Major differences in the metabolism of vitamin B6 in various cancers compared to their normal cellular counterparts have been documented. In particular, pyridoxine- 5'-phosphate oxidase (PNPO), the rate-limiting enzyme in pyridoxal 5'-phosphate (PLP) biosynthesis, is absent in liver and neurally-derived tumors. We show that the expression of PNPO is developmentally regulated not only in liver but also in brain. Specifically, PNPO activity in fetal brain tissue is 7.5-fold lower than that found in adult brain tissue. Furthermore, the isolation and characterization of a PNPO cDNA are described. The isolated cDNA was verified to be the authentic PNPO cDNA on the basis of two criteria. First, the translated product from the PNPO cDNA is immunologically reactive to a polyclonal PNPO antibody. Second, PNPO negative hepatoma cell lines stably transfected with the PNPO cDNA express enzymatically active PNPO protein. The availability of these biological reagents will not only facilitate in depth investigations of the reasons for the absence of PNPO in liver and brain malignancies but also aid in an understanding of the biochemical regulation of B6 metabolism in development.