Vitamin B6 is an essential cofactor of enzymes involved in critical biochemical processes such as amino acid metabolism, and neurotransmitter and haemoglobin biosynthesis. While most microorganisms and plants can synthesise metabolically active vitamin B6 de novo, animals lack this ability and instead rely on salvage pathways for its production. In their study published in The FEBS Journal, Kito et al. identify a new vitamin B6 ‘salvage enzyme’ in mammals, thus providing important insights into B6 metabolism and regulation.
Vitamin B6 is an essential cofactor of enzymes involved in amino acid metabolism, and in neurotransmitter and haemoglobin biosynthesis. It is comprised of six interconvertible molecules or vitamers, including pyridoxal (PL) and its phosphorylated form pyridoxal 5-phosphate (PLP). PLP is the metabolically active form of vitamin B6. While most microorganisms and plants can synthesise PLP de novo, animals lack such a biosynthetic pathway, and instead rely on dietary sources and intestinal bacteria to produce intracellular PLP from metabolising other B6 vitamers through salvage pathways.
Thus, the ‘salvage enzymes’ that catalyse B6 vitamer metabolism maintain intracellular PLP homeostasis, and a deficiency or disruption in their activity can result in pathologies such as B6-dependent epilepsy and polyneuropathy. A deeper understanding of B6 metabolism is therefore required for proper elucidation of its biological roles, and for developing therapeutic approaches to combat vitamer imbalances.
While there are three known enzymes that can metabolise B6 vitamers in mammals, recent work has suggested the existence of an additional, yet to be identified salvage enzyme. For instance, in a recent study, zebrafish deficient in a B6 salvage enzyme still showed an increase in pyridoxine (PN) and its phosphorylated form when given PLP.
In their study published in The FEBS Journal, Kito et al. set out to identify this mystery enzyme in mammals. Their initial experiments focused on determining if a PL-to-PN conversion pathway existed in mice, as this pathway has previously only been described in a limited number of species. They administered PL to mice and analysed the levels of B6 vitamers in the plasma, liver and kidney. Excitingly, they found increased PN concentrations in all three, thus confirming the presence of this conversion pathway. Building on their previous work demonstrating that bacterial pyridoxal reductase enzymes can metabolise extracellular PL to PN, they sought to investigate if this murine B6 vitamer conversion could be attributed to the intestinal microbiota rather than to the mice themselves. They treated the mice with antibiotics and repeated their experiments, finding that PN formation still occurred in the antibiotics-treated or ‘bacteria-free’ mice, thus confirming that the PL-to-PN conversion occurred in the mammalian host, rather than via bacterial enzymes. Digging deeper into the mechanism underlying PL-to-PN conversion, the authors found that this reaction was dependent on the presence of NADPH.
Now that the existence of such a mammalian enzyme had been verified, Kito and co-authors next sought to identify the enzyme itself. Drawing from the knowledge that all previously characterised pyridoxal reductase enzymes belong to the AKR superfamily, they selected 8 AKR family members, and screened their ability to metabolise PL. They found that only AKR1C1 could catalyse the formation of PN from PL, but also found that this reaction produced an additional unidentified compound, ‘X’. The authors demonstrated that compound X was 4-pyridoxolactone (4-PLA), also a B6 vitamer, and that the PL-to-4-PLA reaction was enhanced in the presence of NADP. They thus concluded that AKR1C1 is a bifunctional enzyme that can catalyse the NADPH-dependent conversion of PL to PN, and the NADP+-dependent oxidation of PL to 4-PLA.
Their findings identify AKR1C1 as the first mammalian pyridoxal reductase enzyme which can catalyse the reduction of PL to PN, and point to the complexity of B6 metabolism and regulation. Graziani et al. discuss the meaning of these findings further in their accompanying Commentary published in the journal, and offer additional perspectives on the importance of this study for combating pathologies linked to B6 vitamer imbalances.