Further Studies of the Enzyme Composition of Mutant Cells in X-Linked Uric Aciduria

Abstract

Assay procedures have been developed for the determination of hypoxanthine guanine phosphoribosyltransferase (HGPRT) and adenine phosphoribosyltransferase (APRT) activity in erythrocytes and for HGPRT and inosinic acid dehydrogenase activity in fibroblasts. In normal erythrocytes the mean activity for HGPRT using hypoxanthine as substrate was 39.09 m\g=m\mols/hr/\g=m\lof cells. The mean value for patients with the Lesch-Nyhan syndrome was 0.003 m\g=m\mols/hr/\g=m\l. Nearly all of the obligate heterozygotes studied had normal levels of HGPRT activity in both erythrocytes and fibroblasts. The normal erythrocyte mean level of APRT activity was 23.56 m\g=m\mols/hr/\g=m\l.All subjects with complete or partial deficiency of HGPRT and heterozygotes had elevated APRT activity.There were no differences among the inosinic acid dehydrogenase (IMPDH) activities of fibroblasts from subjects who were normal, HGPRT-deficient, or hyperuricemic without HGPRT deficiency.The syndrome described by Lesch and Nyhan' consists clinically of mental retardation, athetoid cerebral palsy, and aggressive, compulsive be¬ havior most dramatically character¬ ized by self-mutilation.2 This is an in¬ herited metabolic disease. The locus for the mutant gene is on the X chro¬ mosome.3 It was shown by Seegmiller et al4 that the primary expression of the mutant gene is to determine the activity of the enzyme hypoxanthine guanine phosphoribosyltransferase (HGPRT). Summaries of experience in their laboratory with the enzyme and its defects in the erythrocyte and in the fibroblasts have been presented by Kelley.5·6 It is the purpose of this report to describe our experience with the activity of HGPRT and other en¬ zymes of purine metabolism in con¬ trol individuals and in patients with disorders of purine metabolism.The methods employed for the as¬ say of HGPRT were different from those previously described. Levels of activity of HGPRT have been deter¬ mined using both hypoxanthine and guanine as substrates. Adenine phosphoribosyltransferase (APRT) activity has also been determined using similar methodology. Assess¬ ment of these activities has been car¬ ried out in erythrocytes of control in¬ dividuals and in patients with the Lesch-Nyhan syndrome, in whom the erythrocyte activity of HGPRT is vir¬ tually absent. Erythrocytes have also been studied in patients with partial deficiency of HGPRT in whom quite a different clinical syndrome has been observed.7 8 Patients described by Kogut and colleagues7 had 5% of normal activity of HGPRT and suffered from urinary tract stone disease in child¬ hood. There were no central nervous system abnormalities. We have also studied women who were obligate hétérozygotes and patients with hy¬ peruricemia of unknown cause. The activity of HGPRT has also been de¬ termined using hypoxanthine as sub¬ strate in fibroblasts in cell culture from the same populations of sub¬ jects. The activity of inosinic acid dehydrogenase (IMPDH) has also been assayed in fibroblasts cultured from some of these subjects. MethodsThe assays emp...