6533b830fe1ef96bd1296604

RESEARCH PRODUCT

All-trans to 11-cis retinol isomerization in nuclear membrane fraction from bovine retinal pigment epithelium

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Abstract Isomerization of all-trans to 11-cis retinol has been studied in a membrane preparation from the nuclear fraction of bovine retinal pigment epithelium. When the nuclear membrane preparation deprived of endogenous retinoids is incubated with 4·5 μ m all-trans-retinol, the mean value calculated for the isomerase activity is 1·32 nmol 11-cis retinol formed hr−1 mg protein−1. Simultaneous formation of all-trans and 11-cis retinyl esters is also observed in the nuclear preparation. When assayed under the same experimental condition, RPE 150 000 g post-nuclear sediment shows about 70% of the isomerase activity found in the nuclear membrane fraction. Treatment of the nuclear membrane fraction with 0·5% (w/v) CHAPS produces a 200 000-g supernatant retaining 80% of the total isomerase activity and leads to a modest purification of the enzyme activity. Apparent values for Km and Vmax of the solubilized enzyme are 1·6 μ m and 2·5 nmol 11-cis retinol formed h−1 mg protein−1, respectively. Bovine serum albumin and β-lactoglobulin effectively stimulate the isomerization reaction. The mechanism underlying this activating effect remains unclarified at present. Some hypotheses are discussed.