Human Hypoxanthine Phosphoribosyltransferase. Purification and Properties

Hypoxanthine phosphoribosyltransferase (IMP:pryophosphate phosphoribosyltransferase, EC 2.4.2.8) from human erythrocytes has been purified 13 000-fold to apparent homogeneity. The native enzyme has a sedimentation coefficient of 5.9 S, determined by analytical ultracentrifugation, and a molecular weight of 81 000-83 000, determined by sedimentation equilibrium centrifugation. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicates a subunit molecular weight of 26 000, suggesting that the enzyme is a trimer. Isoelectric focusing resolves three peaks of enzyme activity at pH 5.6, 5.7 and 5.9. The amino acid composition of hypoxanthine phosphoribosyltrasferase is 17 Lys, 5 His, 12 Arg, 0 Trp, 31 Asx, 12 Thr, 14 Ser, 16 Glx, 14 Pro, 19 Gly, 12 Ala, 5 Cys, 18 Val, 5 Met, 11 Ile, 20 Leu, 10 Tyr, and 9 Phe. The enzyme appears to have a blocked N terminus.