 6 are present in the circulation (plasma). Under fasting conditions,
the two aldehyde forms compose 70–90% of the total B 6 vitamers in plasma, with PLP
making up 50–75% of the total (Table 7). The next most abundant forms are PN, PMP,
and PM. Interestingly, pyridoxine-5′-phosphate is essentially absent in plasma.

Table 6 Activity of Human Liver Enzymes
Involved in Vitamin B 6 Metabolism
Enzyme (activity)
Pyridoxal kinase (nmol/min)
Pyridoxine-5′-P-oxidase (nmol/min)
Pyridoxal-5′-P-phosphatase (nmol/min)
Pyridoxal oxidase (nmol/min)
Source: Data taken from Refs. 94 and 99.

Per gram liver
11.2
2.4
0.1–2.1

5Ϯ9

0
n.d.
n.d.

19 Ϯ 33
32 Ϯ 7
n.d.

8Ϯ8
3Ϯ3
n.d.

2Ϯ2
6Ϯ1
n.d.

a

From Ref. 103.
From Ref. 104.
c
From Ref. 105.
n.d., none detected.
All data obtained by HPLC methods.
b

Within the circulating fluid (primarily blood), the erythrocyte also appears to play
an important role in the metabolism and transport of vitamin B 6 . However, the extent of

plasma PLP concentration with time in 10 control females and 11 oral contraceptive users
fed a diet low in vitamin B 6 (0.19 mg, 1.1 µmol) for 4 weeks (95). There was an initial


Vitamin B 6

353

Fig. 5 Semilog plot of plasma pyridoxal-5′-phosphate concentration over 4 weeks of feeding a
vitamin B 6-deficient diet and 4 weeks of repletion with pyridoxine in control subjects and oral
contraceptive users. (From Ref. 86.).

rapid decline in plasma PLP concentration followed by a slower decrease. Extrapolation
of the slope for the slowly decreasing portion of the curve for each of the two groups and
determination of the plasma t 1/2 PLP revealed a value of 28 days for the control females
and 46 days for the oral contraceptive users. The value for controls is consistent with the
data of Shane (117). The longer t 1/2 for oral contraceptive users may reflect higher levels
of enzymes with PLP bound to them (118). Coburn (119) has discussed the turnover and
location of vitamin B 6 pools, based in part on modeling calculations.
Muscle has been suggested as a possible storage site for vitamin B 6 . This is based
in part on the B 6 content of muscle and the total muscle mass of animals. As previously
mentioned, in muscle a majority of vitamin B 6 is present as PLP bound to glycogen phosphorylase (114,115). In contrast, glycogen phosphorylase accounts for only about 10%
of the vitamin B 6 content of liver (120). Black and co-workers examined the storage of
vitamin B 6 in muscle by studying the activity of muscle glycogen phosphorylase (121).
In their studies, Black et al. found that feeding rats a diet high in vitamin B 6 (70 g of
vitamin B 6 per kilogram of diet) resulted in a high vitamin B 6 content and a high glycogen
phosphorylase content in muscle. This increase in content and enzyme level occurred
in concert for 6 weeks, whereas the level of alanine and aspartic aminotransferase increased for the first 2 weeks and then plateaued. In subsequent work (122), these same
researchers found that muscle phosphorylase content (and thus vitamin B 6 content) decreased only when there was a caloric deficit and not necessarily with a deficiency of
vitamin B 6 . This observation of muscle not acting as a mobile reservoir during a vitamin

