Lesch-Nyhan Syndrome: The Synthesis of Inosine 5′-Phosphate in the Hypoxanthine-Guanine Phosphoribosyltransferase-deficient Erythrocyte by Alternate Biochemical Pathways

Publisher Website
Google Scholar
Abstract
Summary: Erythrocytes, obtained from a normal adult male and from a patient with Lesch-Nyhan syndrome, were incubated with [8− 14C]adenine and [8-14C]hypoxanthine (Table 1). The labeled adenine was utilized to about the same extent for the synthesis of AMP by the normal subject's and the patient's erythrocytes. Deamination of AMP to IMP occurred to about the same extent in both samples. In contrast, hypoxanthine was utilized extensively for IMP synthesis in the normal erythrocyte only. The amount of total label in the IMP was about 100 times that of the Lesch-Nyhan erythrocyte, a consequence of the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity in the syndrome. No significant labeling of the AMP occurred. When aliquots of erythrocytes from both sources were incubated with 4-amino-5-imidazoIccarboxamide (AICA) and sodium [14C]formate, extensive labeling of the IMP occurred in normal and in Lesch-Nyhan erythrocytes. The data suggest that AICA serves as a substrate for the adenine phosphoribosyllrans- ferase (APRT) of the Lesch-Nyhan erythrocyte and that the ribotide of AICA, 5′-phosphoribosyl-5-aminoimidazole-4-car- boxamide (AICAR), undergoes formylation by labeled N10- formyl tetrahydrofolic acid formed from the reaction of sodium [14C]formate with the tetrahydrofolic acid of the cell. The formyl-AICAR undergoes ring closure to IMP by a series of reactions comparable to those described for the normal erythrocyte. When 5-amino-1-ribosyl-4-imidazolccarboxamide (rAICA) and sodium [14C]formate were incubated with erythrocyte suspensions, extensive utilization for IMP synthesis was also observed in normal erythrocytes and in erythrocytes from Lesch- Nyhan patients (Table 2). The reaction sequence is somewhat different from that of AICA. rAICA is not a substrate for the purine nucleoside phosphorylase of rabbit or human erythrocytes. The mechanism of rAICA utilization is visualized as a direct phosphorylation of the ribosyl compound, possibly by the adenosine kinasc of the human cell. The ribotide, AICAR, formed by this mechanism, undergoes formylation and ring closure, yielding IMP. The glutamine antagonist, diazooxonorlcucinc (DON), was added to aliquots of patients' cells incubated with rAICA and sodium [14C]formate. DON is an effective inhibitor of the conversion of IMP to GMP and its presence in an incubation suspension resulted in a somewhat greater radioactivity of the total cellular IMP. The extension of the current studies to Lesch-Nyhan cells in culture may serve to assist in the direct evaluation of the regulatory role of IMP in the de novo pathway of purine nucleotide biosynthesis. Because of the substrate requirements of the reactions, the metabolism of AICA and rAICA may also serve to differentiate the roles of purine nuclcotidcs and of phosphoribo- sylpyrophosphate (PRPP) in the pathway regulation. The findings presented also offer a possible therapeutic approach to the early treatment of the disease in the afflicted neonate. The administration of AICA and/or rAICA, possibly supplemented with folic acid and allopurinol, by providing a route to IMP and GMP, with or without concomitant reduction of the elevated PRPP levels, might eliminate, delay, or reduce the development of the symptoms and manifestations of the genetic disease. Speculation: The erythrocytes of patients with Lesch-Nyhan syndrome, although lacking the enzymie capacity for conversion of the 6− ketopurines, hypoxanthine and guanine, to the corresponding ribonucleotides, can convert AICA and its ribosyl derivative (rAICA) to IMP by two different reaction mechanisms, thus bypassing the enzymic deficiency. The pathways described should be applicable to studies of regulatory mechanisms in de novo purine nucleotide synthesis and in purine nucleotide inter- conversions. The imidazole compounds may also have therapeutic application in the early clinical treatment of the syndrome.