Farnesyl diphosphate synthase is a cytosolic enzyme in Leishmania major promastigotes and its overexpression confers resistance to risedronate.

Farnesyl diphosphate synthase is the most likely molecular target of aminobisphosphonates (e.g., risedronate), a set of compounds that have been shown to have antiprotozoal activity both in vitro and in vivo. This protein, together with other enzymes involved in isoprenoid biosynthesis, is an attractive drug target, yet little is known about the compartmentalization of the biosynthetic pathway. Here we show the intracellular localization of the enzyme in wild-type Leishmania major promastigote cells and in transfectants overexpressing farnesyl diphosphate synthase by using purified antibodies generated towards a homogenous recombinant Leishmania major farnesyl diphosphate synthase protein. Indirect immunofluorescence, together with immunoelectron microscopy, indicated that the enzyme is mainly located in the cytoplasm of both wild-type cells and transfectants. Digitonin titration experiments also confirmed this observation. Hence, while the initial step of isoprenoid biosynthesis catalyzed by 3-hydroxy-3-methylglutaryl-coenzyme A reductase is located in the mitochondrion, synthesis of farnesyl diphosphate by farnesyl diphosphate synthase is a cytosolic process. Leishmania major promastigote transfectants overexpressing farnesyl diphosphate synthase were highly resistant to risedronate, and the degree of resistance correlated with the increase in enzyme activity. Likewise, when resistance was induced by stepwise selection with the drug, the resulting resistant promastigotes exhibited increased levels of farnesyl diphosphate synthase. The overproduction of protein under different conditions of exposure to risedronate further supports the hypothesis that this enzyme is the main target of aminobisphosphonates in Leishmania cells.