Cholesterol Ester Metabolism in Rat Brain: A CHOLESTEROL ESTER HYDROLASE SPECIFICALLY LOCALIZED IN THE MYELIN SHEATH

Abstract

Abstract The cholesterol ester hydrolase with a pH optimum of 6.6, that we previously reported in rat brain, consists of two distinct cholesterol ester hydrolases, one localized in microsomes and the other in the myelin sheath. The microsomal hydrolase has a pH optimum of 6.0 and is highly activated by both sodium taurocholate and Triton X-100. The myelin hydrolase has a pH optimum of 7.2 and is activated by taurocholate but slightly inhibited by Triton X-100. Standard assay systems for these enzymes have been developed. A differential assay system which permits determination of either of the two cholesterol ester hydrolases in a mixture of both has been established. Of the total activity of the myelin cholesterol ester hydrolase in the starting homogenate, 70 to 80% was recovered in the purified myelin fraction with a relative specific activity of 10. Gray matter, 7-day-old rat whole brain, and brains of myelin-deficient mutant mice, Jimpy and Quaking, all contained much less activities of the myelin cholesterol ester hydrolase compared to white matter, adult rat whole brain, or brains of the control littermates of the mutant mice. The activities of the microsomal cholesterol ester hydrolase were similar in all of these samples. These findings strongly indicate that the taurocholate-activated, Triton X-100-inhibited cholesterol ester hydrolase with the pH optimum of 7.2 is highly localized in the myelin sheath. This is the second enzyme found to be myelin-specific, the first one being cyclic 2',3'-nucleotide 3'-phosphohydrolase. Together with the most acidic cholesterol ester hydrolase localized in the crude mitochondrial fraction, we have demonstrated three distinct cholesterol ester hydrolases in rat brain.