Evidence for the prolonged photoactivated lifetime of an analogue visual pigment containing 11 -cis 9-desmethylretinal

Abstract

AbstractFollowing bright flashes, rod photoreceptors exhibit a period of photocurrent saturation that increases linearly with the logarithm of flash intensity. In a recent report, Pepperberg et al. (1992) presented evidence that the slope of the function relating the saturation period (T) to the natural logarithm of flash intensity (In If) represents the exponential lifetime (τ) of photoactivated visual pigment: τ = ΔT/Δ[ln If]. In salamander rods, 11 -cis 9-desmethylretinal combines with opsin to form 9-desmethyl rhodopsin. Dim flash responses mediated by this analogue visual pigment exhibited slow recovery kinetics relative to those of native pigment (Corson et al., 1991). This observation raises the hypothesis that the physiological lifetime of photoactivated 9-desmethyl rhodopsin is substantially longer than that of native visual pigment. To test this hypothesis, we have examined the relation between the period of photocurrent saturation and flash intensity in salamander rods containing a mixture of the two pigments. Brief stimuli at two widely separated wavelengths (440 and 640 nm) elicited saturating photocurrent responses that were preferentially mediated by 9-desmethyl rhodopsin or residual native pigment, respectively. Plots of T vs. In If revealed a linear increase in the period of response saturation over a large range of saturating intensities at both wavelengths. However, the slope of the relation between T and In If with 440-nm flashes was more than twice as large (4.1 ± 0.5 s, n = 5) as that measured with 640-nm flashes (1.7 ± 0.4 s). For rods subjected only to bleaching of the native pigment, or to bleaching and resensitization with 11-cis retinal, the slope of the relation between T and In If remained independent of wavelength and indistinguishable from that of native pigment in unbleached cells. The data provide support for the hypothesis that the slope parameter τ represents the lifetime of photoactivated pigment, and specifically suggest that the lifetime of photoactivated 9-desmethyl rhodopsin is abnormally long.