Photic reactions of chicken iodopsin and rhodopsin at liquid helium temperature

Bathoiodopsin, a primary intermediate of iodopsin at physiological temperature

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 degrees 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 wave-length slightly shorter than that measured at low temperature (lambda max, 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.

Primary intermediates of rhodopsin studied by low temperature spectrophotometry and laser photolysis. Bathorhodopsin, hypsorhodopsin and photorhodopsin

The primary photochemical processes of rhodopsin studied by low temperature spectrophotometry and picosecond laser spectroscopy in our group was summarized. Low temperature spectroscopic experiments demonstrated that the retinylidene chromophores of hypso- and bathorhodopsins are in a twisted all-trans forms. Excitation of rhodopsin with 532 nm laser pulse (width: 25 psec) yielded a new bathochromic photoproduct "photorhodopsin"; its spectrum was located at longer wavelengths than that of bathorhodopsin. Photorhodopsin decays to bathorhodopsin with time constants of about 200 psec in squid and 40 psec in cattle. Squid and octopus hypsorhodopsins were produced within 25 psec by high energy pulse, but not by low energy pulse. Thus hypsorhodopsin is produced by two photon reactions (sequential two photochemical reactions) and decayed to bathorhodopsin with time constant of 125 psec.

Activation of phosphodiesterase by chicken iodopsin

Activation of guanosine 3',5'-cyclic monophosphate phosphodiesterase in outer-segment membrane of chicken retina was investigated. Irradiation of dark-adapted chicken outer segment membrane for bleaching of iodopsin increased the enzyme activity twice as much as that in the dark in the presence of GTP. Further irradiation of the sample for bleaching of rhodopsin in the membrane induced some additional activation of the enzyme. However, chicken iodopsin activated the enzyme in frog rod outer segment membrane without irradiation, while chicken rhodopsin did not. Irradiation of chicken iodopsin increased the enzyme activity twice as much as that in the dark.

Formation of hyposorhodopsin in frog retina

Hypsorhodopsin was formed in frog retina by irradiation at liquid helium temperature and converted into bathorhodopsin above about 29K.

Accessibility of the iodopsin chromophore

Iodopsin can replace its chromophore (11-cis retinal) by added 9-cis retinal, resulting in the formation of isoiodopsin. NaBH4 bleaches iodopsin in the dark. In a relatively low concentration of digitonin, the scotopsin (the protein moiety of chicken rhodopsin) removes 11-cis retinal from iodopsin in the dark. These facts suggest that the linkage of the chromophore to opsin in the iodopsin molecule (presumably a Schiff-base linkage) is accessible to these reagents, which is different from the situation in rhodopsin.