A deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase(HGPRT) is associated with a varying clinical picture which may include hyperuricaemia, neurological abnormalities and bizarre self-mutilating behaviour. Due to technical problems with the usual in vitro enzyme assays, it has not been possible to establish a correlation between the degree of the enzyme deficiency and the severity of the clinical manifestations. In this study, the HGPRT activity of 12 patients with various clinical features was measured by quantitative analysis of the incorporation of radioactive precursors into purine compounds in intact fibroblasts. The results demonstrate that a correlation between the severity of the clinical symptoms and the degree of the enzyme deficiency as measured in intact fibroblasts does in fact exist.

Abstract. In women heterozygous for hypoxanthine guanine phosphoribosyl transferase deficiency, the activity of this enzyme in the erythrocyte is usually normal. In a key kindred two such obligate heterozygotes were also heterozygous for glucose-6-phosphate dehydrogenase types A and B. The AB genotype was confirmed in one by assay of skin fibroblasts. Erythrocytes were exclusively of type B. These observations suggest the clonal origin of the hematopoietic system in these women from a primordial cell line with a single active X chromosome.Patients with complete deficiency of the activity of the enzyme hypoxanthine guanine phosphoribosyl transferase (HGPRT) (E.C.2.4.2.8) present clinically with the Lesch-Nyhan syndrome, in which mental retardation, choreoathetosis, spastic cerebral palsy, and self-destructive biting occur along with hyperuricemia." 2 The disorder is transmitted as an X-linked recessive trait.' In accordance with the Lyon hypothesis, the female carrier of the gene should be a mosaic, in which there are two populations of cells, one in which the activity of HGPRT is normal and the other in which the activity of HGPRT is completely absent. This has been shown to be the case in clones of fibroblasts grown in cell culture after biopsy of the skin of mothers of boys with the disease. 5 If the inactivation of one of the X chromosomes in the female were a random process, the mean activity of an enzyme determined by the X chromosome in a series of mothers of deficient males should approximate 50 per cent of normal, unless there were some form of regulation after inactivation. However, assay of HGPRT activity in the erythrocytes of a group of obligate heterozygotes has indicated that activity is normal in virtually all of them.6 Similarly, in autoradiographic studies of hypoxanthine incorporation in lymphocytes of heterozygous females, Dancis and colleagues' found no evidence of mosaicism. Observations on a family in which there were two types of glucose-6-phosphate dehydrogenase (G6PD) (E.C.1.1.1.49) as well as two types of HGPRT indicate that in the hematopoietic system there is selection for the cell with normal activity of HGPRT.Materials and Methods. Fibroblast cultures were established following punch biopsy of the skin and carried in Eagle's minimal essential medium with 20% fetal calf serum.

A B S T R A C T An inherited, structurally abnormal and superactive form of the enzyme 5-phosphoribosyl 1-pyrophosphate (PP-ribose-P) synthetase (EC 2.7.6.1) has been characterized in fibroblasts cultured from a 14-yr-old male (S.M.) with clinical manifestations of uric acid overproduction present since infancy. PPribose-P synthetase from the cells of this child showed four-to fivefold greater than normal resistance to purine nucleotide (ADP and GDP) feedback inhibition ofenzyme activity and hyperbolic rather than sigmoidal inorganic phosphate (Pi) and by an accelerated, age-related decrement in enzyme activity in lysates of erythrocytes separated by specific density. Despite the diminished amount of PP-ribose-P synthetase in the S.M. erythrocyte population, S.M. erythrocytes had increased PPribose-P concentration and increased rates of incorporation of ['4C]adenine and hypoxanthine into acid-soluble nucleotides during incubation at 1 mM Pi. These findings provided further confirmation of the extent to which PP-ribose-P synthesis is modulated in the normal cell at physiological Pi concentration by purine nucleotide inhibition of PP-ribose-P synthetase.The activity and kinetic characteristics of PPribose-P synthetase from fibroblasts of the mother of patient S.M. indicated that this woman was a heterozygous carrier of the enzyme defect expressed in hemizygous manner by her son. INTRODUCTIONThe high-energy sugar phosphate 5-phosphoribosyl I-pyrophosphate (PP-ribose-P)' is an intermediate in the synthesis of purine, pyrimidine and pyridine nucleotides. The synthesis of PP-ribose-P from ATP and ribose-5-phosphate (Rib-5-P) is catalyzed by the enzyme PP-ribose-P synthetase (E.C. 2.7.6.1) in a reaction requiring Mg2+ and inorganic phosphate (Pi). Studies of PP-ribose-P production by intact cells (1, 2) and analyses of the kinetic characteristics of purified microbial (3-5) and mammalian (6-10) PP-ribose-P synthetases indicate that a substantial number of effector compounds including substrates, inhibitors, activators, and products influence enzyme activity.