Light isomerizes the chromophore of rhodopsin, ll-cis retinal (formerly retinene), to the all-trans configuration. This introduces a succession of unstable intermediates--pre-lumirhodopsin, lumirhodopsin, metarhodopsin --in which all-trans retinal is still attached to the chromophoric site on opsin.Finally, retinal is hydrolyzed from opsin. The present experiments show that metarhodopsin exists in two tautomeric forms, metarhodopsins I and II, with ),m~ 478 and 380 m#. Metarhodopsin I appears first, then enters into equilibrium with metarhodopsin II. In this equilibrium, the proportion of metarhodopsin II is favored by higher temperature or pH, neutral salts, and glycerol The change from metarhodopsin I to II involves the binding of a proton by a group with pK 6.4 (imidazole?), and a large increase of entropy. Metarhodopsin II has been confused earlier with the final mixture of all-trans retinal and opsin (Xm,~ 387 m/z), which it resembles in spectrum. These two products are, however, readily distinguished experimentally.On exposure to light, rhodopsin bleaches over transient intermediates to an eventual mixture of retinaP and opsin. If one begins with cattle rhodopsin, for example, at the temperature of liquid nitrogen (-195°C), the product of irradiation is pre-lumirhodopsin with km~x 543 m#. On warming above -140°C, this goes over in the dark to lumirhodopsin (km,x 500 m/z), and above -40°C, to metarhodopsin (km,x 478 m#) (of. Yoshizawa and Wald, 1963).Above -15°C, metarhodopsin fades to a yellow compound, and since this reaction occurs only in the presence of water, we have heretofore assumed that it corresponds to the hydrolysis of metarhodopsin to all-trans retinal (km,x about 387 m#) and opsin (cf. Wald, Durell, and St. George, 1950;Hubbard and Kropf, 1958).This hydrolysis, however, is slow, requiring about an hour for completion i We shall use the nomenclature proposed by the Commission on the Nomenclature of Biological Chemistry of the International Union of Pure and Applied Chemistry (1960), according to which vitamin A is now known as retinol, and rcfinene (vitamin A aldehyde), as retinal. We shall, however, refer to refinenc oximc as retinaldehydc oximc, and on occasion to refinene as retinaldehyde, in order to avoid the confusion which might be engendered by the use of "retinal" in its adjectival form.
