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The covalent binding of pyridoxal5'-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 ['HIPLP, 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 peptidc Leu-Asp-Glu-Lcu-Arp-As~GIu-Gly-Xaa-Als-Ser-Ser-Ala-Lys which corresponds to residues 182-195 of HSA. The data indicate that LyslYo 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 (LysJz5) and acctyla.tion by aspirin (L~s'~~).Pyridoxal 5'.phosphate; Vitamin B6; Binding site; Albumin (human)