Biochemical aspects of the visual process XL. Spectral and chemical analysis of metarhodopsin III in photoreceptor membrane suspensions

Abstract

The late photointermediates of rhodopsin photolysis have been analyzed spectrally and chemically in bovine rod outer segment membrane suspension at 25°C and pH 6.5. The decay of metarhodopsin II follows two spectrally distinct routes, resulting 40 min after illumination in a stable mixture of photoproducts with absorbance maxima around 380 and 452 nm, free retinal and metarhodopsin III, respectively. Chemical analysis shows that three different products are involved: free retinal (approx. 34%), protein-bound retinal (approx. 51%) and lipid-bound retinal (approx. 15%). The latter fraction consists of retinylidene-phosphatidylethanolamine exclusively. Photolysis of membranes reconstituted with various phospholipids gives a qualitatively normal spectral picture, but the production of metarhodopsin III may vary with the phospholipid composition, i.e. with the percent of phosphatidylethanolamine present. Chemical analysis shows that with increasing phosphatidylethanolamine content of the membrane, the retinylidene phosphatidylethanolamine fraction increases proportionally at the expense of free retinal, while the fraction of protein-bound retinal remains unaffected. The results indicate that under these conditions metarhodopsin III (defined as a long wavelength product of metahrodopsin II decay) is composed of two chemically distinct components: opsin-bound retinal and retinylidene phosphatidylethanolamine.