in humans. A variety of methods have been utilized to assess vitamin B 6 status. These
methods are given in Table 8 and are divided into direct, indirect, and dietary methods
(128–130). Direct indices of vitamin B 6 status are those in which one or more of the B 6
vitamers or the metabolite 4-pyridoxic acid are measured. These are usually measured in
plasma, erythrocytes, or urine samples because tissue samples are not normally available.
Indirect measures are those in which metabolites of metabolic pathways in which PLP is
required for specific enzymes are measured, or in which activities of PLP-dependent enzymes are determined. In this latter case, an activity coefficient is often determined by
measuring the enzyme activity in the presence and absence of excess PLP.
Dietary intake of vitamin B 6 itself is not sufficient to assess vitamin B 6 status, especially if only a few days of dietary intake are obtained. In addition to the inherent problems
in obtaining accurate dietary intakes, the nutrient databases used in determining vitamin
B 6 content of diets are often incomplete with respect to values for vitamin B 6 . Thus, reports
of vitamin B 6 status based only on nutrient intake must be viewed with caution. Some of
the suggested values for the evaluation of status given in Table 8 are based on the relationship of vitamin B 6 and tryptophan metabolism (95). Plasma pyridoxal-5′-phosphate concentration is considered one of the better indicators of vitamin B 6 status (131). Lumeng
et al. (104) have shown that plasma PLP concentration is a good indicator of tissue PLP
levels in rats. In humans, plasma PLP concentration is significantly correlated with dietary
vitamin B 6 intake (97). Table 9 contains mean plasma PLP values reported by several
laboratories for males and females. These are selected references drawn from reports in
which the sex of the subjects was clearly identified. The means reported range from 27
to 75 mnol/L for males and 26 to 93 µmol/L for females. These ranges should not necessarily be considered as normal since the values given in Table 9 reflect studies in which
dietary intake was controlled and other studies in which dietary intake was not assessed.


Vitamin B 6

355

Table 8 Methods for Assessing Vitamin B 6 Status and
Suggested Values for Adequate Status
Index
Direct

Ͼ3.0 µmol/day
Ͼ0.5 µmol/day
Ͼ1.25 b
Ͼ1.80 b
Ͻ65 µmol/day
Ͻ351 µmol/day
NV
Ͼ1.2–1.5 mg/day
Ͼ0.02
NV
NV

a
Reference values in this table are dependent on sex, age, and protein intake
and represent lower limits (130).
b
For each aminotransferase measure, the activity coefficient represents the ratio
of the activity with added PLP to the activity without PLP added.
NV, no value established; limited data available, each laboratory should establish its own reference with an appropriate healthy control population.

As discussed by Shultz and Leklem (97), dietary intake of both vitamin B 6 and protein
influences the fasting plasma PLP concentration. Miller et al. (136) have shown that
plasma PLP and total vitamin B 6 concentrations in males were inversely related to protein
intake (see Table 9) in males whose protein intake ranged from 0.5 to 2 g/kg per day.
Similar results from metabolic studies in women support these findings in men (151).
Other factors that may influence plasma PLP and should be considered when using
this index as a measure of vitamin B 6 status include the physiological variables of age
(133,147,153), exercise (124), and pregnancy (143). Rose et al. determined the plasma
PLP concentration in men ranging in age from 18 to 90 years (133). They observed a
decrease in plasma PLP with age, especially after 40 years of age. However, one must

5
8

—c
20–34
35–49
18–29
30–39
40–49
50–59
60–69
70–79
80–89
—
27 Ϯ 3
27 Ϯ 4

35
4
7
5
9

38 Ϯ 14
22–35
16 Ϯ 1
27 Ϯ 6
25 Ϯ 4

Miller (136)

77
6
3
4
58
?
?
4

—
20–34
35–49
—
20–29
29 Ϯ 8
22 Ϯ 2
22 Ϯ 2
20–34
20–34
—
—
24–32

Ref.
Males
Wachstein (132)
Chabner (38)
Rose, 1976 (133)

Contractor (134)

SS, NF
—
SS, F
SS, F
Met, (1.55), F
SS, (2.0 Ϯ 8), F
Met, (1.60), F
SS, F
Met, (1.60), F
SS, F
Met, F
Met 1, (1.6)LP, F
Met, (1.6)MP, F
Met, (1.6)HP, F
Met, (1.6)F
SS, (1.9), F
SS, (1.5), F
Met, (1.1), F
(2.3), F
(2.7), F
SS, (1.34), F
SS, (1.96), F
SS, (2.88), F
—
SS, F
SS, F
SS, —
SS, F
SS, NF
Met, (0.8), F

27.9
38.8
45.5
39.2
27.5
55.0
114.5
25.7
40.3
48.0

Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ

18.7
6.5
36.0
23.3
19.4
9.0
11.5
10.9
15.0
22.4
2.7
5.7
7.0
5.1
7.2
9.4

34.0 Ϯ 10.1
67.9 Ϯ 14.6
46.1 Ϯ 13.8
36.8 Ϯ 8.9
25.9 Ϯ 15.4
38.0 Ϯ 17.0
22.9 Ϯ 13.9
60.7 Ϯ 20.2
68.4
43.4
51.8 Ϯ 30.7
46.5 Ϯ 24.3
38.4 Ϯ 15.8

