Abstract
A mutant hypoxanthine-phosphoribosyltransferase (EC 2.4.2.8) from a patient with gout was examined. The activity of the erythrocyte enzyme was about 5% of normal in this case. Immunoprecipitation studies using antiserum against highly purified human hypoxanthine-phosphoribosyltransferase revealed that the patient''s erythrocytes contained a normal amount of cross-reacting material. The mutant enzyme had an altered net charge as shown by preparative isoelectric focusing (pI (isoelectric point) values of 5.75 and 4.55). The influence of chemical modification on enzymic activity was studied using a number of different reagents directed against SH, NH2 and guanidino groups. Compared with normal hypoxanthine-phosphoribosyltransferase the mutant enzyme showed a generally lowered susceptibility to active site-directed inhibition. The patient''s enzyme is apparently the product of a structural mutation.

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