ACYL-COA - CHOLESTEROL ACYLTRANSFERASE AND 3-HYDROXY-3-METHYLGLUTARYL-COA REDUCTASE IN CARP-LIVER MICROSOMES - EFFECT OF COLD-ACCLIMATION ON ENZYME-ACTIVITIES AND ON HEPATIC AND PLASMA-LIPID COMPOSITION

Hepatic microsomal activities of acyl-CoA: cholesterol acyltransferase (ACAT) and 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, rate-limiting enzymes in cholesterol esterification and cholesterol synthesis, and the concentrations and compartimentalization of esterified and unesterified cholesterol, were studied in carp acclimated to 10 and 30-degrees-C. Irrespective of acclimation temperature, carp-liver ACAT is characterized by an apparent K(m)-value for oleoyl-CoA of 11-15 muM and displays an optimum activity at pH 7.4. The enzyme activity is reduced approx. 2-fold upon preincubation of microsomes with alkaline phosphatase. Arrhenius plots of ACAT-activity are curvilinear, with curvatures considerably affected by the acclimation temperature of the fish. Carp HMG-CoA reductase has been characterized previously by Teichert and Wodtke ((1987) Biochim. Biophys. Acta 920, 161-170). When measured at 30-degrees-C, ACAT activities from 30-degrees-C- and 10-degrees-C-acclimated carp are identical (approx. 6 pmol/min per mg protein), whilst 'expressed' HMG-CoA reductase activity (18.1 +/- 12.2 pmol/min per mg protein for 30-degrees-C-acclimated carp vs. 159.8 +/- 106.6 pmol/min per mg protein for 10-degrees-C-acclimated carp) is enhanced 9-fold in the cold environment. This disparity indicates that cold-acclimation results in a massive increase in the capacity for hepatic cholesterol synthesis relative to hepatic cholesterol esterification. At the same time, hepatic compositional analysis reveals identical contents of unesterified cholesterol in either groups of carp but significantly decreased (3-fold) amounts in cholesterol ester (and also in triacylglycerol, 4-fold) in cold-acclimated carp. Moreover, microsomal fractions display lower cholesterol to phospholipid ratios in the cold. In contrast, concentrations of either cholesterol fractions (and of triacylglycerols) in plasma - the mobile compartment for lipoprotein transport - do not differ in cold- and warm-acclimated carp. Based on current concepts of cholesterol metabolism, it is concluded that the cold-enhanced expression of hepatic HMG-CoA reductase activity is a homeostatic response directed against and compensating for a cold-induced but not yet characterized deficiency in hepatic cholesterol availability.