Disparate enzyme activity in erythocytes and leukocytes. A variant of hypoxanthine phosphoribosyl-transferase deficiency with an unstable enzyme

A family is reported in which each of two sisters has a son with no detectable hypoxanthine phosphoribosyltransferase (HPRT) (EC 2. 4. 2. 8) in his erythrocytes, a finding considered pathognomonic of Lesch-Nyhan disease. However, neither has the stigmata of the disease. One boy is neurologically normal, and the other is moderately retarded. There was only a slight increase in urinary uric acid, but the amounts of hypoxanthine and xanthine, and their ratios, were similar to those found in Lesch-Nyhan disease, strongly indicating that excesses of these last two oxypurines are not responsible for the symptomatology in that disease. In contrast to the nondetectable HPRT activity in the red blood cells, leukocyte lysates from the two boys have 10-15% of normal activity, possibly reflecting continuing synthesis of an unstable enzyme. This hypothesis is supported by the demonstration that at 4 degrees C HPRT activity was rapidly lost in the propositus while the activity increased in control subjects. The mother's cells were intermediate between the two. The intact and disrupted leukocytes of the hemizygote, in the absence of added phosphoribosyl converted as much hypoxanthine to inosinate as the normal cell, and appropriate tests indicated that under these circumstances enzyme concentration is not rate limiting whereas the concentration of the cosubstrate, phosphoribosyl pyrophosphate, is. The capacity for normal function in the intact mutant cell is more representative of in vivo conditions than the lysate, which may explain the important modification of clinical symptomatology, the relatively mild hyperuricosuria, and the presence of mosaicism in the circulating blood cells of the heterozygotes. A similar explanation may apply to other genetic diseases in which incomplete but severe enzyme deficiencies are found in clinically normal individuals. An associated deficiency in glucose-6-phosphate dehydrogenase in this family permitted confirmation of previous observations on linkage with hypoxanthine phosphoribosyltransferase.

Studies on the coordinate activity and liability of orotidylate phosphoribosyltransferase and decarboxylase in human erythrocytes, and the effects of allopurinol administration

A coordinate relationship between the activities of two sequential enzymes in the de novo pyrimidine biosynthetic pathway has been demonstrated in human red cells. The two enzymes, orotidylate phosphoribosyltransferase and decarboxylase are responsible for the conversion of orotic acid to uridine-5'-monophosphate. Fractionation of red cells, on the basis of increase of specific gravity with cell age, has revealed that these two enzymes have a marked but equal degree of lability in the ageing red cell. It is postulated that orotidylate phosphoribosyltransferase and decarboxylase form an enzyme-enzyme complex, and that the sequential deficiency of these two enzymes in hereditary orotic aciduria may reflect a structural abnormality in this complex. In patients receiving allopurinol, the activities of both enzymes are coordinately increased, and this increase appears to be due, at least in part, to stabilization of both orotidylate phosphoribosyltransferase and decarboxylase in the ageing red cell. Allopurinol ribonucleotide is an in vitro inhibitor of orotidine-5'-monophosphate decarboxylase and requires the enzyme hypoxanthineguanine phosphoribosyltransferase for its synthesis. However, the administration of allopurinol to patients lacking this enzyme results in orotidinuria and these patients have elevated orotidylate phosphoribosyltransferase and decarboxylase activities in their erythrocytes. Evidence is presented that the chief metabolite of allopurinol, oxipurinol, with a 2,4-diketo pyrimidine ring is capable of acting as an analogue of orotic acid. It is postulated that the in vivo formation of oxipurinol ribonucleotide, catalyzed by orotidylate phosphoribosyltransferase, after allopurinol administration, leads to inhibition of orotidine-5'-monophosphate decarboxylase. This inhibition results in the urinary excretion of excessive amounts of orotidine and orotic acid, and "pseudo-substrate" stabilization of orotidylate phosphoribosyltransferase and decarboxylase.