Identification and characterization of an acyl-CoA:triterpene acyltransferase activity in rabbit and human tissues

Rabbit and human tissues contain substantial amounts of an unusual lipid, a fatty acid ester of a pentacyclic triterpene, that is a potent in vitro inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT). A possible origin of the triterpene ester is via dietary absorption of plant triterpenes (which have a similar structure to the triterpene moiety of the animal triterpene ester), followed by fatty acid esterification of the triterpene in animal tissues. To support this idea, homogenates of rabbit and human enterocytes and liver are now shown to contain an acyl-CoA:triterpene acyltransferase activity (ATAT) which esterifies triterpene to a fatty acid. The enzyme activity was stimulated by exogenous triterpene and required ATP and coenzyme A when fatty acid was used as substrate; ATP and coenzyme A were not required when fatty acyl-CoA was used. ATAT was not inhibited by two structurally different ACAT inhibitors, which may indicate that ACAT and ATAT are different enzymes. Rat enterocytes and liver contained very little ATAT activity, consistent with the finding that rat liver contained very little triterpene ester. To establish that triterpene esterification occurs in vivo, [3H]triterpene was shown to be incorporated into triterpene ester in several organs and tissues from a rabbit given a gastric bolus of the labeled triterpene. These data provide support for the hypothesis that triterpene esters in animal tissues arise from the dietary absorption of triterpenes followed by the esterification of the triterpenes by an enzymatic activity in the animal tissues.