Cholesterol Esterification by Mammary Gland Microsomes from the Lactating Rat

Cholesterol esterification by fatty acyl-CoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) has been demonstrated in microsomes prepared from breast tissue of the lactating rat, employing the incorporation of [1-¹⁴C]oleoyl coenzyme-A into cholesteryl[¹⁴C]oleate. The regulation of this activity in vitro was studied by supplementing microsomes with unesterified cholesterol supplied either as a dispersion in acetone or by incubating microsomes with cholesterol-enriched serum lipoproteins prior to enzyme assay. ACAT activity was increased by more than 80% when the ratio of unesterified cholesterol to microsomal protein was increased from 29 to 48 μg/mg protein, indicating that the normal cholesterol content of mammary gland microsomes does not saturate this enzyme. Cholesterol esterification could be inhibited nearly completely in vitro by addition of the polar steroid progesterone (93% decrease in ACAT activity with 75 μM progesterone) and, to lesser extents, by estradiol and retinol. Enzyme assays were also performed after incubating microsomes with N-acylamides that inhibit cholesteryl ester synthesis in other systems. Under conditions where the mammary gland ACAT reaction was inhibited by more than 82%, the esterification of retinol was reduced by less than 30% and incorporation of [¹⁴C]oleoyl-CoA into triglycerides was not inhibited at all. These studies indicate that the lactating mammary gland contains ACAT activity having properties similar to ACAT in other organs. The presence of ACAT activity in the lactating mammary gland provides a possible mechanism for the synthesis of cholesteryl esters found in milk. It can also be inferred that ACAT in mammary gland microsomes is likely to be distinct from the microsomal acyltransferases that catalyze the esterification of retinol and glycerides.