Extensive incorporation of (2-14C) mevalonic acid into cholesterol precursors by human platelets in vitro.

Human platelets convert [l-W]or [2J4C]acetate to labeled CO2 and fatty acids; the latter, in turn, become esterified to acylglycerols and phospholipids. However, acetate apparently does not reach mevalonate in the sterol pathway. By contrast, with [2J4C]mevalonate as substrate, there is labeling of farnesol, farnesoic acid, squalene, lanosterol, and dihydrolanosterol in the chloroform-soluble fraction. Proof of the identity of these five labeled compounds was established by gas-liquid radiochromatography; identities of squalene and lanosterol were further verified by gas-liquid chromatography-mass spectrometry, as well as by recrystallization to constant specific activity. Platelets share this pattern of mevalonate uptake with human reticulocytes, buffy coat, bone marrow, saphenous vein, coronary artery, and aorta. On an equivalent protein basis human platelets direct 14% as much label beyond mevalonate as does rat liver, a heavy producer of sterols. However, the platelet is able to carry the label only as far as lanosterol and dihydrolanosterol, with considerable accumulation of radioactivity in farnesol and farnesoic acid, the degradative products of farnesyl pyrophosphate, whereas the liver puts 76% of utilized mevalonate into the end product, cholesterol. The platelets cannot produce any WO? whatever from [‘“Cllanosterol, as evidence of demethylation, whereas the liver has no problem with CO, and cholesterol production from the same substrate. Interestingly, platelets can convert [14C]desmosterol to cholesterol, although not nearly as efficiently as liver. This fact suggests that a major block exists in vitro in demethylation of lanosterol by human platelets.