Guanine phosphoribosyltrans

Lesch-Nyhan syndrome, an overproduction hyperuricemia characterized by frequent episodes of uric acid hthiasis and a bizarre syndrome of self-mutilation, reflects a defect in hypoxanthme-guanine phosphoribo-syl transferase, an enzyme of purine salvage (Figure 34—4). The accompanying rise in intracellular PRPP results in purine overproduction. Mutations that decrease or abohsh hypoxanthine-guanine phosphoribosyltrans-ferase activity include deletions, frameshift mutations, base substitutions, and aberrant mRNA splicing. 

Lesch-Nyhan syndrome Hypoxanthine-guanine phosphoribosyltrans-f erase Purines, uric acid Mental retardation, self-mutilation... 

Mechanisms of action and resistance Mercaptopurine and thioguanine are purine antimetabolites. Both drugs are activated by hypoxanthine-guanine phosphoribosyltrans-ferases (HGPRTases) to toxic nucleotides that inhibit several enzymes involved in purine... 

J. E. Seegmiller. Hypoxanthine-guanine phosphoribosyltrans-ferase deficiency in gout. Ann.Intern.Med., 70 165 (1969). 

Cultured human skin fibroblasts are commonly utilized in the detection of hemizygosity and heterozygosity for hypoxanthine-guanine phosphoribosyltrans-ferase (HGPRT) deficiency. Two obstacles are encountered in the determination of HGPRT and adenine phosphoribosyl-transferase (APRT) in extracts of cultured skin fibroblasts the sensitivity of these enzymes in dilute cell suspension to freezing and thawing (l), and the presence of nucleotidase activity (2). 

Action of 6-MP and allopurinol on the hypoxanthine-guanine-phosphoribosyltrans ferase... 

The deficiency of hypoxanthine guanine-phosphoribosyltrans-ferase (HGPRT) activity of patients with the Lesch-Nyhan syndrome is accompanied by an accelerated rate of purine biosynthesis de novo. This elevated rate of purine biosynthesis is reflected in fibroblasts cultured from the skin of these patients and is accompanied by elevated levels of phosphoribosylpyrophosphate (PRPP). 

Hypoxanthine, on the other hand, which accounts for only a fifth or so of the urinary uric acid is an active intermediate. It is degraded to xanthine and then to uric add by xanthine oxidase. This enzyme is found mainly in liver, kidney, and bowel, while guanase is widely distributed and would quickly deaminate any guanine formed. The product xanthine is a poor substrate for hypoxanthine phosphoribosyltrans-ferase (HPRT). Most of the hypoxanthine formed is reutiliiced by conversion to inosinic acid. Similar conclusions were reached by Ayvazian and Skupp in 1965 when they administered C-labeled purines to patients (A2). Furthermore, these studies and those earlier studies show that the xanthine is converted to hypoxanthine, presumably at the nucleotide level, and on the basis of what we know about microorganisms, we would assume it to be via guanine nucleotides (M2). Since label was found in urinary 7-methylguanine as early as 4 hours after administration of C-labeled purines, and since methylation of RNA occurs at the macromolecular level (B13), interconversion must be rapid and incorporation of some of these products into nucleic acids must also occur quickly. 

PROPERTIES OF ERYTHROCYTE PURINE PHOSPHORIBOSYLTRANS-FERASES IN PARTIAL HYPOXANTHINE-GUANINE PHOSPHORIBOSYL-TRANSFERASE DEFICIENCY... 

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