ABSENCE OF PYRIDOXINE-5'-PHOSPHATE OXIDASE (PNPO) ACTIVITY IN NEOPLASTIC-CELLS - ISOLATION, CHARACTERIZATION, AND EXPRESSION OF PNPO CDNA

Major differences in the metabolism of vitamin Bg in various cancers c ompared 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 liv er and neurally-derived tumors. We show that the expression of PNPO is developmentally regulated not only in liver but also in brain. Specif ically, PNPO activity in fetal brain tissue is 7.5-fold lower than tha t found in adult brain tissue. Furthermore, the isolation and characte rization 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 PNP O 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 Bg metabolism in development.