Acyl coenzyme A:cholesterol acyltransferase in neonatal chick brain

An acyl coenzyme A:cholesterol acyltransferase activity which directly incorporates palmitoyl coenzyme A into cholesterol esters using endogenous cholesterol as substrate was demonstrated in microsomal preparations from neonatal chick brain. The enzyme showed, at pH 7.4, about 2-fold greater activity than that observed at pH 5.6. Nearly 10-times higher esterifying activity was found in brain microsomes using palmitoyl coenzyme A than that with palmitic acid. The acyltransferase activity was clearly different from the other cholesterol-esterifying enzymes previously found in brain, which incorporated free fatty acids into cholesterol esters and did not require ATP or coenzyme A as cofactors. Chick brain microsomes also incorporated palmitoyl coenzyme A into phospholipids and triacylglycerols. However, most of the radioactivity from this substrate was found in the fatty acid fraction, due to the presence of an acyl coenzyme A hydrolase activity in the enzyme preparations. Therefore, the formation of palmitate was tested during all the experiments. The brain acyltransferase assay conditions were optimized with respect to protein concentration, incubation time and palmitoyl coenzyme A concentration. Microsomal activity was independent of the presence of dithiothreitol in the incubation medium and microsomes can be stored at -40 degrees C for several weeks without losing activity. Addition of fatty acid-free bovine serum albumin to brain microsomal preparations produced a considerable increase in the acyltransferase activity, while acyl coenzyme A hydrolase was clearly inhibited. Results obtained show the existence in neonatal chick brain of an acyl coenzyme A:cholesterol acyltransferase activity similar to that found in a variety of tissues from different species but not previously reported in brain.