Characterization of a cytosolic protein in rat liver inhibiting neutral cholesteryl ester hydrolase

Neutral cholesteryl ester hydrolase activity (EC 3.1.1.13) present in microsomes isolated from lactating rat mammary glands was found to be inhibited by a factor (or factors) occurring in the cytosolic fraction of male rat liver. The inhibitor was heat-labile, non-dialyzable, destroyed by proteolysis, and was stable following preparation of an acetone/diethyl ether powder of the cytosolic fraction. The protein also inhibited the activity of hormone-sensitive lipase (HSL) (from bovine adipose tissue) and esterase from Candida cylindracea, but seemed to be more active against the neutral hydrolase found in rat liver microsomes. For the mammary gland microsomal cholesteryl ester hydrolase, the extent of the inhibitory effect was dependent on the concentration of the cytosolic protein, 50% inhibition being achieved by about 100 micrograms of cytosolic protein, and on the method of initiating the enzyme assay. Kinetic analysis indicated that, under circumstances where the reaction was initiated by the addition of substrate, the inhibition was characterized as "uncompetitive." When an inhibitor/substrate complex was allowed to form in the absence of enzyme, an element of "competitive" inhibition was introduced into the reaction. Food withdrawal reduced the activity of the inhibitor in liver by 56%, but activity was fully restored by short-term re-feeding. In contrast, feeding a diet high in fat led to a 34% increase in activity. The present findings suggest that the inhibitory factor(s) may be involved in the regulation of the hydrolysis of cholesteryl esters in the liver and also in other cell types.