Studies on the flash photolysis of visual pigments. 1. Pigments present in frog-rhodopsin solutions after flash-irradiation

CHANGES IN ELECTRICAL CONDUCTANCE OF RHODOPSIN ON PHOTOLYSIS

The change in electrical conductance of rhodopsin solutions was studied with flash-photolysis techniques. The whole pattern of the conductance change on illumination consists of three different processes: (I) the initial decrease, (II) the increase, and (III) the slow decrease, which are in decreasing order of reaction rate. The processes I, II, and III can be most distinctly recognized on flash illumination of acid, slightly acid, and alkaline rhodopsins, respectively. On the other hand, the bleaching of rhodopsin also shows at least three successive phases of different rates, but none of them corresponds in reaction rate to any of the processes of the conductance change. The conductance change may be related to conformational changes of opsin following photoisomerization of retinene, being due to hydrogen or hydroxyl ions and some other inorganic electrolytes. The amount of the change, especially the initial decrease, is proportional to the amount of rhodopsin bleached, even when the photochemical back reaction towards rhodopsin and isorhodopsin occurs in the chromophore depending on the intensity of illumination. Of the three processes, the slow decrease is most severely affected by aging, but the initial decrease and increase are slightly affected. These two processes promptly caused by illumination are connected closely to the conformational changes during the conversion of rhodopsin to metarhodopsin, and perhaps to the initial stage of excitation of rod cells.

The gecko visual pigments. The behavior of opsin

The 521-pigment extracted out of the retina of the Tokay gecko has the typical stereospecificity of the vertebrate visual pigments. This is true for the pigment in the chloride-depleted, "blue-shifted" state as well as for the normal pigment with added chloride. While in the chloride-deficient state, pigment regeneration occurred with both 11-cis- and 9-cis-retinals and the regenerated photopigments were also in the blue-shifted, chloride-depleted state. As with the native pigment, these regenerated pigments were bathochromically shifted to their normal positions by the addition of chloride. Chloride-deficient opsin by itself also responded to chloride for the pigment regenerated with 11-cis-retinal from such chloride-treated opsin was in the normal 521-position. Regeneration was always rapid, reaching completion in less than 5 min, and was significantly faster than for cow rhodopsin regenerating under the same conditions. This rapid rate was found with or without chloride, with both 11-cis- and 9-cis-retinals and in the presence of the sulfhydryl poison, p-hydroxymercuribenzoate (PMB). Like the native chloride-deficient pigment, the regenerated chloride-depleted photopigments responded to PMB by a blue shift beyond the position of the chloride-deficient state. The addition of chloride to these "poisoned" regenerated pigments caused a bathochromic shift of such magnitude as to indicate a repair of both the PMB and chloride-deficient blue shift. In this discussion the possible implications of these results to phylogenetic considerations are considered as well as to some molecular properties of the 521-pigment.

Visual adaptation: effects of externally applied retinal on the light-adapted, isolated skate retina

Incubation with externally applied 11-cis retinal induces a marked increase of visual sensitivity within partially bleached skate photoreceptors. This activity of 11-cis retinal is duplicated by 9-cis retinal, but not by all-trans retinal. The sensitization of photoreceptors promoted by 11-cis and 9-cis retinal is accompanied by the formation of rhodopsin and isorhodopsin, respectively.

Rhodopsin flash photolysis in man

1. Human rhodopsin in vivo was flash bleached by a 600 musec xenon flash which could deliver to the retina up to 15 rod-equivalent quanta per rhodopsin molecule, and the fraction bleached measured by fundus reflexion densitometry. 2. The curve relating fraction rhodopsin bleached to intensity of flash saturates at 0.5 to 0.6. Thus, 40-50% of the rhodopsin is left photo-regenerated by the brightest flashes. 3. Three types of densitometry experiments confirm the saturation of the bleaching curve. 4. The kinetic constants required to account for the observed photo-regeneration were somewhat discrepant with in vitro and in situ estimates from infrahuman species. Specifically, (i) the quantum efficiency of the back reaction, metarhodopsin I hv leads to rhodopsin, was inferred to be nearly as high as that of the forward reaction; and (ii) the rate of the metarhodopsin I leads to metarhodopsin II dark reaction was inferred to be less than 500 sec(-1).