PURINE SALVAGE ENZYMES IN TRICHOMONAS VAGINALIS: 133

Abstract

Trichomonas vaginalis, a pathogenic protozoa, is incapable of de novo purine synthesis and is thus dependent on preformed purines. Unlike other protozoa, T. uaginalis is devoid of purine phosphoribosyltransfer activity but has both purine nucleoside phosphorylase (PNP) and purine nucleoside kinase activity. These enzymes comprise a potential route for the salvage of purines in this organism. Both enzymes have been purified and characterized.The purified PNP (mol. wt. 95,000) catalyzed the synthesis and cleavage of guanosine, adenosine and inosine at maximal velocities (μmol/min/mg) of 590:360:240 and 81:16:390 respectively. Km values ranged from 17-54 μM for these purine nucleosides and 21-25 μM for the purine bases. Initial velocity studies in both the synthetic and cleavage direction indicate a sequential mechanism for this enzyme.As a result of the extreme lability of the nucleoside kinase, only a limited purification was possible. The purified enzyme (mol. wt. 16,000) had a specific activity of 34 nmol of GMP formed/min/mg. It was free of interfering activities and catalyzed the phosphorylation (Km μM/Rel. Vmax) of guanosine (1/100), adenosine (200/111) and inosine (20/67). This enzyme appears to be the only purine ribonucleoside kinase activity in extracts of this organism.The finding that both of these enzymes catalyze reactions involving the common purine nucleosides, guanosine and adenosine, suggests that they may act as a coordinated set of enzyme activities to salvage purines and purine ribonucleosides.