3-Hydroxy-3-methyl-glutaryl-CoA reductase in Trypanosoma (Schizotrypanum) cruzi: subcellular localization and kinetic properties

The subcellular localization of 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) reductase, which catalyzes the first committed step of the mevalonate pathway, was investigated in Trypanosoma cruzi epimastigotes using well-established cell fractionation procedures. It was found that ca. 80% of the activity of the enzyme was associated with the glycosomes, microbody-like organelles unique to kinetoplastid protozoa which contain most of the enzymes of the glycolytic pathway, while the rest of the activity was found in the soluble (cytoplasmatic) fraction, with almost no activity associated with microsomes. The glycosome-associated enzyme is not membrane-bound as it was recovered quantitatively in the aqueous phase of the biphasic system formed by Triton X-114 at 30 degrees C. Studies with digitonin-permeabilized intact epimastigotes demonstrated the presence of two pools of soluble HMG-CoA reductase in these cells, associated to the cytoplasmic and glycosomal compartments. Steady-state kinetic studies of the glycosome-associated enzyme indicated classical Michaelis-Menten behavior with Km,app (HMG-CoA) 28 +/- 3 microM, Km,app (NADPH) 37 +/- 4 microM, and Vm,app 3.9 +/- 0.2 nmol/min mg protein; the transition-state analog lovastatin behaved as a competitive inhibitor with respect to HMG-CoA with Kis 23 nM and a noncompetitive inhibitor toward NADPH with Kii 29 nM. The results are in complete agreement with recent gene cloning and expression studies which showed that T. cruzi HMG-CoA reductase lacks the NH2-terminal membrane-spanning sequence. This is the first demonstration of a soluble eukaryotic HMG-CoA reductase and also the first report on the presence of an enzyme of the isoprenoid biosynthesis pathway in glycosomes.