Behaviour of octopus rhodopsin and its photoproducts at very low temperatures

Abstract

Many aspects of light energy transduction in photoreceptors are under study^1,2. One important problem concerns the light-initiated primary event in rhodopsin and the identity of the primary photoproduct. It is usually assumed that the red-absorbing pigment bathorhodopsin³ is the first photoproduct. In cattle rhodopsin, it is produced within 6 ps of excitation⁴ and is stable at 77 K. However, Yoshizawa et al.^5–7 have found a blue-absorbing pigment, hypsorhodopsin, in photosteady-state mixtures formed with orange light at liquid helium temperatures (∼5 K). On warming the hypsorhodopsin in the dark to liquid nitrogen temperatures (77 K), it converts to bathorhodopsin. These two results suggested that hypsorhodopsin might be a precursor of bathorhodopsin, but that presents difficulties. For example, hypso-type intermediates have not been seen for all visual pigments⁶. Moreover, when cattle rhodopsin was irradiated with blue light at liquid helium temperatures, it was converted to bathorhodopsin without forming detectable hypsorhodopsin^6,7. FinaUy, rapid kinetic experiments to determine the time course of formation of bathorhodopsin and hypsorhodopsin have given ambiguous results^4,8–10; this may be due to the use of rhodopsins from different species. Because squid rhodopsin seemed to have more hypsorhodopsin in photosteady states than cattle rhodopsin⁷, we have studied the photo products of another cephalopod rhodopsin at carefully controlled low temperatures (±0.2 K). We report here that octopus rhodopsin has a hypso intermediate that is easily formed on irradiation but that the batho-product appears before significant amounts of the hypso-product accumulates. Moreover, the thermal conversion of hypsorhodopsin to bathorhodopsin must be fitted with two rate constants, and the activation enthalpy of the faster process is almost zero, that is, it is temperature independent over the range studied.