Structure and Synthesis of Milk Fat. IV. Role of the Mammary Gland with Special Reference to the Cholesterol Esters

Summary The extent to which milk lipids are synthesized within the mammary gland is not known. The objective of the research was to provide a better definition of what the gland accomplishes, as distinguished from what may be contributed by lipid metabolism in the rest of the body. Using intramammary infusion of both labeled and unlabeled fatty acids, it was demonstrated that cholesterol esters, glycerides, and phospholipids of milk are all made from fatty acids within the gland. Diverse evidence from this and other studies suggests that most, if not all, of the ester lipids of milk are synthesized from a common pool of fatty acids within the mammary gland. A remarkably active metabolic role is indicated for the cholesterol esters of milk. In the case of both labeled (palmitate-1-C 14 ) and unlabeled (linoleate) infusates, uptake by the cholesterol ester fraction was more intense and, by tracer, more rapid than for the glycerides or phospholipids. Some compositional data on cholesterol esters of milk are also presented.