Ross A C
J Lipid Res. 1982 Jan;23(1):133-44.
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 [(3)H]retinol dispersed in dimethylsulfoxide, some [(3)H]retinol esterification was observed (147 pmol/5 min per 0.5 mg protein). However, 3- to 7-fold increases in retinyl ester 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 [(3)H]-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 micro M palmitoyl CoA, and was linear with time of incubation for at least 5 min. Retinyl ester synthesis increased with the apparent concentration of [(3)H]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 is esterified in this organ before secretion of retinyl esters in milk.-Ross, A. C., Retinol esterification by mammary gland microsomes from the lactating rat.
由于牛奶中的维生素A主要以酯化视黄醇的形式存在，而血浆中主要含有未酯化的视黄醇，因此进行了实验以确定泌乳乳腺的膜是否能够在体外合成视黄酯。当将来自泌乳7至14天的大鼠的微粒体与分散在二甲基亚砜中的[（3）H]视黄醇一起孵育时，观察到了一些[（3）H]视黄醇的酯化现象（每0.5mg蛋白质147pmol/5分钟）。然而，通过提供脂肪酸辅酶A生成系统或预先形成的脂肪酸辅酶A硫酯，可以使视黄酯合成增加3至7倍；因此，体外的主要活性具有脂肪酸辅酶A：视黄醇酰基转移酶的特征。长链和中链脂肪酸辅酶A酯均能刺激体外[（3）H]标记的视黄酯合成。与此一致的是，对大鼠乳汁视黄酯模式的分析表明存在八种不同的视黄醇酯，其脂肪酰链长度从8到18个碳不等。微粒体对视黄醇的酯化在中性pH（7.1）、存在约50μM棕榈酰辅酶A的情况下最大，并且在孵育至少5分钟的时间内呈线性。视黄酯合成随着[（3）H]视黄醇的表观浓度增加至约200nmol/ml而增加，但也取决于孵育混合物中视黄醇相对于总微粒体蛋白的比例。这些实验首次证明了乳腺膜对视黄醇的酯化作用，并提出了这样的假设，即血浆中的游离视黄醇在该器官中被酯化，然后再分泌到乳汁中的视黄酯中。——罗斯，A.C.，泌乳大鼠乳腺微粒体对视黄醇的酯化作用。
