A lack of esterification of lanosterol and other methyl sterols in human serumin vitro

Human serum contains small amounts of free and esterified cholesterol precursors, such as lanosterol and other methyl sterols. Incubating the serum for 24 h at room temperature without and with different additions of free methyl sterols was not followed by any detectable esterification of these sterols during the subsequent 24 h incubation period, though the amount of cholesterol esters was markedly increased. [3H]dihydrolanosterol added to serum was not esterified in any lipoprotein fraction either, whereas the [14C]cholesterol added was found to be esterified during the 24 h incubation period. The lack of methyl sterol esterification in vitro suggests that these sterols are not proper substrates for serum lecithin: cholesterol acyltransferase and that esterified serum methyl sterols originate from tissues.