STUDIES ON THE ELECTROPHORETIC VARIANTS OF HUMAN HYPOXANTHINE-GUANINE PHOSPHORIBOSYLTRANSFERASE

Abstract

ABSTRACT. During the purification of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) from normal human male erythrocytes, three distinct peaks of HGPRT activity (I-pI 5.65; II-pI 5.80; III-pI 6.01) have been reproducibly distinguished by preparative isoelectric focusing and subsequently purified to homogeneity. With these highly purified preparations of variants I, II, and III we have found that: (1) the three variants are interconvertible as determined by polyaery1amide gel electrophoresis; (2) the three variants are immunologically identical; (3) the three variants have similar substrate utilization and end-product inhibition; (4) the three variants have the same native molecular weight (68,000) and Stokes radius (36a) and each is composed of two non-covalently bound subunits of equal molecular weight (34,000) and net charge; (5) the amino acid composition of variants II and III is nearly identical. We conclude that the electrophoretic variants of human erythrocyte HGPRT most likely result from a non-genetic post-transcriptional alteration of one or both subunits of the HGPRT enzyme molecule. Although the exact nature of the post-transcriptional alteration is not known, differential sialiation, differential binding of ribose-5-phosphate or ampholytes and association of the subunits into trimers, tetramers, etc. have been excluded.