Studies on the effects of bleaching amphibian rod pigments in situ: II. The kinetics of the slow bleaching reactions in axolotl red rods

The kinetics of the slow bleaching reactions in axolotl retinas either rich in rhodopsin or porphyropsin were studied by measuring flash-induced changes in absorbance with ultraviolet (380 or 400 nm) and blue light (470 and 480 nm). The results were analyzed in terms of four kinetic models, three of which could satisfactorily simulate the data. The models require that the extinction coefficients of some of the bleaching products in situ should differ from their reported solution values.For the most plausible model, the rate constants for rhodopsin bleaching at 19–20°C are: metarhodopsin II → metarhodopsin III, 0·32 min^?1; metarhodopsin II → “retinal”, 0·77 min^?1; metarhodopsin III → retinol, 0·19 min^?1; “retinal” → retinol, 0·087 min^?1; the rate constants for porphyropsin bleaching at 22–23°C are: metaporphyropsin II → metaprophyropsin III, 0·46 min^?1; metaporphyropsin II → “3-dehydroretinal”, 0·72 min^?1; metaporphyropsin III,→ 3-dehydroretinol, 0·20 min^?1; “3-dehydroretinal” → 3-dehydroretinol, 0·13 min^?1.