Retinol esterification by mammary gland microsomes from the lactating rat

Because vitamin A in milk is largely present as esterified retinol while blood plasma predominantly contains unesterified retinol, experiments were conducted to determine whether membranes from the lactating mammary gland are able to synthesize retinyl esters in vitro. When microsomes from rats lactating for 7 to 14 days were incubated with [³H]retinol dispersed in dimethylsulfoxide, some [³H]retinol esterification was observed (147 pmol/5 min per 0.5 mg protein). However, 3- to 7-fold increases in retinyl esters synthesis could be achieved by supplying either a fatty acyl CoA-generating system or preformed fatty acyl CoA thioesters; thus, the major activity in vitro has the characteristics of a fatty acyl CoA: retinol acyltransferase. Both long-chain and medium-chain fatty acyl CoA esters stimulated [³H]-labeled retinyl ester synthesis in vitro. Concordantly, analysis of the retinyl ester pattern of rat milk demonstrated the presence of eight different esters of retinol ranging in fatty acyl chain length from 8 to 18 carbons. Retinol esterification by microsomes was maximal at neutral pH (7.1) in the presence of approximately 50 μM palmitoyl CoA, and was linear with time of incubation for at least 5 min. Retinyl ester synthesis increased with the apparent concentration of [³H]retinol to approximately 200 nmol/ml, but was also dependent on the ratio of retinol relative to total microsomal protein in the incubation mixture. These experiments demonstrate for the first time retinol esterification by mammary gland membranes and point to the hypothesis that free retinol from plasma in esterified in this organ before secretion of retinyl esters in milk.