On the light-stimulated coupling between rhodopsin and its disk membrane environment.

Disks from bovine ROS undergo a rapid shrinkage when flash illuminated with green light (Uhl, R., et al. (1977) Biochim. Biophys. Acta 469. 113-122). This can be monitored as a light scattering transient, referred to as the P signal. In this paper the P signal is studied at various temperatures and pH. The temperature dependence of the kinetics reveals that "P" consists of two sequential reaction steps. Both appear to occur within the receptor molecule rhodopsin. The actually observed event, the shrinkage of the disk, is therefore not rate limiting under the tested conditions. Both steps of "P" take place while there is only one spectroscopically detectable reaction of the rhodopsin molecule, the metarhodopsin I-metarhodopsin II transition. This implies that there are intermediates of the rhodopsin photolytic cycle which are not evident as spectroscopically separate species. The amplitude of "P", i.e., the extent of the disk shrinkage, is independent of the state of the equilibrium between the two photoproducts absorbing at 478 and 380 nm respectively and called MI and MII. A scheme is suggested in which the irreversible decay of MI (478) triggers the disk shrinkage (and maybe transduction), and in which there is an equilibrium between MII (380) and a proposed isochromic photoproduct MI' (478).