Pyridoxal 5'-phosphate in plasma: source, protein-binding, and cellular transport.

Abstract

Abstract Pyridoxal 5′-phosphate (PLP), the coenzyme form of vitamin B 6 , has a rapid turnover in human plasma and is in a dynamic equilibrium between synthesis and cellular extraction and/or degradation. In studies of the effect of organ ablation in anesthetized dogs, the liver was demonstrated to be the principal, if not the sole, organ responsible for the formation of plasma PLP from either pyridoxine or pyridoxal. Thus it appears that the liver possesses a unique transport mechanism absent in other tissues, e.g., the red cells, which provides for the efflux of PLP into the plasma. At physiologic concentrations, PLP in human plasma is principally complexed to albumin through Schiff's-base formation. Albumin-bound PLP cannot enter human red cells directly, but it can be hydrolyzed to pyridoxal by alkaline phosphatase. Hydrolysis occurs even when the phosphatase is separated from albumin-bound PLP by a semipermeable membrane, thus indicating that a small amount of free PLP normally exists in equilibrium with the albumin-PLP complex. The results are consistent with the view that pyridoxal is an important transport form of the vitamin B 6 compounds.