RELATIONSHIP BETWEEN THE ENDOPLASMIC RETICULUM-GOLGI MEMBRANE SYSTEM AND UBIQUINONE BIOSYNTHESIS

The involvement of the various segments of the endoplasmic reticulum ( ER)-Golgi system in ubiquinone biosynthesis in rat liver was investiga ted using subcellular fractionation. In addition to preparing rough (R ) and smooth microsomes and three different Golgi fractions, a procedu re was developed to isolate a smooth vesicle fraction, designated as s mooth II (SII) microsomes. The electron micrographs, chemical composit ion, distribution of marker enzymes, pattern of glycosidases and glyco syltransferases and participation in cholesterol transport suggest tha t the vesicle components of this latter fraction are intermediary betw een the endoplasmic reticulum and Golgi system. Both R and smooth I(SI ), but not SII microsomes nor Golgi vesicles demonstrate trans-prenylt ransferase activity, which synthesizes the side-chain of ubiquinone fr om geranyl pyrophosphate (GPP). The subsequent enzyme, which transfers solanesyl pyrophosphate (sol-PP) to 4-hydroxybenzoate, is absent from R and SI microsomes, but present in SII microsomes and exhibits high levels of activity in all of the Golgi fractions. Thus, ubiquinone is synthesized sequentially in the ER-Golgi system and thereafter translo cated from this compartment to other cellular membranes.