Winston A, Rando R R
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115, USA.
Biochemistry. 1998 Feb 17;37(7):2044-50. doi: 10.1021/bi971908d.
While the overall biosynthetic pathway leading from all-trans-retinoids to 11-cis-retinoids in the visual cycle is understood, little is known about which step(s) may be rate-limiting and how control is exerted. One possible target for control is the isomerohydrolase, which processes all-trans-retinyl esters into 11-cis-retinol. The basal rate of 11-cis-retinol synthesis from all-trans-retinyl esters is extremely slow using bovine retinal pigment epithelial membranes [3.5 pmol of 11-cis-retinol min-1 (mg of protein)-1], and only small amounts of 11-cis-retinyl ester are formed. However, the addition of retinol binding proteins stimulates 11-cis-retinol formation by a factor of approximately 13. Specific protein-protein interactions are probably unimportant because bovine serum albumin and the physiologically relevant cellular retinaldehyde binding protein (CRALBP) both stimulate 11-cis-retinol formation to the same extent, although CRALBP does so at much lower concentrations. The relatively rapid rate of isomerization in the presence of binding proteins [44.3 pmol of 11-cis-retinol min-1 (mg of protein)-1] suggests that the rate-limiting enzyme in the visual cycle need not be the isomerohydrolase. Also, 11-cis-retinol is shown to inhibit isomerohydrolase, providing a simple mechanism for regulation of the visual cycle and the stimulating effect of binding proteins.
虽然视觉循环中从全反式视黄醛到11-顺式视黄醛的整体生物合成途径已为人所知，但对于哪些步骤可能是限速步骤以及如何进行调控却知之甚少。一个可能的调控靶点是异构水解酶，它将全反式视黄酯加工成11-顺式视黄醇。使用牛视网膜色素上皮细胞膜，从全反式视黄酯合成11-顺式视黄醇的基础速率极慢[3.5皮摩尔11-顺式视黄醇·分钟-1·(毫克蛋白质)-1]，并且仅形成少量的11-顺式视黄酯。然而，添加视黄醇结合蛋白可使11-顺式视黄醇的形成增加约13倍。特异性蛋白质-蛋白质相互作用可能并不重要，因为牛血清白蛋白和生理相关的细胞视黄醛结合蛋白(CRALBP)都能同等程度地刺激11-顺式视黄醇的形成，尽管CRALBP在低得多的浓度下就能做到。在存在结合蛋白的情况下异构化的相对快速速率[44.3皮摩尔11-顺式视黄醇·分钟-1·(毫克蛋白质)-1]表明，视觉循环中的限速酶不一定是异构水解酶。此外，11-顺式视黄醇被证明可抑制异构水解酶，这为视觉循环的调控以及结合蛋白的刺激作用提供了一种简单机制。
