Isomerization of all-trans-retinoids to 11-cis-retinoids in vitro.

Abstract

The key biochemical process of the vertebrate visual cycle required for rhodopsin regeneration, 11-cisretinoid production from all-trans-retinoids, is shown to occur in vito. A 600 x g supernatant from a frog retina/pigment epithelium homogenate transforms added all-trans-[3H]retinol, in a time-dependent fashion, to a mixture of 11-cis-retinol, 11-cis-retinal, and 11-cis-retinyl palmitate. 13-cis-Retinoids are formed in only minor amounts by nonspecific processes. Studies using washed particulate fractions of the 600 x g supernatant indicate that all-trans-[3H]retinol is isomerized to 11-cis-retinoids much more effectively than is all-trans-[3H]retinal or all-trans-[3H]retinyl palmitate. The li-cis-retinoid biosynthetic activity is heat-labile, sedinmentable by highspeed centrifugation, and largely found in the pigment epithelium rather than in the neural retina.The absorption of light by rhodopsin in the vertebrate eye results in the cis-to-trans photoisomerization of its 11-cisretinal chromophore, bound as a protonated Schiff base to lysine, eventually leading to the hydrolysis and release of the all-trans-retinal (1,2). One of the rhodopsin conformers during this bleaching process, metarhodopsin II, catalyzes the binding of GTP in exchange for GDP by a retinal GTP-binding (G) protein, thus initiating the process of visual transduction (3, 4). Under bright light conditions the alltrans-retinal liberated by bleaching is reduced, esterified to long-chain fatty acids, and stored in the pigment epithelium of the eye (5). When a light-adapted animal encounters a dark environment, 11-cis-retinoids must be regenerated from the stores of all-trans-retinoids, and the 11-cis-retinal produced can then combine with opsin to form rhodopsin. The ocular biosynthesis of 11-cis-retinal in the dark is, by definition, a thermal process. It is also an endergonic process because at thermal equilibrium 11-cis-retinoids represent only 0.1% of the retinoids (6), while in a dark-adapted eye at least 75% of the retinoids are in the 11-cis form, with the remaining retinoids in the all-trans form (5). The driving force cannot simply arise from the stereospecific combination of 11-cisretinal with opsin because in many higher vertebrates such as man and amphibians, there is a 2-to 3-fold excess of retinoids over opsin in the eye (5, 7).A particularly vexing problem in visual science has been the mechanism of 11-cis-retinoid biosynthesis in the eye. Major hurdles include the identification of the substrate for isomerization (retinol, retinal, or retinyl ester), the nature of the energy source that drives the biosynthesis of 11-cisretinoids, and most importantly, the identification of the isomerizing system capable of producing 11-cis-retinoids in vivo. No system that can produce 11-cis-retinoids in vitro in darkness has yet been confirmed (8). Over the years several attempts have been made at the identification of an isomerase enzyme specific for retinal (9, 10); however, these attempts have suffered from a misidentification of rho...

Maintenance of Retinoid Metabolism in Human Retinal Pigment Epithelium Cell Culture

“…Previous studies have shown that the all-trans-retinol substrate is not completely used within the 2 hr incubation (Davis et al, 1995). Also, isomerization to 13-cis-retinol has been attributed to nonspecific thermal isomerization during the assay and extraction process (Bernstein, Law and Rando, 1987b). To increase RDH activity, NAD + was added to the reaction substrate in some experiments.…”

Retinoic acid suppresses parathyroid hormone (PTH) secretion and PreproPTH mRNA levels in bovine parathyroid cell culture.

“…One caveat to this line of reasoning is that the parathyroid cells may express enzymes that isomerize retinoic acid, which would confound the interpretation of these data. Such isomerases are expressed in the visual system that act on retinol to regenerate the visual chromophore (30,31 ).…”

Disruption of the 11-cis-Retinol Dehydrogenase Gene Leads to Accumulation of cis-Retinols andcis-Retinyl Esters

“…The isomerization of all-trans-retinol to 11-cis-retinol occurs enzymatically without an exogenous source of energy (1). Hydrolysis of all-trans-retinyl carboxylic ester provides the 4 kcal/ mol energy needed for isomerization, and retinyl esters were proposed to be essential intermediates in 11-cis-retinol formation.…”