Origins of the cholesterol in milk.

Studies were conducted to investigate the origin of milk cholesterol in the ruminant. In the first experiment, [1(-14)C]sodium acetate was infused into one side of the udder of a lactating goat via the test canal whereas in the second, (1,2-3H]cholesterol was injected intravenously and concurrently with a [14C]acetate intramammary infusion. In both experiments, blood and milk samples were collected at intervals for 6 days postinjection. Maximum unesterified cholesterol specific activity (sp act) in whole milk appeared at 78 hr after intravenous injections of 3H cholesterol and within 3-7 hr after infusion of [14C]acetate. Virtually all the tritium in milk was associated with unesterified cholesterol. The sp act of 14C-labeled cholesterol was only 20% of gland-synthesized decanoic acid. Decanoic acid is known to be completely synthesized in the mammary gland, and, like cholesterol, acetate is its precursor. The results indicate that, although some milk cholesterol is synthesized in the mammary gland, it is derived principally from serum cholesterol. The data show also that serum cholesterol equilibrates with membrane cholesterol of the lactating cell prior to its secretion in milk.