Goodman D S
J Natl Cancer Inst. 1984 Dec;73(6):1375-9.
Current knowledge about the metabolism of naturally occurring retinoids and carotenoids was summarized. Dietary provitamin A carotenoids are largely converted to retinol (vitamin A) during intestinal absorption in the mucosal cell. In humans, a limited amount of carotenoids can be absorbed intact, along with retinyl esters (newly synthesized or from dietary vitamin A), mainly via lymph chylomicrons. Carotenoids are stored in several tissues, particularly liver and fat. They are transported in plasma by lipoproteins (density less than 1.21 g/ml), particularly by the low-density lipoproteins. Plasma carotenoids are usually a mixture of compounds with and without provitamin A activity; beta-carotene is about 20-25% of the total. Newly absorbed vitamin A is stored in the liver as retinyl esters. Storage involves both the hepatic parenchymal cells and the nonparenchymal stellate cells. Vitamin A is mobilized from liver stores and transported in plasma as retinol bound to a specific transport protein, retinol-binding protein (RBP). Retinol mobilization is highly regulated by factors that control the rates of RBP synthesis and secretion. Much is known now about the chemical structure, metabolism, and biologic roles of RBP, RBP delivers retinol to peripheral target tissues; delivery may involve cell surface receptors for RBP. Tissues of rats, humans, and other species contain soluble binding proteins with specificity for either retinol (cellular retinol-binding protein) or retinoic acid (cellular retinoic acid-binding protein). These intracellular proteins have been purified from several tissues and partly characterized. From both immunoassay and immunocytochemical studies, information is available about their tissue distribution and levels. Retinoic acid is mainly absorbed through the portal system and transported in plasma as the anion bound to serum albumin. Nonspecific and unregulated delivery of retinoids to biologic membranes apparently leads to vitamin A (retinoid) toxicity.
总结了关于天然存在的类视黄醇和类胡萝卜素代谢的现有知识。膳食中的维生素A原类胡萝卜素在黏膜细胞的肠道吸收过程中大部分转化为视黄醇（维生素A）。在人类中，少量类胡萝卜素可以与视黄酯（新合成的或来自膳食维生素A）一起完整吸收，主要通过淋巴乳糜微粒。类胡萝卜素储存在多个组织中，特别是肝脏和脂肪组织。它们通过脂蛋白（密度小于1.21 g/ml）在血浆中运输，特别是通过低密度脂蛋白。血浆类胡萝卜素通常是具有和不具有维生素A原活性的化合物的混合物；β-胡萝卜素约占总量的20%-25%。新吸收的维生素A以视黄酯的形式储存在肝脏中。储存涉及肝实质细胞和非实质星状细胞。维生素A从肝脏储存中动员出来，并以与特定转运蛋白视黄醇结合蛋白（RBP）结合的视黄醇形式在血浆中运输。视黄醇的动员受到控制RBP合成和分泌速率的因素的高度调节。现在对RBP的化学结构、代谢和生物学作用已经了解很多，RBP将视黄醇输送到外周靶组织；输送可能涉及RBP的细胞表面受体。大鼠、人类和其他物种的组织含有对视黄醇（细胞视黄醇结合蛋白）或视黄酸（细胞视黄酸结合蛋白）具有特异性的可溶性结合蛋白。这些细胞内蛋白已从多个组织中纯化并部分表征。从免疫测定和免疫细胞化学研究中，可以获得有关它们的组织分布和水平的信息。视黄酸主要通过门静脉系统吸收，并以与血清白蛋白结合的阴离子形式在血浆中运输。类视黄醇向生物膜的非特异性和无调节输送显然会导致维生素A（类视黄醇）毒性。
