The formation of metarhodospin380 in the retinal rods of the frog.

1. The formation of metarhodopsin380 (metarhodopsin II) was studied in isolated frog retinas exposed to intense flashes of 120 mus duration. 2. A rapid increase in absorbance at 375 nm during the flash was followed by a slower absorbance increase in the subsequent dark period. The slower increase showed virtual completion after 5 ms. 3. The fast absorbance increase during the flash was due to the formation of metarhodopsin478. The rate of this reaction was dependent on the time course of the flash and on the decay rate of lumirhodopsin. 4. Kinetic analysis indicates that three consecutive reactions occur: the light-controlled formation of lumirhodopsin, its first-order decay to metarhodopsin478 and the conversion of metarhodopsin478 into metarhodopsin380. At 21 degrees C, the decay constants were 2 X 10(4) S-1 (lumirhodopsin) and 1 X 10(3) S-1 (metarhodopsin478), respectively.