Bathoiodopsin, a primary intermediate of iodopsin at physiological temperature.

Abstract

Measurement of the primary photochemical reaction of iodopsin, a chicken red-sensitive cone visual pigment, was carried out at room temperature by using picosecond (ps) laser photolysis. Excitation of iodopsin with a ps green pulse (pulse width, 21 ps) caused the instantaneous formation of a bathochromic product, which was stable on a ps time scale. This product may correspond to "bathoiodopsin," which was detected by low-temperature spectrophotometry. Although bathoiodopsin produced at the temperature of liquid nitrogen or helium reverted to the original pigment (iodopsin) on warming (above -170'C), the bathoiodopsin produced at physiological temperature decayed to all-trans-retinal and R-photopsin (the protein moiety of iodopsin) presumably through several intermediates. The absorption maximum of bathoiodopsin at room temperature was at 625 nm, a wavelength slightly shorter than that measured at low temperature (A4., 640 nm). The extinction coefficient of bathoiodopsin at room temperature was lower than that at low temperature and close to that of the original iodopsin at room temperature.The vertebrate retina contains two kinds of photoreceptor cells, rods and cones. A rod is a photoreceptor cell that has evolutionally acquired an extremely high photosensitivity so as to act under twilight conditions. The visual pigment in rods is called rhodopsin. Cones, however, act under daylight conditions and have a photoresponse with a wider dynamic range and a faster latency (1). Since there are at least three types of cones, each with its own distinct visual pigment possessing characteristic absorption maximum (2), integration of the photoresponses from cones generates the sensation of color in the visual cortex.The primary photoreceptive mechanism in visual transduction has been extensively investigated in rods, which contain rhodopsin as the visual pigment. Rhodopsin has an 11-cis-retinylidene chromophore attached by a protonated Schiff base linkage to a specific lysine residue of the apoprotein opsin. The primary photochemical event in rhodopsin is an isomerization around the C11=C12 bond of the 11-cisretinylidene chromophore to form photorhodopsin, a highly twisted all-trans photoproduct (3,4 (6, 7).On absorption of light, iodopsin bleaches to all-transretinal and R-photopsin (the protein moiety of iodopsin) by way of lumiiodopsin and then metaiodopsin, according to low-temperature spectrophotometry (8). Yoshizawa and Wald (9) observed that irradiation of iodopsin at the temperature of liquid nitrogen produced bathoiodopsin. Prolonged irradiation of this sample formed a steady-state photo mixture composed of iodopsin, isoiodopsin, and bathoiodopsin. This photochemical behavior of iodopsin is very similar to that of rhodopsin (Fig. 1). On warming, however, an unexpected difference between iodopsin and rhodopsin was observed; unlike bathorhodopsin, bathoiodopsin was not thermally converted to lumiiodopsin but reverted back to iodopsin above -170°C. Thus the question arises whether bathoiodopsin is a physiolo...

Ultra‐fast Laser Spectroscopy of Visual Pigments

“…On the other hand, the extinction coefficient of bathorhodopsin at room temperature is 0.76 times smaller than that at low temperature. Similar observations have been reported in bacteriorhodopsin (Shichida et al, 1983) and chicken iodopsin (Kandori et al, 1990). These facts suggest that there exists some difference in potential surface between bathorhodopsins at room and low temperatures.…”

“…The primary photochemical reaction of iodopsin, the chicken red sensitive cone visual pigment, has been recently investigated by means of picosecond laser photolysis (Kandori et al, 1990). The excitation of iodopsin with a picosecond green pulse (wavelength: 532 nm, pulse width: 21 ps) produced an instantaneous formation of a bathochromic photoproduct which was stable on a picosecond time scale.…”

Chimeric Nature of Pinopsin between Rod and Cone Visual Pigments

“…As for green, only the decay of meta III (closed triangles) was observed (time constant, 3.9 min) due to a rapid conversion of meta II to meta III intermediates at this temperature. , and red (crosses), among which those of rhodopsin, red, green, and blue are reproduced from previous reports (9,11,31,32,41,42).…”

Thermal Recovery of Iodopsin from Photobleaching Intermediates†

“…Laser photolyses of iodopsin at room temperature have identified bleaching intermediates of iodopsin formed at physiologic temperatures (22,26). After relaxation of the photoexcited state, bathoiodopsin ( λ max = 625 nm), BL‐iodopsin (571 nm), lumiiodopsin (535 nm), metaiodopsin I (<500 nm) and metaiodopsin II (390 nm) are successively formed.…”

“…The reverse reaction of iodopsin‐Cl − is not observed at room temperature (22,26), indicating that the cooling slowed the forward reaction more than the reverse reaction. The rate constant ( k ) of the thermal reaction depends on temperature ( T ), activation enthalpy (Δ H ‡ ), activation entropy (Δ S ‡ ), gas constant ( R ), Planck constant ( h ) and Boltzmann constant ( k B ) as follows.…”