In vivo regulation of microsomal HMG CoA reductase activity was investigated in freshly isolated mononuclear leukocytes from 26 healthy adult males. Reductase activity exhibited a diurnal rhythm and decreased during fasting. Enzyme activity was also modulated in vivo by alterations in dietary and plasma cholesterol, suggesting the existence of an operative cholesterol feedback regulatory system. A single, high cholesterol meal decreased reductase activity within 2 h. In addition, rapid depletion of circulating cholesterol levels by plasmapheresis led to an approximately twofold elevation in enzyme activity within 90 min of treatment. Finally, reductase activity was inhibited by dichloroacetate, a compound known to lower plasma cholesterol in man and inhibit the human leukocyte enzyme in vitro. The regulatory mechanisms controlling HMG CoA reductase activity in the human mononuclear leukocyte in vivo thus are similar to those that modulate the mammalian liver enzyme in vivo. Assessment of mononuclear leukocyte reductase activity may provide insight into the in vivo regulation of human cholesterol metabolism.

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) controls the rate of cholesterol biosynthesis and is itself modulated through feedback suppression by internalized low density lipoprotein (LDL) cholesterol. We measured HMG CoA reductase protein concentration and microsomal enzyme activity in freshly isolated mononuclear leukocytes from normal individuals and patients with heterozygous or homozygous familial hypercholesterolemia (FH). Reductase protein concentration was similar in normal and heterozygous subjects, but was over twofold elevated in patients with homozygous FH. Reductase protein concentration was inversely related to LDL receptor status. Total activity and catalytic efficiency of reductase, however, were decreased in heterozygous and homozygous FH patients. The decrease in catalytic efficiency was not due to enzyme phosphorylation or thiol-disulfide formation. Reduction of plasma cholesterol concentration over 2 h by plasmapheresis increased reductase activity, the degree of which was directly proportional to the LDL-receptor status of the subjects.Decreased HMG CoA reductase activity and catalytic efficiency in mononuclear leukocytes and perhaps other cells in FH may represent a fundamental abnormality in the regulation of this enzyme independent of that induced by the LDL-receptor defect and may provide new insight into the control of cholesterol metabolism in FH.

When measuring human leukocyte HMG-CoA reductase activity, special care must be taken to prevent erythrocyte contamination of the leukocyte layer during isopycnic centrifugation. Contamination during leukocyte isolation and subsequent erythrocyte lysis during NH4Cl treatment results in increased leukocyte microsomal HMG-CoA reductase activity. Increased enzyme activity is not due to enzyme dephosphorylation, thiol-disulfide reduction or increased enzyme protein concentration. Leukocyte populations containing granulocytes appear to be most sensitive. Prevention of erythrocyte contamination during isopycnic centrifugation should aid in accurate measurement of human leukocyte HMG-CoA reductase activity.

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