Human hypoxanthine-guanine phosphoribosyltransferase. Purification and characterization of mutant forms of the enzyme.

Erythrocyte hypoxanthine-guanine phosphoribosyltransferase has been highly purified from five unrelated patients with a deficiency of this enzyme. Affinity chromatography using either GMP-Sepharose or an immunoadsorbent was the most productive step in the purifications. The specific activity of the purified enzyme was unchanged for patients L. P. and G. S., and slightly decreased for patient R. H., as compared to control subjects. Enzyme from patient I. V. and from patient E. S. exhibited markedly reduced specific activities when purified to near homogeneity. The level of immunoreactive protein in patient I. V. appeared to be significantly higher than normal. The apparent subunit molecular weight of the enzyme from patient G. S. was decreased by approximately 1000 while it was increased by approximately 400 from patient I. V. The isoelectric points of the subunit isozymes were shifted to higher pH values from patients I. V. and E. S., and to lower pH values from patient L. P.; the subunit isozymes from patient G. S. were identical with normal. These studies provide direct evidence for the existence of at least four different mutations in the structural gene for hypoxanthine-guanine phosphoribosyltransferase. The four different mutant forms of human hypoxanthine-guanine phosphoribosyltransferase that have been identified are named as follows: patient L. P., HPRTToronto; patient G. S., HPRTLondon; patient E. S., HPRTKinston; and patient I. V., HPRTMunich.