Kakkad B P, Ong D E
Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232.
J Biol Chem. 1988 Sep 15;263(26):12916-9.
Cellular retinol-binding protein, type II (CRBP (II], an abundant protein of the rat small intestine, has recently been shown to be able to bind retinaldehyde in addition to retinol (MacDonald, P.N., and Ong, D. E. (1987) J. Biol. Chem. 262, 10550-10556). Retinaldehyde is produced in the intestine by oxidative cleavage of beta-carotene. The next step in the intestinal metabolism of vitamin A is the reduction of retinaldehyde to retinol which is then esterified for incorporation into chylomicrons. In the present study retinaldehyde bound to CRBP(II) was found to be available for reduction by microsomal preparations from rat small intestinal mucosa. The microsomal activity was about 8 times greater than the activity observed for an equal amount of cytosolic protein. Retinaldehyde reduction utilized either NADH or NADPH as cofactor, with NADH being slightly more effective. The apparent Km for retinaldehyde-CRBP(II) was 0.5 microM, and the Vmax was approximately 300 pmol/min/mg protein, a rate more than sufficient for the needs of the animal. The product retinol remained complexed to CRBP(II). The microsomal enzyme activity reduced free and bound retinaldehyde to approximately the same extent, although the aldehyde function of retinaldehyde bound to CRBP(II) was less accessible to chemical reducing agents than that of free retinaldehyde. Retinol bound to CRBP(II) could not be oxidized by the microsomal activity in the presence of NAD+, while free retinol or retinol bound to bovine serum albumin was oxidized to retinaldehyde. The more favorable reduction versus oxidation of retinoid bound to CRBP(II) consequently favored the reaction known to be required for the ultimate conversion of beta-carotene to retinyl esters for export from the gut.
细胞视黄醇结合蛋白II型（CRBP（II））是大鼠小肠中的一种丰富蛋白质，最近研究表明，它除了能结合视黄醇外，还能结合视黄醛（MacDonald，P.N.和Ong，D.E.（1987年）《生物化学杂志》262卷，第10550 - 10556页）。视黄醛是由β - 胡萝卜素在肠道中氧化裂解产生的。维生素A在肠道代谢的下一步是视黄醛还原为视黄醇，然后视黄醇被酯化以纳入乳糜微粒。在本研究中，发现与CRBP（II）结合的视黄醛可被大鼠小肠黏膜微粒体制剂还原。微粒体活性比等量胞质蛋白的活性大约高8倍。视黄醛还原利用NADH或NADPH作为辅因子，NADH的效果稍好一些。视黄醛 - CRBP（II）的表观Km为0.5微摩尔，Vmax约为300皮摩尔/分钟/毫克蛋白，该速率足以满足动物的需求。产物视黄醇仍与CRBP（II）结合。微粒体酶活性将游离和结合的视黄醛还原到大致相同的程度，尽管与CRBP（II）结合的视黄醛的醛基功能比游离视黄醛更难被化学还原剂作用。在NAD + 存在的情况下，与CRBP（II）结合的视黄醇不能被微粒体活性氧化，而游离视黄醇或与牛血清白蛋白结合的视黄醇被氧化为视黄醛。因此，与CRBP（II）结合的类视黄醇更有利于还原而非氧化，这有利于β - 胡萝卜素最终转化为视黄酯以便从肠道输出的已知反应。
