Localisation of a 3-hydroxy-3-methylglutaryl-coenzyme A reductase in the mitochondrial matrix of Trypanosoma brucei procyclics

Contrary to Leishmania spp. and Trypanosoma cruzi, Trypanosoma brucei blood stream forms do not synthesise their own sterols but take these compounds i n the form of cholesterol directly from the mammalian host. However, procyc lic insect stages synthesise ergosterol rather than cholesterol. Here the s ub-cellular localisation of the first committed enzyme of this pathway of i soprenoid synthesis 3-hydroxy-3-methylglutaryl-coenzyme A reductase in TT: brucei procyclics (0.9 nmol. min(-1). mg(-1) protein) was carried out using both cell-fractionation by isopycnic centrifugation and digitonin-titratio n experiments. The majority of the NADP(+)-linked 3-hydroxy-3-methylglutary l-coenzyme A reductase is a soluble enzyme present in the mitochondrial mat rix with some additional membrane-associated activity in glycosomes and pos sibly in the endoplasmic reticulum. It is suggested that the active metabol ism of threonine and/or leucine as preferred 2-carbon source for the incorp oration of acetyl units into lipids and/or sterols in the mitochondrion of T. brucei procyclics is the explanation for a high 3-hydroxy-3-methylglutar yl-coenzyme A reductase activity in these protozoan organelles.