Influence of dietary cholesterol, saturated and unsaturated lipid on 3-hydroxy-3-methylglutaryl CoA reductase activity in rabbit intestine and liver.

To characterize further the behavior of the rate-limiting enzyme of cholesterol biosynthesis in animal species, we studied the kinetic properties and the influence of dietary lipid on intestinal and hepatic HMG-CoA reductase activity in the rabbit. In intestinal crypt and villous cells isolated by a dual buffer technique, the KM value was 4.2 and 4.6 microM, respectively for DL-HMC-CoA. The specific activity of HMG-CoA reductase in the jejunum was 0.86 nmol/mg per hr, and evenly distributed between crypt and villous cells. By contrast, reductase activity was considerably lower in the ileum: in villous cells it was 0.40 nmol/mg per hr, and in crypt cells only 0.26 nmol/mg per hr. Liver microsomes had a KM value of 3.0 microM, while the reductase activity averaged 2 nmol/mg per hr. An unexpected finding was the uneven distribution of HMG-CoA reductase in the various lobes of the liver in the single animal. The addition of 1% cholesterol to the diet for 48 hours was followed by an average decline of 73% (P less than 0.005) of HMG-CoA reductase activity in villous and crypt cells of the jejunum. In the ileum, the decrease was less marked (38%, P less than 0.01). Whereas the addition of 5% corn oil to a 1% cholesterol diet did not have an additional suppressant effect on intestinal reductase, the addition of 5% coconut oil to 1% cholesterol caused further decrease of HMG-CoA reductase in jejunum and ileum (P less than 0.05). The 1% cholesterol diet resulted in a 25% decrease of hepatic reductase after 24 hours, whereas after 6 days, the enzyme activity was reduced by 90% of normal. Both 5% corn oil or 5% coconut oil, in addition to 1% cholesterol, further suppressed hepatic reductase activity. The weight of the experimental evidence presented in these studies suggests that cholesterol has a major regulatory effect on both intestinal and hepatic reductase in the rabbit.