The effect of insulin and catecholamines on the activities of 3-hydroxy-3-methyl glutaryl coenzyme A reductase and acyl-coenzyme A: cholesterol-o-acyltransferase in isolated rat hepatocytes

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Abstract
This study was concerned with the effect of insulin and catecholamines on the rate limiting enzymes of cholesterol metabolism in rat hepatocytes. Insulin was found to increase the activity of 3-hydroxy-3-methyl glutaryl coenzyme A reductase and to have no effect on the activity of acyl-coenzyme A: cholesterol-o-acyltransferase. Noradrenaline and isoprenaline increased the activities of both 3-hydroxy3-methyl glutaryl coenzyme A reductase and acyl-coenzyme A: cholesterol-o-acyltransferase. The effect of noradrenaline or isoprenaline in the presence of insulin was that of a lower stimulatory response on 3-hydroxy-3-methyl glutaryl coenzyme A reductase but comparable to that found with either catecholamine alone. The combination of either catecholamine with insulin had no effect on the activity of acyl-coenzyme A: cholesterol-o-acyltransferase. These observations suggest that the activities of 3-hydroxy-3-methyl glutaryl coenzyme A reductase and acyl-coenzyme A: cholesterol-o-acyltransferase are regulated independently by insulin in the presence or absence of catecholamines. By contrast, catecholamines appear to regulate both enzyme activities in a similar fashion.