The transformation in vitro of the plasma free cholesterol first observed by Sperry has been further studied. The drop in free cholesterol occuring on the incubation of rat plasma has been shown to be due to esterification by fatty acids. Radioactive cholesterol incubated in vitro in rat or human plasma becomes incorporated into each of the cholesterol ester subfractions obtained by silicic acid column chromatography. Under similar conditions of incubation no incorporation of free fatty acids into any of the plasma ester fractions has been demonstrated. However, when [14C]linoleic acid-labeled lecithin is incubated in vitro with rat or human plasma, radioactive cholesterol linoleate is formed as well as 14C free fatty acid, triglycerides, mono- and diglycerides, and probably also cephalin. Similarly, when [14C]palmitic acid-labeled tripalmitin is incubated with rat or human plasma, [14C]cholesterol palmitate results as well as 14C-labeled free fatty acid, mono- and diglycerides, cephalin and lecithin. It is concluded that the preformed fatty acid esters of the plasma are the source of the fatty acids for the cholesterol esterification reaction in vitro.
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