Adenine phosphoribosyltransferase specificity

HUMAN ADENINE PHOSPHORIBOSYLTRANSFERASE PURIFICATION, SUBUNIT STRUCTURE AND SUBSTRATE SPECIFICITY... 

The most highly purified enzyme preparation was studied for substrate specificity (Table 2). The results are expressed as per cent of nucleotide formed from each substrate as compared to adenine. Adenine phosphoribosyltransferase shows substantial activity toward 4-amino-5-imidazolecarboxamide and 2,6-diaminopurine but not hypoxanthine, guanine, 6-mercaptopurine, or adenosine. 

Thomas, C.B., Arnold, W.J. and Kelley, W.N. 1973. Human adenine phosphoribosyltransferase Purification, subunit structure, and substrate specificity. J. Biol. Chem. 248 2529-2535. 

Fig ire 2 shows the possible interconversions between purine bases, nucleosides and nucleoside monophosphates. Prom genetic and enzymatic studies it has been shown that the conversion of the free bases(adenine, hypoxanthine,xanthine and guanine) to the monophosphate level is carried out by at least three different enzymes (the phosphoribosyltransferases)(1,5). This is in contrast to htunan tissues, which have one specific adenine phosphoribosyltransferase and one transferase with activity towards hypoxanthine,xanthine and guanine. 

Adenine phosphoribosyltransferase activity assayed by an analogous method using 0.6 mM adenine-8-l C (specific activity 4 mCI/mmole) was not significantly altered in any of the HGPRT-deficient clones (Table II). Starch gel electrophoresis using a method only slightly modified from that of Watson et. al. (19) showed no difference in electrophoretic mobility of mutants partially deficient in HGPRT (Fig. 3) but could not be used to examine the clones with severely deficient enzyme activity. 

Recent advances in the understanding of human purine metabolism have been stimulated by the discovery of specific inborn errors of this pathway in man. In particular, the demonstration of the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) in the Lesch-Nyhan syndrome and in some patients with gout has contributed essential information on the regulation of purine biosynthesis novo and on the critical role of this reutilization pathway in central nervous system function in man. The search for other disorders led to the description of a partial deficiency of adenine phosphoribosyltransferase (APRT) in four members in three generations of one family. Each of the subjects partially deficient in APRT exhibited a normal serum urate concentration and the propositus had a normal excretion of uric acid (Kelley, et al., 1968). We have investigated a second family partially deficient in APRT (Fox and Kelley, in press). 

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. 

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