The regeneration of rhodopsin from all-trans-retinal: solubilization of an enzyme system involved in the completion of the visual cycle

Studies on visual receptors both in vivo and in vitro have highlighted the existence of at least two broad biochemical processes. The first involves the combination through a Schiff-base linkage (Akhtar et al., 1967, 1968; Hirtenstein & Akhtar, 1970; Bownds, 1967) of 1 1-cis-retinal with the protein opsin to give rhodopsin (A.,.1. 498nm). In the second process on absorption of light rhodopsin participates in the transmission of impulses responsible for the visual sensation, and itself is converted into opsin and all- trans-retinal (for reviews see Bridges, 1970; Morton & Pitt, 1969). In order to complete the visual cycle there must be present in vivo an enzyme system that catalyses the conversion of all-trans-retinal into 1 -cis-retinal. Hubbard ( However, attempts to repeat this particular work have been unsuccessful in this laboratory and elsewhere (Plante & Rabinovitch, 1972). We have since shown (Amer & Akhtar, 1972) that carefully prepared rod outer segments possess the ability to regenerate rhodopsin from all-trans-retinal in a completely dark reaction and at a physiologically significant rate. This observ- ation has provided the first convincing evidence for the existence of the retinal isomerase system that catalyses the conversion of all-trans-retinal into 11- cis-retinal. Several properties of this system within the rod outer segments have been studied. We now report on the solubilization of the enzyme system participating in the formation of rhodopsin from all-trans-retinal.