Reconstitution of squid and cattle rhodospin by the use of metaretinochrome in their respective membranes

Abstract

Retinochrome is, even in membranes, converted to metaretinochrome by exposure to orange light. Upon incubation of metaretinochrome in membranes with cattle opsin in rod outer segment membranes, cattle rhodopsin is reconstituted in the dark. When opsin is present in molar excess to metaretinochrome, about 80% of the prosthetic retinal of retinochrome present initially is utilized for the reconstitution of cattle rhodopsin. One reason why all of the prosthetic retinal is not used for the rhodopsin reconstitution is that metaretinochrome transforms slowly to retinochrome during incubation in the dark and another is that metaretinochrome is in a photoequilibrium mixture with a trace of retinochrome after exposure to orange light. Squid rhodopsin is reconstituted when a mixture of metaretinochrome and squid opsin in their respective membranes is incubated in the dark. The reconstituted rhodopsin is converted to acid or alkaline metarhodopsin by exposure to orange light at neutral or alkaline pH, respectively. Three possible mechanisms for the transference of 11- cis retinal from metaretinochrome in a membrane to opsin in a different membrane were considered: (1) the migration of 11- cis retinal through an aqueous medium between the separate membranes, (2) the migration of 11- cis retinal from metaretinochrome to opsin in a fused membrane and (3) the transfer of retinal from membrane to membrane in close contact. In conclusion, the first two mechanisms were inapplicable and the third appeared to explain the present experimental findings. The possibility is discussed that the photoproduct of retinochrome may contribute to the rhodopsin synthesis as an effective donor of 11- cis retinal to opsin in the squid retina.