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Journal of Lipid Research, Vol. 18, 582-591, September 1977
Copyright © 1977 by Lipid Research, Inc.

Effect of dietary fat saturation on acylcoenzyme A: cholesterol acyltransferase activity of Ehrlich cell microsomes

Douglas E. Brenneman , Terry Kaduce , and Arthur A. Spector

Departments of Biochemistry and Medicine, University of Iowa, Iowa City, IA 52242

Ehrlich cells grown in mice fed coconut oil diets (highly saturated) contain about twice as much cholesteryl ester as those grown in mice fed sunflower oil diets (highly polyunsaturated). Acylcoenzyme A: cholesterol acyltransferase (ACAT) activity was 30-100% higher in microsomes prepared from the cells grown on coconut oil (M_c) than in those prepared from the cells grown on sunflower oil (M_s). Increased ACAT activity was noted in M_c with either [1-¹⁴C]palmitoyl CoA or [1,2-³H]cholesterol as the labeled substrate. This occurred at all acyl CoA concentrations tested and, in the [1,2-³H]cholesterol assay, with palmitoyl, oleoyl, or linoleoyl CoA as the substrate. The pH optimum for ACAT activity was the same with M_c and M_s, pH 7.0. ACAT activity obeyed Michaelis-Menten kinetics at palmitoyl CoA concentrations between 1 and 10 µM. Substrate inhibition occurred at higher concentrations. Kinetic analysis with [1-¹⁴C]palmitoyl CoA as the substrate indicated that the apparent K_m for M_c was 33% smaller than for M_s. There was no difference, however, in apparent V_max values. The cholesterol and phospholipid contents of M_c and M_s were similar, but their fatty acid compositions differed considerably. M_c contained 2.7 times more monoenoic fatty acid and only half as much polyenoic fatty acid as M_s. Our results indicate that dietary modification of the microsomal fatty acid composition is associated with alterations in the activity of ACAT, an enzyme that is tightly bound to the microsomes. These changes in ACAT activity may be partly responsible for the differences in cholesteryl ester contents of Ehrlich cells grown in mice fed the coconut and sunflower oil diets.

Supplementary key words cholesteryl esters • fatty acids • phospholipids • membranes

Submitted on January 5, 1977
Accepted on April 20, 1977

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