Purine nucleoside phosphorylase deficiency: altered kinetic properties of a mutant enzyme.

Erythrocyte purine nucleoside phosphorylase from two brothers had 0.5% of normal activity. It differed from the normal enzyme by a tenfold increase in the Michaelis constant for inosine, an inability of inosine to protect against thermal lability, and a more positive net charge. The altered kinetic properties may account for the milder disease in the patients compared to the previously described cases. The data provide evidence for a structural gene mutation and genetic heterogeneity in the new disease of purine nucleoside phosphorylase deficiency and T cell dysfunction.