Human adenine phosphoribosyltransferase. Immunochemical quantitation and protein blot analysis of mutant forms of the enzyme.

We have studied the catalytic, immunochemical, and electrophoretic properties of adenine phosphoribosyltransferase in hemolysates from 30 patients with a deficiency of this enzyme in six unrelated families. We have found that: 1) the level of adenine phosphoribosyltransferase enzyme activity and immunoreactive protein in the four homozygous deficient patients was less than 1% of control values; 2) adenine phosphoribosyltransferase enzyme activity was uniformly decreased to approximately 25% of normal in all 26 heterozygotes studied while the level of adenine phosphoribosyltransferase immunoreactive protein was consistent within each kindred but ranged in value from 22% to 112% of control; and 3) protein blot analysis revealed a single isoelectric form of the adenine phosphoribosyltransferase subunit in hemolysate from normal controls and from every heterozygote except for patient M.R. who exhibited both a normal and a more acidic adenine phosphoribosyltransferase subunit species. These studies provide the first evidence for the existence of a variety of different mutations in the structural gene for adenine phosphoribosyltransferase in patients exhibiting a deficiency of enzyme activity. We further conclude from our data that: 1) the variant enzymes are more labile in vivo and/or catalytically nonfunctional, and 2) heterozygotes express only 25% of normal enzyme activity because the normal and variant enzyme subunits form a hybrid dimer which is either more labile or less catalytically active than the normal adenine phosphoribosyltransferase dimer.