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Assay Method Information

Assay Name:  Isomerase Assay
Description:  The isomerase assay was performed in 10 mM bis-tris-propane (BTP) buffer, pH 7.5, 0.5% BSA (diluted in BTP buffer), 1 mM sodium pyrophosphate, 20 μM all-trans retinol (in ethanol), and 6 μM apo-CRALBP. The test compounds (2 μl) (final 1/15 dilution of serial dilution stocks) were added to the above reaction mixture to which RPE microsomes were added. The same volume of ethanol was added to the control reaction (absence of test compound). Bovine RPE microsomes (9 μl) (see above) were then added, and the mixtures transferred to 37 C. to initiate the reaction (total volume=150 μl). The reactions were stopped after 30 minutes by adding methanol (300 μl). Heptane was added (300 μl) and mixed into the reaction mixture by pipetting. Retinoid was extracted by agitating the reaction mixtures, followed by centrifugation in a microcentrifuge. The upper organic phase was transferred to HPLC vials and then analyzed by HPLC using an Agilent 1100 HPLC system with normal phase column: SILICA (Agilent Technologies, dp 5μ, 4.6 mmX, 25 CM; running method had flow rate of 1.5 ml/min; injection volume 100 μl). The solvent components were 20% of 2% isopropanol in ethyl acetate and 80% of 100% hexane. The area under the A318 nm curve represented the 11-cis retinol peak, which was calculated by Agilent Chemstation software and recorded manually. The IC50 values (concentration of compound that gives 50% inhibition of 11-cis retinol formation in vitro) were calculated using GraphPad Prism 4 Software (Irvine, Calif.). Each experiment was performed three times in duplicate. Inhibition of isomerase activity (IC50) was determined for each compound.The concentration dependent effect of the compounds disclosed herein on the retinol isomerization reaction can also be evaluated with a recombinant human enzyme system. In particular, the in vitro isomerase assay was performed essentially as in Golczak et al. 2005, PNAS 102: 8162-8167, ref. 3). A homogenate of HEK293 cell clone expressing recombinant human RPE65 and LRAT were the source of the visual enzymes, and exogenous all-trans-retinol (about 20 μM) was used as the substrate. Recombinant human CRALBP (about 80 ug/mL) was added to enhance the formation of 11 cis-retinal. The 200 μL Bis-Tris Phosphate buffer (10 mM, pH 7.2) based reaction mixture also contains 0.5% BSA, and 1 mM NaPPi. In this assay, the reaction was carried out at 37 C. in duplicates for one hour and was terminated by addition of 300 μL methanol. The amount of reaction product, 11-cis-retinol, was measured by HPLC analysis following Heptane extraction of the reaction mixture. The Peak Area Units (PAUs) corresponding to 11 cis-retinol in the HPLC chromatograms were recorded and concentration dependent curves analyzed by GraphPad Prism for IC50 values.
Affinity data for this assay
 

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Last update November 1, 2007
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