Molecules 2010, 15, 442-459; doi:10.3390/molecules15010442
Vitamin B6: A Molecule for Human Health?
Hanjo Hellmann * and Sutton Mooney
Washington State University, Abelson 435, P.O. Box 66224, Pullman, WA, USA
* Author to whom correspondence should be addressed; E-Mail: email@example.com.
Received: 5 November 2009; in revised form: 16 January 2010 / Accepted: 20 January 2010 /
Published: 20 January 2010
Abstract: Vitamin B6 is an intriguing molecule that is involved in a wide range of
metabolic, physiological and developmental processes. Based on its water solubility and
high reactivity when phosphorylated, it is a suitable co-factor for many biochemical
processes. Furthermore the vitamin is a potent antioxidant, rivaling carotenoids or
tocopherols in its ability to quench reactive oxygen species. It is therefore not surprising
that the vitamin is essential and unquestionably important for the cellular metabolism and
well-being of all living organisms. The review briefly summarizes the biosynthetic
pathways of vitamin B6 in pro- and eukaryotes and its diverse roles in enzymatic reactions.
Finally, because in recent years the vitamin has often been considered beneficial for human
health, the review will also sum up and critically reflect on current knowledge how human
health can profit from vitamin B6.
Keywords: vitamin B6; PDX; de novo; salvage; health
The B vitamins are a group of water soluble, chemically quite distinct compounds to which other
than vitamin B6, vitamin B1 (thiamine), B2 (riboflavin), B3 (niacin or niacin amide), B5 (pantothenic
acid), B7 (biotin), B9 (folic acid), and B12 (various cobalamins) also belong . Historically, it was
believed that only one vitamin B existed with a critical function for maintenance of growth and health
and prevention of characteristic skin lesions in animals and human . However, with ongoing
research it became obvious that vitamin B actually comprised a group of compounds that was
collectively called the ‘vitamin B complex’.
Molecules 2010, 15
Vitamin B6 (vitB6 from here on) itself is an enzymatic co-factor required for more than 140
biochemical reactions including transaminations, aldol cleavages, α-decarboxylations, racemizations,
β- and γ- eliminations, and replacement reactions. Most of these reactions are related to amino acid
biosynthesis and degradation, but vitB6 is also involved in other processes including sugar and fatty
acid metabolism . It comprises a set of three different pyridine derivatives called pyridoxine (PN;
1), pyridoxal (PL; 2), and pyridoxamine (PM; 3). They differ in a variable group present at their 4-
position with PN carrying a hydroxymethyl group, and PL (2) and PM (3) having an aldehyde and an
aminomethyl group, respectively. Furthermore, all three B6 vitamers are phosphorylated by a kinase,
which is a requirement for their role as cofactors in enzymatic reactions (Scheme 1). While
pyridoxamine-5’-phosphate (PMP; 4) has been reported to function as a co-factor, it is pyridoxal 5’-
phosphate (PLP; 5) that is the biologically most active form [4,5].
A growing number of interesting and helpful new resources have been established in the last years
that focus primarily on vitB6 related issues. For example, an online database has been launched that
allows searching whole genomes for PLP-dependent enzymes, and which also provides information on
critical aspects such as the biochemical pathways requiring PLP (5) and the classification of PLP-
dependent enzymes (http://bioinformatics.unipr.it/cgi-bin/bioinformatics/B6db/home.pl) . In
addition, a database has been established that allows searching for mutated PLP-dependent enzymes in
various organisms (http://www.studiofmp.com/plpmdb/home.htm) .
2. Suggested Reaction Mechanisms of VitB6 for Amino Acid Metabolism
In most cases PLP (5) is covalently bound to the ε-amino group of a conserved lysine residue in the
active center of a PLP-dependent enzyme, with its 5’-phosphate group being buried in a conserved
phosphate-binding cup . It is suggested that reactions are initiated by the formation of a geminal
diamine intermediate between the aldehydic carbon atom of PLP (5) and an amino group of the
substrate. This is followed by its rapid breakdown and the formation of an external aldimine (Schiff
base) between PLP (5) and the substrate causing the release of the lysine residue of the enzyme from
PLP (5). From this point on subsequent reactions mainly depend on the specific, participating enzymes
that guide and modulate the next steps leading to e.g. racemisations, β- and γ- eliminations.
