Hypoxanthine-guanine phosphoribosyltransferase (EC 18.104.22.168) (HGPRT) of human red blood cells has been separated into three major isoenzymes, the relative quantities of which change as the cell ages. The predominant isoenzyme in the youngest circulating red blood cells, reticulocytes, has the same isoelectric point as the single enzyme of lymphoblasts. This lymphoblast-like enzyme is diminished in older red cells, and the major fraction of HGPRT activity is recovered in the two more acidic isoenzymes. The HGPRT enzymes of human lymphoblasts and red cells have been purified to apparent homogeneity, as evidenced by the criterion of subunit molecular weight in NaDodSO4 gels. The lymphoblast enzyme dissociates to a single subunit (a) upon isoelectric focusing in 8 M urea and is presumed to be a homo dimer (αα). The red cell isoenzymes dissociate to two subunits, one with the same isoelectric point as that in lymphoblasts (α) and one more negatively charged (α′). We infer that the three major red cell isoenzymes, I–III, correspond to enzyme species with none (αα), one (αα′), or both (α′α′) subunits modified. Tryptic peptide maps of these iodo[2-14C]acetamide-labeled enzyme subunits indicate that the one red cell subunit (α) is identical with that in lymphoblasts and that the second subunit (α′) differs from these in only one of the five cysteine-containing tryptic peptides. These results indicate that the HGPRT subunit is subject to at least one covalent and site-specific modification in human erythroid cells.
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