Human hypoxanthine-guanine phosphoribosyltransferase. Tryptic peptides and post-translational modification of the erythrocyte enzyme.

We describe the isolation and characterization of tryptic peptides of human erythrocyte hypoxanthine-guanine phosphoribosyltransferase. The digest was separated by reverse-phase high pressure liquid chromatography into 30 peaks, 26 of which contained purified peptides. The four complex peaks were resolved by high pressure liquid chromatography with a different reverse-phase column. Each peptide predicted from the recently described amino acid sequence of human erythrocyte hypoxanthine-guanine phosphoribosyltransferase was isolated from this peptide map. Sequence analysis of the purified peptides identified two peptides, 14 and 14d, that differed by an Asn/Asp heterogeneity at the position corresponding to residue 106 of the intact protein. Additional studies indicate that this heterogeneity is due to deamidation of the protein in vivo. This post-translational modification appears to be responsible for some of the electrophoretic heterogeneity observed in the normal erythrocyte enzyme.