The Vitamin B6 Salvage Pathway: BiomedicalAspects of Pyridoxal 5’-Phosphate Availability

Pyridoxal 5’-phosphate (PLP), the catalytically active form of vitamin B6, has a crucial role in several cellular processes, acting as cofactor in more than 140 different enzyme reactions, including the synthesis and degradation of neurotransmitters. In humans, as in all animals, PLP is recycled from foods and degraded B6-enzymes in a “salvage pathway”, which involves two enzymes: pyridoxal kinase and pyridoxine phosphate oxidase. Once PLP is made, it is targeted to dozens of different apo-B6-enzymes that are being synthesized in the cell. The mechanism and regulation of the salvage pathway and the mechanism of addition of PLP to the newly synthesized apo-B6-enzymes are poorly understood and represent a very challenging research field. The free PLP concentration in the cell is quite low, probably in order to prevent unspecific reactions of the cofactor aldehyde group, and is not enough to meet the requirements of the many B6-enzymes (1,2). This raises the intriguing question of how the cell supplies sufficient PLP, with high specificity, to B6-enzymes. A very attractive hypothesis is that PLP may be transferred from either pyridoxal kinase or pyridoxine phosphate oxidase to apo-B6-enzymes by direct channeling (2-4), an efficient, exclusive and protected means of delivery of the highly reactive PLP. Polymorphisms in enzymes of the salvage pathway are known to reduce the availability of PLP in the cell, causing severe neurological disorders in humans. At the same time, polymorphisms of PLP-dependent enzymes are known to affect their capability to bind the cofactor and may determine disease states. Several drugs, as well as natural substances, are known to antagonize the action of vitamin B6. Some classes of substances achieve these effects by inhibiting the activity of pyridoxal kinase, causing seizures, headache, agitation, convulsions, unconsciousness, paralysis and even death. Our presentation will focus on several aspects of the above-mentioned problems and will report: i) studies on serine hydroxymethyltransferase as a model of PLP addition to apo-B6-enzymes; ii) characterization of human pyridoxine phosphate oxidase polymorphisms and studies on the hypothetical channelling of PLP to apo- B6-enzymes; iii) studies on human pyridoxal kinase: regulation mechanism and structural determination of the interaction with drugs. Studies on the hypothetical channelling of PLP to apo- B6-enzymes.