Identification of ubiquinone-50 as the major methylated nonpolar lipid in human monocytes. Regulation of its biosynthesis via methionine-dependent pathways and relationship to superoxide production

Human blood monocytes incorporated the methyl group from methionine into their neutral lipids. The major methylated product was identified as ubiquinone-50 in monocytes, lymphocytes, and a variety of human tumor cell lines by several analytical procedures including TLC or high performance liquid chromatography and as ubiquinone-45 in a mouse tumor cell line. Up to three methyl groups were shown to be derived from methionine by mass spectrometry. The rate of synthesis of ubiquinone-50 by monocytes as assessed by measuring labeled methyl group incorporation was shown to be linear over a 3-h period. Degradation of ubiquinone proceeded slowly; 80% of the labeled compound persisted after 18 h. The dependence of ubiquinone-50 synthesis upon methionine concentration was established in monocytes, with an estimated apparent Km for methionine of about 20 microM. The tumor promoter, tetradecanoate phorbol acetate, a potent stimulator of superoxide anion (O2-) production in phagocytic cells, inhibited ubiquinone-50 synthesis at nanomolar concentrations in monocytes, but not in lymphocytes, under conditions where oxidation of methionine takes place. Degradation of the labeled ubiquinone was unaffected. Formylmethionylleucyl-phenylalanine, a chemoattractant peptide which stimulates O2- production in phagocytic cells, also inhibited ubiquinone-50 synthesis. The degree of inhibition by either stimulus was increased when the methionine concentration in the medium was low. These findings demonstrate that in human monocytes ubiquinone-50 biosynthesis is regulable and that methionine concentration modulates both its rate of synthesis and the inhibitory effects of two stimuli of O2- production.