HGPRTase

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. 

The product of this reaction, 6-TGMP, can eventually be converted to deoxy-6-thioguanosine-triphos-phate (dTGTP), which has been shown to be incorporated into DNA. Resistance of human leukemia cells to thioguanine has been correlated with decreased activity of HGPRTase and to increased inactivation of the thio nucleotides by alkaline phosphatase. 

Resistance to mercaptopurine may be a result of decreased drug activation by HGPRTase or increased inactivation by alkaline phosphatase. 

A serious genetic disorder is associated with the salvage pathways, the Lesch-Nyhan syndrome. It is believed that it is caused by a failure to control the de novo purine biosynthetic pathway. In the Lesch-Nyhan syndrome, the enzyme HGPRTase is severely depressed. Because the de novo pathway is controlled largely via feedback effects of purine nucleotides, the pathway is derepressed and excessive quantities of purine nucleotides and their degradation product, uric acid, are accumulated. This results is neurologic effects, self-mutilation, and mental retardation. 

The Lesch-Nyhan syndrome is caused by a deficiency of the phosphoribosyltransferase that is involved in the salvage pathway for hypoxanthine and guanine (HGPRTase). The accumulation of P-Rib-PP stimulates purine biosynthesis. 

The reaction mixture contained 50 /xM guanine, 50 fiM hypoxanthine, 100 /xM PRibPP, and 1 mM MgCh in potassium phosphate (pH 7.4). The reaction was initiated by the addition of the HGPRTase activity. An intervals the reaction was terminated by heating in a boiling water bath for 1 minute. Denatured protein was removed by centrifugation, and the sample was purified... 

Salvage pathway is a useful term to refer to that collection of biochemical reactions whose transformations result in the phosphorylation of purines. As a consequence of this phosphorylation, purines are not secreted by cells but, in fact, are returned to the cellular metabolic pool. One of these salvage enzymes is hypoxanthine-guanine phosphoribosyltransferase (HGPRTase),... 

The enzyme 5 -nucleotidase dephosphorylates IMP to inosine and P. Thus, since this reaction represents a possible fate for the IMP formed by the transferase (Fig. 10.7), reconstitution studies were undertaken with the nucleotidase. These studies were carried out using the HPLC assay method developed for the HGPRTase activity. A reaction mixture was prepared that contained hypoxanthine and PRibPP as substrates. The reaction was started by the addition of purified HGPRTase enzyme. Samples were removed and were analyzed by HPLC. The chromatographic profiles obtained at 0,10, 20, and... 

Weiss EB, Forman P, Rosenthal IM. Allopurinol-mduced arteritis in partial HGPRTase deficiency. Atypical seizure manifestation. Arch Intern Med 1978 138(11) 1743. ... 

Resistance may resnlt from the loss of HGPRTase activity (inability to convert thioguanine to TGMP) or increased catabolism of TGMP by a nonspecific phosphatase. Althongh it is variable, cross-resistance with mercaptopu-rine nsnally occnrs. 

There are three distinct PRTases in L. donovani (30). One has its major activity with hypoxanthine and guanine a second with adenine and a third with xanthine. Pyrazolo (3,4-d)pyrimidines, such as allopurinol, are efficient substrates for the HGPRtase. Promastigotes accumulate large quantities of allopurinol ribonucleoside-5 -phosphate when exposed to allopurinol (31). A separate PRTase for xanthine is unusual in a eukaryotic cell. XPRTase is also present in L. mexicana and L. amazonensis as well as in four non-pathogenic trypanosomatids (32,33). 

T.b. gambiense bloodstream forms have APRTase, HGPRTase, adenosine kinase and adenylosuccinate synthetase but lack adenosine deaminase. Two phosphorylase activities have been described for the bloodstream forms of T.b. brucei (42,50). One catalyzes the reversible phosphorolysis of adenosine, inosine and guanosine and the other is specific for adenosine and methylthioadenosine. Guanine deaminase is present whereas both adenosine and adenine deaminase are absent (8). Similar results have been reported for T. congolense (51). T. vivax is unique among the other trypanosomes in that it has an adenine deaminase (51). 

The schistosomule HGPRTase has been purified and characterized (68). It is twice as active with guanine as with hypoxanthine and cannot use xanthine. The gene for this protein has been cloned and expressed at a high level in Escherchia coli (69). 

Allopurinol is a good substrate for HGPRTase in trypanosomes but not mammals. Recall that allopurinol is also an inhibitor of xanthine oxidase and is used in gout and cancer chemotherapy. The answer is (A). 

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... 

A) Resistance to fluorouracil occurs via decreased activity of hypoxanthine-guanine phos-phoribosyl transferase (HGPRTase)... 

To exert anticancer activity, mercaptopurine (and thioguanine) must first be activated to nucleotides by HGPRTase. If mercaptopurine were an irreversible inhibitor of this enzyme, this bioactivation process could not occur. The answer is (D). 

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