Characteristics of the Cholesterol-Esterifying Activity in Normal and Atherosclerotic Rabbit Aortas

Abstract

Esterification of cholesterol with [l-14 C]palmityl-CoA by an atherosclerotic cell-free homogenate was approximately 16-50-fold greater than that by a normal cell-free homogenate for a given amount of protein in the homogenate. This difference was due to hyperactivity of the cholesterol-esterifying system in the atherosclerotic cell-free homogenate rather than to depletion of radioactive palmityl-CoA in the reaction mixture containing normal homogenate. Neither an activator of cholesterol esterification in the soluble fraction of the atherosclerotic aortic homogenate nor an inhibitor in the soluble fraction of the normal aortic homogenate could be demonstrated. The pH optimum within the pH range covered for esterification and the apparent K m values were approximately the same in normal and atherosclerotic microsomes, suggesting that the enzymes were probably the same. The results suggested a higher concentration or a higher activity of the enzyme in or on atherosclerotic microsomes. An alternative possibility is that high concentrations of free cholesterol in the atherosclerotic microsomes were responsible for the augmented cholesterol esterification. This possibility seems unlikely, because the observed 2.3-fold increase in the free cholesterol concentration should not produce a 25-fold increase in cholesterol esterification. The rate of cholesterol esterification by atherosclerotic microsomes varied with the substrate: oleyl-CoA ] > palmitylCoA > linoleyl-CoA. KEY WORDS pH optimum microsomes acyl-CoA cholesterol concentration atherosclerosis cholesterol esterification cell-free homogenate K• Cholesteryl ester of atherosclerotic aortas is derived in part from plasma and in part from in situ synthesis. The latter has been shown by the incorporation of radioactivity into cholesteryl ester using acetate (l) and fatty acids (2) as substrates. This process contributes at least 13% and perhaps much more to the total cholesteryl ester in atherosclerotic aortas of rabbits (3). Two pathways for cholesterol esterification in atherosclerotic arterial tissues have been reported: one involves direct esterification of cholesterol with acyl-CoA (4) and the other transesterification of cholesterol 176 with the /3-fatty acid of lecithin (5). The rate of direct esterification is enhanced 28-40-fold in atherosclerotic tissue (6), suggesting that this pathway is probably responsible for the accumulation of cholesteryl ester in the atheroma. The present paper describes additional studies comparing the cholesterol-esterifying activity of normal and atherosclerotic aortic homogenates. We attempted to demonstrate the existence of an activator in atherosclerotic homogenates or an inhibitor in normal homogenates. Moreover, we investigated the effects of pH and palmityl-CoA concentration, the relation of free cholesterol concentration of microsomes to cholesterol-esterifying activity, and the relative utilization of CoA esters of palmitic, stearic, oleic, and linoleic acids in cholesterol esterification. MethodsMale New Zealand rabbits (...

Cholesteryl Ester Synthesis in Normal and Atherosclerotic Aortas of Rabbits and Rhesus Monkeys

“…The increased activity we observed as a result of atherosclerosis is in agreement with the findings of other workers (8,10,27). Added exogenous cholesterol did not increase oleic acid esterification and did not become esterified to endogenous fatty acids.…”

Properties and Subcellular Distribution of Acyl-CoA: Cholesterol Acyltransferase (ACAT) in Guinea-pig Liver

“…In our experiments we always kept the molar ratio between acyl-CoA and albumin less than 7 without detecting any activity of ACAT. We suggest that the inhibition we observed when acyl-CoA esters were added to the incubation mixture was due to a detergent effect as reported by Hashimoto et al (9).…”

Reversible accumulation of cholesteryl esters in macrophages incubated with acetylated lipoproteins.

“…Female NCS mice (25)(26)(27)(28)(29)(30) ) . All other tissue culture supplies, thin-layer and gas-liquid chromatographic materials, and reagents for assays were obtained from sources as previously reported (2,6,10) .…”