Nanosecond laser photolysis of iodopsin, a chicken red-sensitive cone visual pigment

The photobleaching process of iodopsin (a chicken red-sensitive cone visual pigment) purified in a detergent system containing CHAPS and phosphatidylcholine was investigated by means of nanosecond laser photolysis at room temperature. Excitation of iodopsin with a nanosecond laser pulse (wavelength, 560 nm; pulse width, 17 ns) resulted in the formation of at least four intermediates on the nanosecond to millisecond time scale. The earliest intermediate detected had an absorption maximum at 571 nm, which was very close to that of original iodopsin (lambda max = 567 nm), and remarkably blue-shifted as compared with that of bathoiodopsin [lambda max = 625 nm; Kandori et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8908-8912]. The intermediate, named BL-iodopsin, converted to the next intermediate, lumiiodopsin (lambda max = 535 nm), with a time constant of 130 ns. The BL intermediate had an absorption maximum just between batho- and lumiiodopsins, and an extinction coefficient comparable with these intermediates. These properties are different from those of the corresponding intermediate of rhodopsin [BL(BSI)-rhodopsin], suggesting that the binding of chloride to iodopsin, but not to rhodopsin, has an influence upon changes of the chromophore-opsin interaction in the early stage of photobleaching of iodopsin. Lumiiodopsin converted to metaiodopsin I (lambda max < 500 nm) with a time constant of 230 microseconds, and then to metaiodopsin II (lambda max = 390 nm) with a time constant of 6 ms. A thermal equilibrium between metaiodopsin I and II was established, but unlike meta intermediates of rhodopsin, they showed little temperature dependence.