Enzymatic esterification of vitamin a in the pigment epithelium of bovine retina

Berman, E.R.; Horowitz, Jack; Segal, N.; Fisher, Seth; Feeney-Burns, L

doi:10.1016/0304-4165(80)90135-x

Cited by 64 publications

(12 citation statements)

References 21 publications

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“…tLRAT is thus not inhibited by the presence of retinyl heptanoate which is accumulated in the reaction medium during this period of time. This result is different from that obtained by Berman et al [36] who showed that the kinetics of esterification by LRAT was only linear for the first 15 min. This behavior might result from an irreversible surface denaturation of LRAT at the lipid-water interface.…”

Section: Kinetics Behavior Of Tlrat With the Dhpc And Retinol Substratescontrasting

confidence: 57%

Enzymatic activity of Lecithin:retinol acyltransferase: A thermostable and highly active enzyme with a likely mode of interfacial activation

Horchani

Bussières

Cantin

et al. 2014

Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics

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Lecithin retinol acyltransferase (LRAT) plays a major role in the vertebrate visual cycle. Indeed, it is responsible for the esterification of all-trans retinol into all-trans retinyl esters, which can then be stored in microsomes or further metabolized to produce the chromophore of rhodopsin. In the present study, a detailed characterization of the enzymatic properties of truncated LRAT (tLRAT) has been achieved using in vitro assay conditions. A much larger tLRAT activity has been obtained compared to previous reports and to an enzyme with a similar activity. In addition, tLRAT is able to hydrolyze phospholipids bearing different chain lengths with a preference for micellar aggregated substrates. It therefore presents an interfacial activation property, which is typical of classical phospholipases. Furthermore, given that stability is a very important quality of an enzyme, the influence of different parameters on the activity and stability of tLRAT has thus been studied in detail. For example, storage buffer has a strong effect on tLRAT activity and high enzyme stability has been observed at room temperature. The thermostability of tLRAT has also been investigated using circular dichroism and infrared spectroscopy. A decrease in the activity of tLRAT was observed beyond 70°C, accompanied by a modification of its secondary structure, i.e. a decrease of its α-helical content and the appearance of unordered structures and aggregated β-sheets. Nevertheless, residual activity could still be observed after heating tLRAT up to 100 °C. The results of this study highly improved our understanding of this enzyme.

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Section: Kinetics Behavior Of Tlrat With the Dhpc And Retinol Substratescontrasting

confidence: 57%

Enzymatic activity of Lecithin:retinol acyltransferase: A thermostable and highly active enzyme with a likely mode of interfacial activation

Horchani

Bussières

Cantin

et al. 2014

Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics

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“…The fact that 11-cis-[3H]retinoid biosynthesis occurred in the absence of any added cofactors means that a sufficient endogenous energy source is available for the accumulation of the radioactive l1-cis-retinoids. An analogous situation arises with the formation of radioactive all-trans-retinyl palmitate using pigment epithelium microsomes and radioactive alltrans-retinol in the absence of an added energy source to drive the esterification (27). Further purification of the isomerizing system will clarify its energy requirements and its other characteristics.…”

Section: Resultsmentioning

confidence: 99%

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

Bernstein¹,

Law²,

Rando³

1987

Proc. Natl. Acad. Sci. U.S.A.

154

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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...

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“…302 The distribution of retinoids in cone-dominant retinas is substantially different from that of rod-dominant species, with an abundance of 11- cis -retinyl esters found in the retina as opposed to the stores of all-trans -retinyl esters in the RPE. 303,304 Three enzymatic activities, namely isomerase, 11- cis - and all-trans -retinyl ester synthase, and retinol dehydrogenase/reductase, are associated with membrane fractions from chicken neural retina. 302 Cultured primary Müller cells from chickens were found to convert all-trans -retinol added to the media into 11- cis -retinol and 11- cis -retinyl esters, suggesting that this cell type could be a site of visual chromophore production in vivo .…”

Section: Transformation Of Retinoids Within the Eyementioning

confidence: 99%