Pyridoxal 5'-phosphate in plasma: source, protein-binding, and cellular transport
Lumeng, L.; Brashear, R.E.; Li, T.K.
Journal of Laboratory and Clinical Medicine 84(3): 334-343
1974
ISSN/ISBN: 0022-2143 PMID: 4853981 Document Number: 74303
Ablation of organs in anaesthetised dogs indicated that the liver is the primary source of pyridoxal 5'-phosphate (PLP) in plasma, synthesised from either pyridoxine or pyridoxal. Although pyridoxal kinase activity was present in almost all tissues examined, only the liver contributed PLP to the plasma pool. Studies in vitro on human erythrocytes substantiated the observations mad in vivo and indicated that in liver there is a unique transport mechanism, absent in other tissues such as erythrocytes, which affects the efflux of PLP into plasma. At physiological concentrations albumin was the principal carrier of PLP in human plasma; the capacity of human serum proteins to bind PLP was more than 800 mu g/ml. Albumin-bound PLP did not enter human red cells directly, but it was hydrolysed to pyridoxal by alkaline phosphatase; hydrolysis occurred even when the phosphatase was separated from albumin-bound PLP by a semipermeable membrane, indicating that a small amount of free PLP normally exists in equilibrium with the albumin-PLP complex.