Studies on the effects of bleaching amphibian rod pigments in situ: III. Linear dichroism in axolotl red rods before and during bleaching

The linear dichroism of axolotl porphyropsin and rhodopsin was studied spectrophotometrically using transverse illumination of retinal folds. Partial bleaching experiments on retinas whose rods contained roughly equal mixtures of the pigments showed that the dichroism of the porphyropsin was lower than that of the rhodopsin. The orientations of the long-lived photoproducts observed in retinas containing predominantly porphyropsin or rhodopsin were determined from absorbance changes at selected wavelengths following flash bleaches. The dichroism of metaporphyropsin II and metaporphyropsin III was found to be higher than that of the porphyropsin they derived from, while metarhodopsin II and III showed similar dichroism to their parent rhodopsin. Kinetic analyses of the near u.v. absorbance changes during bleaching indicated that both “retinal” and “3-dehydroretinal” intermediates were also aligned predominantly parallel to the plane of the disc membrane. In contrast, the final in situ bleaching products, retinol and 3-dehydroretinol, were found to be oriented at a large mean angle to the plane of the disc membrane.