Purine phosphoribosyltransferases phosphoribosyltransferase

V. L. Schramm, and S. C. Almo, The 2.0 A structure of malarial purine phosphoribosyltransferase in complex with a transition-state analogue inhibitor, Biochemistry, 38 (1999) 9872-9880. 

Hassan, H. F. and Coombs, G. H. (1985) Purine phosphoribosyltransferases of Leishmania mexicana mexicana and other flagellate protozoa. Comp. Biochem. Physiol. [B] 82 113-119. 

Queen, S. A., Vander Jagt, D. and Reyes, P. (1988) Properties and substrate specificity of a purine phosphoribosyltransferase from the human malaria parasite, Plasmodium falciparum. Mol. Biochem. Parasitol. 30 123-133. 

M12. Murray, A. W., Purine phosphoribosyltransferase activities in rat and mouse tissues and in Ehrlich ascites-tumour celk. Biochem. J. 100, 664 (1966). 

It was not until 1953 that Goldwasser (9) and Williams and Buchanan (10) showed that purine bases could be converted to ribonucleotides by a one-step process, without the intermediate formation of ribonucleosides. The source of the ribose phosphate moiety was discovered in 1955 to be PP-ribose-P in the course of studies of adenylate synthesis by Kornberg et al. 11), and of inosinate synthesis by Korn et al. 12) extracts of yeast, beef liver, and pigeon liver were employed. The enzymes involved were at first called nucleotide pyrophosphorylases, but are now known as purine phosphoribosyltransferases. The general reaction is... 

Further studies have shown that there are two important purine phosphoribosyltransferases in animal tissues and three in some microorganisms. This topic has recently been exhaustively reviewed by Murray et al. 13), Raivio and Seegmiller I4), and by Murray IS). 

The existence of a separate phosphoribosyltransferase for adenine was first deduced because gentle heating of crude enz3rme preparations destroyed this activity, without decreasing their ability to make inosinate from hypoxanthine and PP-ribose-P. Adenine phosphoribo ltransferases have since been separated from other purine phosphoribosyltransferases by ion-exchange chromatography, electrophoresis, and gel filtration, and the separate identities of these enzymes have also been demonstrated by specific mutations, both in man and in bacteria. 

The purine phosphoribosyltransferases permit cells to use exogenous or dietary purines, and this function is undoubtedly important to some bacteria and to those animal cells (e.g., erythrocytes) which do not synthesize purines de novo. The importance of this role in other animal cells in vivo is far from clear, however, as hypoxanthine is present in serum only at very low concentrations (50), and adenine and guanine have not been detected in normal serum. Purine bases (especially hypoxanthine and guanine) can be produced intracellularly by the catabolism of messenger RNA and soluble purine nucleotides their reutilization via the phosphoribosyltransferases would prevent the loss of these compounds from the cells. 

Table I Summary of activities of purine phosphoribosyltransferases in Salmonella typhimurium.

Purine phosphoribosyltransferase activity nmol/mg protein/hour ... 

Table I Purine phosphoribosyltransferase activities in 30.000 g supernatants and washed pellets obtained from erythrocyte lysates.

PP-ribose-P is a sugar phosphate which is synthesized from ATP and ribose-5-phosphate in a reaction requiring magnesium and inorganic phosphate catalyzed by PP-ribose-P synthetase (Figure 1). The PP-ribose-P formed is a substrate in the first and probable rate-limiting reaction of purine synthesis de novo which is catalyzed by PP-ribose-P amidotransferase (PAT). In addition, PP-ri-bose-P is a substrate in the purine phosphoribosyltransferase reactions which constitute a pathway for the salvage of purine bases. 

Apart from such inter-species differences, many differences have been found in the proportions of enzymes in the different tissues of a sir le organism. Thus aconitase and oxaloacetate transacetase are much more abundant in heart than in skeletal muscle, but the reverse is true of aldolase (Dixon and Webb, 1964 cf. Table 4.5). Similarly, in mouse tumour cells (Ehrlich ascites), the specific activity of purine phosphoribosyltransferase was found to be between 15 and 60 times the activity of that in liver, brain, spleen, heart, or kidneys of the same animal (Murray, 1966). 

Adenine phosphoribosyltransferase (APRT) deficiency is an inherited disorder of purine metabolism and is inherited in an autosomal recessive manner (K18, V7). This enzyme deficiency results in an inability to salvage the purine base adenine, which is oxidized via the 8-hydroxy intermediate by xanthine oxidase to 2,8-di-hydroxyadenine (2,8-DHA). This produces crystalluria and the possible formation of kidney stones due to the excretion of excessive amounts of this insoluble purine. Type I, with virtually undetectable enzyme activity, found predominantly in Caucasians, is found in homozygotes or compound heterozygotes for null alleles. Type II, with significant APRT activity, found only in Japan, is related to a missense mu-... 

The free bases of the purines can be salvaged to spare de novo synthesis. The only hard thing is remembering what the names stand for. HGPRTase is hypoxanthine-guanine phosphoribosyltransferase, and it makes both IMP and GMP. A separate enzyme exists for the salvage of adenine. The salvage pathways are included in Fig. 19-1. 

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