Norum K R, Blomhoff R
Institute for Nutrition Research, Faculty of Medicine, University of Oslo, Norway.
Am J Clin Nutr. 1992 Oct;56(4):735-44. doi: 10.1093/ajcn/56.4.735.
We discuss vitamin A with emphasis on its absorption, transport, cellular uptake, storage, and intracellular metabolism. Dietary retinyl esters are hydrolyzed to retinol in the intestinal lumen before absorption by enterocytes. Carotenoids are absorbed and then partially converted to retinol in the enterocytes. In enterocytes retinol is esterified before incorporation into chylomicrons together with triacylglycerols. Chylomicrons reach the general circulation by way of the intestinal lymph, and chylomicron remnants are formed in the blood capillaries. The remnants, which contain almost all the absorbed retinol, are cleared by the liver parenchymal cells, and to some extent also by cells in blood, bone marrow, adipose tissue, and spleen. The uptake is most probably mediated via surface receptors for low-density lipoproteins or a low-density lipoprotein-receptor-related protein. In the liver parenchymal cells the retinyl esters are rapidly hydrolyzed to retinol, which binds to retinol-binding protein. Normally, most of the absorbed retinol coming into the liver parenchymal cell is transferred on retinol-binding protein to stellate cells, which store retinol as retinyl esters in lipid droplets.
我们将讨论维生素A，重点关注其吸收、转运、细胞摄取、储存及细胞内代谢。膳食视黄酯在被肠细胞吸收前，先在肠腔内水解为视黄醇。类胡萝卜素被吸收后，在肠细胞内部分转化为视黄醇。在肠细胞中，视黄醇在与三酰甘油一起掺入乳糜微粒之前先被酯化。乳糜微粒通过肠淋巴管进入体循环，并在毛细血管中形成乳糜微粒残粒。这些残粒几乎包含了所有吸收的视黄醇，由肝实质细胞清除，血液、骨髓、脂肪组织和脾脏中的细胞也能清除一部分。摄取过程很可能是通过低密度脂蛋白表面受体或低密度脂蛋白受体相关蛋白介导的。在肝实质细胞中，视黄酯迅速水解为视黄醇，视黄醇与视黄醇结合蛋白结合。通常，进入肝实质细胞的大部分吸收视黄醇会通过视黄醇结合蛋白转移到星状细胞，星状细胞将视黄醇以视黄酯的形式储存于脂滴中。
