ISOPRENOID BIOSYNTHESIS IN RAT-LIVER MITOCHONDRIA - STUDIES ON FARNESYL PYROPHOSPHATE SYNTHASE AND TRANS-PRENYLTRANSFERASE

Mevalonate pathway enzyme activities in rat liver mitochondria were in vestigated, and it was found that isopentenyl pyrophosphate can be uti lized for the synthesis of all-trans-polyprenyl pyrophosphates in vitr o. In this reaction sequence intermediate formation of farnesyl pyroph osphate (FPP) predominates, and the FPP synthase activity was studied in more detail. The mitochondrial activity constitutes 13% of the tota l hepatic capacity for FPP synthesis, exceeding the corresponding micr osomal, nuclear, and peroxisomal activities by 10-fold. Mitochondrial FPP synthase exhibits trypsin sensitivity only after sonication of int act mitochondria and upon subfractionation the activity is found local ized in the matrix. FPP synthase activities at different locations res ponded distinctly when rats were treated with a diet enriched in chole sterol or containing mevinolin or cholestyramine. With the high choles terol diet, mitochondrial FPP synthase activity increased 2-fold, whil e the cytosolic activity was slightly decreased. Both mevinolin and ch olestyramine treatment resulted in 3-fold increases in cytosolic FPP s ynthase activities, without altering the mitochondrial activity. FPP w as utilized as substrate for trans-prenyltransferase activity in the i nner mitochondrial membrane. The products formed in this reaction were identified as nona- and decaprenyl-PP, and the reaction was influence d by changes in both substrate and Mg2+ concentration, giving more dec aprenyl-PP when the concentrations of these substances were increased. These results demonstrate that mitochondria utilize endogenously prod uced FPP for isoprenoid biosynthesis and that the biosynthetic steps i n mitochondria are regulated independently from those occurring in oth er subcellular compartments.