PNPO catalyzes the terminal, rate-limiting step in the synthesis of pyridoxal 5'-phosphate, also known as vitamin B6. Vitamin B6 is a required co-factor for enzymes involved in both homocysteine metabolism and synthesis of neurotransmitters such as catecholamine.Vitamin B6, or pyridoxal 5-prime-phosphate (PLP), is critical for normal cellular function, and some cancer cells have notable differences in vitamin B6 metabolism compared to their normal counterparts. The rate-limiting enzyme in vitamin B6 synthesis is pyridoxine-5-prime-phosphate (PNP) oxidase (PNPO; EC 1.4.3.5).[supplied by OMIM].