Enzymes of the mevalonate pathway in rat liver nodules induced by 2-acetylaminofluorene treatment

4To whom correspondence should be addressed Certain enzymes of the mevalonate pathway have been investigated in persistent liver nodules induced in the rat by 2-acetylaminofluorene. In these nodules the doh'chol level was increased 5-fold, the ubiquinone-9 content elevated 6-fold and the amount of cholesterol unchanged. Microsomal (J-hydroxy-P-methylglutaryl-coenzyme A reductase activity was greatly increased compared to control liver tissue, which was also the case for the cytosolic farnesyl pyro-phosphate synthase. A significant elevation of a\\-trans-geranylgeranyl pyrophosphate synthase activity in the cyto-sol was also observed. The branch-point enzyme of micro-somal doh'chol synthesis, i.e. cis-prenyltransferase, was decreased in the nodules; whereas the activity of squalene synthase, the terminal regulating enzyme of cholesterol synthesis, remained unchanged. The dolichol species in nodular tissue were redistributed towards the longer chain length species. One factor regulating the chain length of the polyisoprene products formed in vitro was shown to be the ratio of the concentrations of isopentenyl pyrophos-phate: farnesyl pyrophosphate employed. Other regulatory factors in the terminal steps of this biosynthetic pathway appear to determine the amounts and nature of the final isoprenoid compounds formed in vivo. In contrast to the microsomal frans-prenyltransferase activity, which was unchanged, the activity of nonaprenyl-4-hydroxybenzoate transferase, an enzyme participating in ubiquinone syn-thesis, was greatly elevated. The alterations observed in the activities of enzymes in the mevalonate pathway can at least partially explain the increased levels of dolichol and ubiquinone and the unchanged level of cholesterol found in liver nodules. It is reasonable to propose that this modified mevalonate metabolism will render nodular cells resistant to certain toxic factors and prone to cell proliferation