IDENTIFICATION OF LYS(190)AS THE PRIMARY BINDING-SITE FOR PYRIDOXAL 5'-PHOSPHATE IN HUMAN SERUM-ALBUMIN

The covalent binding of pyridoxal 5'-phosphate (PLP) to human serum albumin (HSA) is important in the regulation of PLP metabolism. In plasma, PLP is bound to HSA at a single high-affinity and at two or more nonspecific sites. To characterize the primary PLP binding site, HSA was incubated with [H-3]PLP, and the Schiff base linkage was reduced with potassium borohydride. Tryptic peptides were purified, and the major labeled peptide was sequenced. Amino acid analysis confirmed a homogeneous peptide Leu-Asp-Glu-Leu-Arg-Asp-Glu-Gly-Xaa-Ala-Ser-Ser-Ala-Lys which corresponds to residues 182-195 of HSA. The data indicate that Lys190 is the primary PLP binding site. This Lys residue is distinct from other sites of covalent adduct formation; namely, the primary sites for nonenzymatic glycosylation (Lys525) and acetylation by aspirin (Lys199).