Submolecular characterization of a mutant human purine-nucleoside phosphorylase

Purine-nucleoside phosphorylase deficiency in humans is associated with a severe defect in thymus-derived lymphocyte function. We have used the consanguineous parents of one purine-nucleoside phosphorylase-deficient patient as a source of mutant protein since the enzyme is totally lacking in the homozygous deficient child. When analyzed by denaturing two-dimensional gel electrophoresis, purified erythrocytic purine-nucleoside phosphorylase from normal individuals has four major and two minor subunits of identical molecular weight (29,700) but different isoelectric points. Similar analysis of phosphorylase purified from the parents of the purine-nucleoside phosphorylase-deficient child reveals the presence of two additional, more alkaline subunits with molecular weights significantly greater (approximately 500) than the normal subunits. Comparison of the two-dimensional thin layer peptide maps of the complete tryptic hydrolysates of the 125I-labeled subunits demonstrates that all eight subunits have a majority of their peptides in common, and thus, the structures of all are closely related. Peptide maps of the major subunits of normal purine-nucleoside phosphorylase are all identical with each other but different in several peptides from those of the normal minor subunits. Peptide maps of the mutant subunits differ from those of the normal major subunits only in the loss of one normal peptide and the presence of two new peptides. Tryptic peptide analysis of overlapping partial proteolytic cleavage fragments of the mutant and normal major subunits of purine-nucleoside phosphorylase has been used to determine the order of the iodinated tryptic peptides in the molecule. These results indicate that the mutant subunits are altered at an internal position rather than at the termini of the protein. A likely explanation of this internal alteration is an insertion of several tandem amino acid residues, perhaps resulting from an unequal genetic crossover or an intron processing abnormality.