3. Three Different Biosynthetic Pathways for VitB6 Are Known
Three different pathways for vitB6 biosynthesis have been described which will be just briefly
summarized, as they were topics of other recent reviews [8,9]. In eubacteria like Escherichia coli, the
vitamin can be de novo synthesized by the concerted activities of the pyridoxine biosynthesis proteins
A and J (PdxA (EC 220.127.116.112) and PdxJ (EC 18.104.22.168), respectively) which use 4-phospohydroxy-L-
threonine (4HPT; 6) and deoxyxylose 5’-phosphate (DXP; 7) to synthesize pyridoxine 5’-phosphate
(PNP; 8) (Scheme 1) [10–12]. In bacteria, archaea, and eukarya a second de novo pathway is known
that synthesizes PLP (5) from ribose 5’-phosphate (9) or ribulose 5’-phosphate (10), in combination
with either glyceraldehyde 3’-phosphate (11) or dihydroxyacetone phosphate (12) and glutamine (13)
(Scheme 1) [13–17].
Molecules 2010, 15
Scheme 1. The three known pathways for PLP biosynthesis: one salvage pathway, and
two de novo pathways, a DXP-dependent one and a DXP-independent one. Chemical
structures: (5) PLP; (7) deoxyxylulose 5’-phosphate, (6) 4-(phosphohydroxy)-L-
threonine; (11) glyceraldehyde 3’-phosphate; (12) dihydroxyacetone phosphate; (9)
ribose 5’-phosphate; (10) ribulose 5’-phosphate, (13) glutamine, (3) PM, (4) PMP, (1)
PN, (8) PNP, (2) PL.
Here two pyridoxine biosynthetic enzymes (PDX) are active: while PDX2 functions as a
glutaminase that deaminates glutamine to glutamate in order to supply nitrogen for the PLP
heterocycle, PDX1 arranges the final ring closure [18–24]. Because of a different sugar precursor used
for the biosynthesis of the vitamin, the de novo pathway from eubacteria is known as the DXP-
dependent pathway, while the other is the DXP-independent pathway . In addition to the two de
novo pathways, most organisms also have a salvage pathway that converts the different B6 vitamers to
PLP (5). This is achieved by the concerted activities of an oxidase, PDXH (EC 22.214.171.124), and a kinase,
PDXK (EC 126.96.36.199) (Scheme 1) [8,25]. Most animal organisms, including humans, have a salvage
Molecules 2010, 15
pathway, however, they lack the enzymatic machinery for de novo synthesis and rely on external
uptake of the vitamin from food .
4. VitB6 and Its Healthy Face
Since its discovery in 1932 by the Japanese scientist S. Ohdake, vitB6 has been discussed in
relationship to health issues . In these early works from, for example, Ohdake or the Hungarian
scientist P. Györgi, vitB6 was associated with pellagra, a skin disease that is based on multi-vitamin
deficiencies that mostly occurs in context with niacin undersupply [27–29]. A search through the
public literature data basis (http://www.ncbi.nlm.nih.gov/) for health aspects associated with vitB6
yields a surprisingly high number of articles (>900). Furthermore, the current Recommended Dietary
Allowance per day by the National Institute of Health (NIH) of the USA is around 2 mg with an
upward tolerance of 100 mg per day for adults. A recent U.S. study, which tested the blood PLP levels
in around 8,000 patients, demonstrated a widespread deficiency of the vitamin among all tested
subgroups, and the authors suggested an increase of the daily allowance from around 2 mg to 3 to
4.9 mg per day . It has been reported for animal models, that continuous uptake of very high doses
(e.g. 400 mg/kg) can lead to peripheral sensory neuropathy and nerve degeneration [31,32]. These
problems are generally reversible when supplementation is stopped. Additionally some studies have
suggested that increased levels of the B6 vitamers and some derivatives can generate toxic
photoproducts as a result of UV irradiation [33–35]. However, the applied daily dosages were far
beyond any physiological concentrations an organism is normally exposed to, making it unlikely that
such vitB6 induced impacts will be observed. Because of the great interest in vitB6 as a therapeutic and
pharmaceutical compound, its reactive capability, and its potent antioxidative characteristics, we
summarize in the following paragraphs some of the relevant topics related to these issues.
