Acylcoenzyme A:
cholesterol acyltransferase (ACAT) was solubilized from Ehrlich
ascites cell microsomes with
Triton X-100. After removal of the
detergent, ACAT activity per mg
protein was reduced by 50 to 65% as compared with untreated microsomes. When this microsomal extract was combined with
liposomes composed of
cholesterol and egg
phosphatidylcholine, the ACAT activity increased 5.4- to 6.7-fold. Under these conditions
sucrose density gradient centrifugation indicated that more than 50% of the added
lipid was incorporated into vesicles having the same density as the ACAT activity, suggesting the formation of a complex. ACAT activity increased 2.9-fold when the
phosphatidylcholine content of the
liposomes was raised from 0.5 to 5.0 mumol/mg microsomal
protein. By contrast, the ACAT activity increased only 42% when the
cholesterol content of the
liposomes was raised from 0.17 to 0.57 mumol/mg microsomal
protein. Addition of
phosphatidylethanolamine to the
liposomes produced little change in ACAT activity, whereas the activity was reduced by 25 and 50%, respectively, when
sphingomyelin or
phosphatidylserine was added. ACAT activity was five times higher when the
liposomes were prepared from
dioleoylphosphatidylcholine than from saturated
phosphatidylcholines, including hydrogenated egg yolk, dimyristoyl or
dipalmitoyl phosphatidylcholine. Likewise, the ACAT activity with
liposomes made from soybean or egg yolk
phosphatidylcholine was almost 3.5-fold greater than with those prepared from the saturated
phosphatidylcholines. These results are consistent with the view that the activity of ACAT can be modified by changes in the composition of the
membrane lipids with which the
enzyme is associated.