TDC
TDC
TDC


Vitamin B 6

357

Table 9 Continued
No.
subjects

Age
(years)

Ubbink (149)
Huang (150)

41
23
29
5
5
5
5
5
5
41 (C) d
32 (C)


6

28.2 Ϯ 2.6

Ribaya-Mercado (139)

4

63.6 Ϯ 0.8

Kretsch (152)

8

21–30

Ref.
Shultz (97)
Guilland (146)
Lee (147)

Driskell (148)

Diet a
SS, (1.6 Ϯ 0.5), F
SS, (1.1), F
Met, (1.0), F
SS, F
SS, F

CF, (2.0)(1.5), F

PLP (nmol/L) Method b
37.7
92.8
52.0
35.5
31.3
61.7
40.5
202
168
48.1
44.5
43.7
46.1
42.1
46.5
31.7
58.19
32.40
38.31
45.43
53.65
27.9
32.4
41.0
58.9
26.5
29.4

Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ

14.7
7.3
4.1
14.8
13.3
25.6
12.2
45
38
17.9
15.8
15.3
15.8
15.8
13.9
19.4
16.28
10.50
9.68
16.24
10.94
11.4
11.6

HPLC
HPLC

TDC

TDC

TDC

a
The notations for diet indicate if the blood samples were obtained from subjects who self-selected (SS) their
diets or were receiving a controlled intake (Met) and the amount of vitamin B 6 consumed (value given as mg/
day in parentheses), if known. F indicates that the blood sample was collected after a fast of at least 8 h; NF
indicates nonfasting. LP, MP, HP refer to grams of protein as 0.5, 1.0, and 2.0 g/kg body weight.
b
TDC, tyrosine apodecarboxylase; HPLC, high-performance liquid chromatography; FL, fluorimetry.
c
A dash indicates data was not given in the respective reference.
d
C, Caucasian; B, black.


358

Table 10

Urinary 4-Pyridoxic Acid and Vitamin B 6 Excretion in Males and Females

Ref.
Males

10
9
6
8

20–25
18–35
16–51
27 Ϯ 4
27 Ϯ 3
22–35
35 Ϯ 14
26 Ϯ 4
26 Ϯ 4
25 Ϯ 4
28 Ϯ 6
27 Ϯ 4

Met (1.66 mg, P ϭ 150)
(1.66 mg, P ϭ 54)
—
Met (1.55)
SS
Met (1.6)
SS (2.0 Ϯ 0.8)
SS
Met (1.60)
Met (1.55)
Met (4.2 Ϯ 0.4)
Met (1.60)


1.75
0.60
1.4
0.85
0.5
1.08
4.34
1.40
0.59
1.10
1.86
0.89
0.54
0.71

15
26
29
6
3
8

18–47
—
25
22 Ϯ 2
22 Ϯ 2
18–23


0.41 (31)

4-Pa
(nmol/day)

UB6
(µmol/day)
—

(50) a
(44)

(63)

(53)
(45)
(LP)
(MP)
(HP)

0.76
0.8
0.76
0.92
0.81
0.76
1.05

—
Ϯ 0/17


Hansen (151)

10

27.5 Ϯ 6.8

6

28.2 Ϯ 2.6

5
4
4

29 Ϯ 6

Hansen (86)
Kretsch, (152)

4

21–30

SS (1.6 Ϯ 0.5)
Met (2.3)
SS
Met, (1.60), F
Met, (0.45), F
Met, (1.26), F

3.23
4.00
5.89
9.51
2.87
3.35
7.88
3.60
4.02

Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ
Ϯ

3.09
0.55


—

1.35
2.29
4.01
5.98

Ϯ
Ϯ
Ϯ
Ϯ

0.47
0.27 (39)
0.25 (49)
0.36 (55)

—

(59)
(50)
(37)
(35)
(33 Ϯ 2)
(33 Ϯ 2)
(33 Ϯ 2)
(60)
(52)
(54)


Vitamin B 6

Shultz (97)
Lee (147)
Ubbink (149)
Huang (150)

0.12
0.10
0.09
0.14
0.08
0.09
0.11
0.082
0.116

a
The number in parentheses refers to the percent of intake excreted as 4-PA.
Abbreviations are as used in Table 9, except for 4-pyridoxic, which is 4-PA and urinary vitamin B 6 , which is UB6.