4.1. Therapeutic applications by using drugs against PLP-dependent enzymes
PLP-dependent enzymes are highly diverse and the reactions they facilitate are estimated to
represent 4% of all known catalytic activities; hence, many of them are being explored as targets for
therapeutic agents (for an excellent overview see ). We chose three major examples for this review
to illustrate the potentials of this approach in disease control: malaria, sleeping sickness, and cancer
One of the most threatening human diseases is malaria, with more than 300-500 million infected
people worldwide and an annual death toll of up to one million people (http://www.unicef.org/health/
index_malaria.html). Several approaches are currently underway in an effort to affect the life cycle or
metabolism of the pathogen Plasmodium falciparum, the cause of malaria. One such approach is to
impair biosynthesis of xanthurenic acid (14), which is essential for gametogenesis and fertility of the
pathogen [36–38]. The acid is synthesized as part of the L-tryptophan (15) degradation pathway from
L-kynurenine (16) via 3-hydroxykynurenine (17) by the activity of the PLP-dependent kynurenine
aminotransferase (EC 188.8.131.52)  (Scheme 2A).
A possible strategy involves developing specific drugs that reduce activity of the aminotransferase.
This might lower the levels of 3-hydroxykynurenine (17) in P. falciparum infected mosquitoes
Molecules 2010, 15
potentially reducing or even preventing malaria transmission to humans. A similar direction was
recently proposed by channeling synthetic pyridoxyl-amino acid adducts into the pathogen, which can
phosphorylate these compounds mediated by PDXK kinase . After binding by a PLP-dependent
enzyme, such phosphorylated compounds should inhibit these enzymes and affect further metabolism.
Müller and co-workers successfully tried pyridoxyl-tryptophan methyl ester to inhibit proliferation of
P. falciparum opening up the possibility for a novel malaria treatment in the future . Because P.
falciparum expresses PDX1/PDX2 proteins, which humans lack, a potential approach can also be to
target these de novo pathway proteins by specific drugs . However, no specific approach has been
reported so far.
African sleeping sickness is another severe epidemic disease with an estimated 300–500 thousand
people affected in various African countries (http://www.sbri.org/diseases/african.asp). It is caused by
the protist Trypanosoma brucei and transmitted by flies of the Genus Glossina. A target to treat
sleeping sickness in affected patients is the PLP-dependent enzyme ornithine decarboxylase (ODC;
E.C. 184.108.40.206). It catalyzes the step from L-ornithine (18) to the diamine putrescine (19), an initial step
in the production of polyamines (Scheme 2B). α-Difluoromethylornithine (DFMO) is a proven
irreversible inhibitor of ODC activity and works by forming a covalent bond with a cysteine residue of
ODC after decarboxylation [42–44]. Although DFMO is an approved drug in treating sleeping
sickness caused by T. brucei, the precise reason for its effectiveness is not fully resolved because
human and T. brucei ODCs are comparably affected by the agent . It is suggested that this effect is
based on metabolic differences: a more rapid turnover of the host’s ODC on the one side, and on the
other side T. brucei’s high demand for the synthesis of the polyamine trypanothione, a specific dithiol
essential for the detoxification system of Trypanosomes and Leishmania parasites [46,47].
Targeting PLP-dependent enzymes is also discussed in context with cancer. Here an interesting
candidate is, for example, serine hydroxymethyltransferase (SHMT; EC 220.127.116.11), which catalyzes the
reversible transfer of the Cβ of serine (21) to tetrahydrofolate (22) to form glycine (23) and 5,10-
methylenetetrahydrofolate (24) (Scheme 2C). Because of 5,10-methylenetetrahydrofolate (24), which
serves as a methyl donor in many reactions, SHMT activity is critical for one-carbon metabolism, the
biosynthesis of methionine, lipids, formyl-tRNA and pyrimidine. The latter is of special interest as
apparently SHMT activity is coupled to some extent with increased demand for DNA biosynthesis. For
example there is evidence in tumors with highly proliferating, mitotically active cells, that serine is
preferentially channeled for DNA biosynthesis [48,49]. Consequently SHMT is a proposed target in
developing drugs for chemotherapy [50,51].