Age-related changes in the metabolism of cholesterol in rat liver microsomes

Abstract

The effects of aging on the hepatic metabolism of cholesterol were studied in 1‐, 6‐ and 24‐month‐old male Sprague‐Dawley rats. Microsomal 3‐hydroxy‐3‐methylglutaryl coenzyme A (HMG‐CoA) reductase activity, which regulates cholesterol biosynthesis, decreased from 835±144 (SEM) pmol/min/mg protein in the youngest group to 219±34 and 205±53 pmol/min/mg protein (p<0.001) in the 6‐ and 24‐month‐old groups, respectively. Cholesterol 7α‐hydroxylase activity, which governs bile acid synthesis, was gradually reduced from 70±14 pmol/min/mg protein in the 1‐month‐old group to 32±7 and 16±3 pmol/min/mg protein (p<0.05) in the 6‐ and 24‐month‐old groups, respectively. Acyl coenzyme A:cholesterol acyltransferase activity, which catalyzes the esterification of cholesterol, averaged 431±47 and 452 ±48 pmol/min/mg protein in the 1‐ and 6‐month‐old groups, respectively, and was increased to 585±55 pmol/min/mg protein (p<0.05) in the 24‐month‐old group. The level of total cholesterol showed an age‐related increase from 1.56±0.16 mg/g liver in the 1‐month‐old group to 1.70±0.15 and 2.20±0.19 mg/g liver (p<0.05) in the 6‐ and 24‐month‐old groups, respectively. The increase was mainly caused by an accumulation of esterified cholesterol. We conclude that a marked decrease in HMG‐CoA reductase occurs between 1 and 6 months of age; thereafter the enzyme activity stays unchanged. The activity of cholesterol 7α‐hydroxylase decreases progressively and drastically with age, whereas the capacity for esterifying cholesterol increases slightly. We speculate that the reduced conversion of cholesterol to bile acids may be one explanation of the age‐related increase of plasma cholesterol seen in rats.