SUBCELLULAR-LOCALIZATION OF SQUALENE SYNTHASE IN RAT HEPATIC CELLS - BIOCHEMICAL AND IMMUNOCHEMICAL EVIDENCE

In the present study we investigated the subcellular localization of s qualene synthase (farnesyl-diphosphate: farnesyl-diphosphate farnesylt ransferase, EC 2.5.1.21). Squalene synthase catalyzes the formation of squalene from trans-farnesyl diphosphate in two distinct steps and is the first committed enzyme for the biosynthesis of cholesterol. Recen tly, a truncated form of the enzyme from rat hepatocytes was purified, and monospecific antibodies for squalene synthase were produced. This enabled the subcellular localization of squalene synthase by three di fferent methods: (i) analytical subcellular fractionation and measurem ents of enzyme activities; (ii) immunodeterminations of squalene synth ase in the isolated subcellular fractions with a monospecific antibody ; and (iii) immunoelectron microscopy. All three methods gave consiste nt results. The data clearly illustrate that squalene synthase enzymat ic activity and squalene synthase are exclusively localized in the end oplasmic reticulum. In rat hepatic peroxisomes we were not able to det ect any squalene synthase. In addition, we also demonstrated that squa lene synthase in the microsomal fraction is dramatically regulated by a number of hypolipidemic drugs and dietary treatments.