3-Hydroxy-3-methylglutaryl coenzyme A reductase activity in the human gastrointestinal tract.

Activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.34) was measured in intestinal mucosa of the human gastrointestinal tract. Activity was highest in gastric mucosa (18.2 pmol per mg per min) and there was a constant low level in the small bowel and colon (approximately 10 pmol per mg per min). Phosphorylation/dephosphorylation modulation of intestinal reductase activity was demonstrated in normal mucosa. Expressed jejunal reductase activity was significantly higher in celiac sprue mucosa and mucosa from defunctionalized intestine of jejunoileal bypass patients. Enzyme activity also increased during 24-hr mucosal organ culture in the absence of exogenous cholesterol. Addition to the culture medium of pure cholesterol or 25-hydroxycholesterol dissolved in a small volume of ethanol suppressed the culture-induced increase to 86 +/- 3% and 69 +/- 5% of paired controls, respectively. This evidence suggests that a moderate degree of feedback regulation of intestinal cholesterol synthesis by luminal sterol occurs in man. Mucosal HMG-CoA reductase activity was also measured in patients with hyperlipoproteinemia. Patients with either predominant hypercholesterolemia or predominant hypertriglyceridemia lipid profiles had "normal" expressed reductase activity, but feedback regulation by free cholesterol could not be demonstrated in either group under these conditions.