Journal of Lipid Research, Vol 32, 1689-1698, Copyright © 1991 by Lipid Research, Inc.
Identification and characterization of an acyl-CoA:triterpene acyltransferase activity in rabbit and human tissues
I Tabas, N Beatini, LL Chen, WC Su, MS Puar, S Dugar and JW Clader
Department of Medicine, Columbia University, New York, NY 10032.
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.
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