A multicompartmental model describing the enterohepatic circulation of conjugated bile acids in man under steady-state conditions is proposed. The model encompasses conjugation; deconjugation and reconjugation; dehydroxylation; sulfation, desulfation and resulfation; dehydrogenation; and stereoselective rehydrogenation. A dynamic description of the enterohepatic circulation and a brief description of bile acid functions in health and dysfunctions in disease are also discussed.
The studies described here suggest the potential physiological role of polypeptide and corticosteroid hormones in the regulation of cholesterol synthesis. Evidence was shown for substantial differences between various cell types in their responses to these agents and for certain degree of independence of the effects on biosynthesis of cholesterol from those on protein and DNA synthesis. Cholesterol synthesis and HMGCoA reductase are stimulated in a number of diploid cell lines following an incubation with insulin or with glucocorticoids for 4 hr or longer. Stimulation of sterol synthesis by insulin and by dexamethasone requires protein synthesis, but the two hormones do not compete for the same site. Addition of glucagon or of dibutyryl cyclic AMP, or elevation of intracellular cyclic AMP by PGE1 does not inhibit cholesterol synthesis in skin fibroblasts. A possibility of a relationship between the mechanisms of the hormonal effects and of feedback control of cholesterol synthesis is suggested.
Kinetic analysis of activation of lipoprotein lipase by apo C-2 indicates that the lipase co-protein increase the rate of lipolysis by adsorption to enzyme at the lipid interface, with formation of a 1:1 molar complex, whose dissociation constant in the presence of triglyceride substrate is about 3 X 10(-13) moles cm-3. Activation by apo C-2, like that by the entire lipoprotein apoprotein moiety (Fielding and Fielding, 1976) is reversible by inorganic salts and the dependence of activation on medium ion-pair activity supports the concept that such inhibition is mediated through a single specific anion-binding site of the lipase co-protein.
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