359


360

Leklem

remains to be determined. There is one controlled metabolic study that has evaluated

for about 40–60% of the intake. Because of the design of most studies and the limited
number of studies done with females compared with males, it is not possible to determine
if there is a significant difference between males and females. The limited data in Table
11 suggest that there is little difference. However, males consistently had higher plasma
PLP and total vitamin B 6 concentrations as well as higher excretion of 4-pyridoxic acid
and total vitamin B 6 . Urinary total vitamin B 6 (all forms, including phosphorylated and
Table 11 Plasma Pyridoxal-5′-Phosphate and Total Vitamin B 6 Concentration, and Urinary 4Pyridoxic Acid and Vitamin B 6 Excretion in Males and Females Consuming 2.2 mg Vitamin B
Subject
Males (n ϭ 4)
Females (n ϭ 4)
a

Mean Ϯ SD.

PLP
(nmol/L)

TB6
(nmol/L)

4-PA
(µmol/day)

UB6
(µmol/day)

78.4 Ϯ 27.0 a
58.5 Ϯ 12.6

86.2 Ϯ 37.1

circumstances provide useful information. The review by Reynolds (155) provides an
excellent critique of methods currently in use for assessment of vitamin B 6 status.
VIII. FUNCTIONS
A. Immune System
The involvement of PLP in a multiplicity of enzymatic reactions (177) suggests that it
would serve many functions in the body. Table 12 lists several of the known functions
of PLP and the cellular systems (137) affected. PLP serves as a coenzyme for serine
transhydroxymethylase (178), one of the key enzymes involved in one-carbon metabolism.
Alteration in one-carbon metabolism can then lead to changes in nucleic acid synthesis.
Such changes may be one of the keys to the effect of vitamin B 6 on immune function
Table 12

Cellular Processes Affected by Pyridoxal-5′-Phosphate

Cellular process or enzyme
One-carbon metabolism, hormone modulation
Glycogen phosphorylase, transamination
Tryptophan metabolism
Heme synthesis, transamination, O 2 affinity
Neurotransmitter synthesis, lipid metabolism
Hormone modulation, binding of PLP to lysine on
hormone receptor

Function/system influenced
Immune function
Gluconeogenesis
Niacin formation
Red cell metabolism and formation
Nervous system
Hormone modulation

within liver and muscle and, in the case of liver, a source of glucose for adequate blood
glucose levels. In rats a deficiency of vitamin B 6 has been shown to result in decreased
activities of both liver (188) and muscle glycogen phosphorylase (114,122,188). Muscle
appears to serve as a reservoir for vitamin B 6 (114,122,123), but a deficiency of the vitamin
does not result in mobilization of these stores. However, Black et al. (122) have shown
that a caloric deficit does lead to decreased muscle phosphorylase content. These results
suggest that the reservoir of vitamin B 6 (as PLP) is only utilized when there is a need for
enhanced gluconeogenesis. In male mice the half-life of muscle glycogen phosphorylase
has been shown to be approximately 12 days (189). In contrast to low intake of vitamin
B 6 , rats given an in injection of a high dose of PN, PL, or PM (300 mg/kg) showed a
decrease in liver glycogen and an increase in serum glucose (190). This effect is mediated
via increased secretion of adrenal catecholamines. The extent to which lower intake of
B 6 vitamers has this effect or if this occurs in humans remains to be determined.
C.

Erythrocyte Function

Vitamin B 6 has an additional role in erythrocyte function and metabolism. The function
of PLP as a coenzyme for transaminases in erythrocytes has been mentioned. In addition,
both PL and PLP bind to hemoglobin (107,108). The binding of PL to the α chain of
hemoglobin (191) increased the O 2 binding affinity (192), while the binding of PLP to
the β chain of hemoglobin S or A lowers the O 2 binding affinity (193). The effect of PLP
and PL on O 2 binding may be important in sickle cell anemia (194).