The Enzymatic Synthesis and Hydrolysis of Cholesterol Esters in Blood Serum*

In recent publications (1, 2) we have shown that bile salts exert a marked influence on the proportion between the cholesterol fractions in incubated human and dog sera. In small concentrations all of the bile salts studied inhibit, increasingly with increasing amounts, the esterification of free cholesterol which occurs normally during incubation of both types of serum. In larger concentrations some of them promote hydrolysis of cholesterol esters in dog serum but not in human serum. It is probable that these reactions are involved, directly or indirectly, in the metabolism of cholesterol in tivo. For example, if esterification of free cholesterol goes on continuously in vivo as it does in vitro, cholesterol esters must be removed continuously from the serum in order to maintain the proportion between combined and free cholesterol within the narrow limits observed in healthy human beings (3). In hepatic disease bile salts, accumulating in the serum, may inhibit the esterification as they do in vitro without affecting the mechanism for removing cholesterol esters. The result would be a decrease in the proportionate amount of combined cholesterol such as is actually found. On the basis of such considerations it seemed worth while to study the reactions in more detail and in the present communication we describe a series of experiments designed to shed further light on the mechanisms involved. 1. The spontaneous esterification of free cholesterol on in-