Biochemical aspects of the visual process. XXXIV. Relation between sulfhydryl groups and properties of rhodopsin studied by means of methylmercuric iodide.

1. Treatment of isolated bovine rod outer segment membranes with the very reactive, small and uncharged sulfhydryl reagent methylmercuric iodide shows all six SH groups of rhodopsin can be modified without loss of its typical absorbance spectrum. 2. The partial loss of regeneration capacity, which occurs under these circumstances, can be attributed to secondary conformational changes following the actual modification reaction. 3. These observations plead against a direct involvement of SH groups in these two important parameters of rhodopsin. 4. Upon modification of SH groups, the normal photolytic sequence of rhodopsin is increasingly disturbed. When four or more SH groups are modified, the appearance of a photointermediate resembling metarhodopsin I dominates under conditions where native rhodopsin yields the transition of metarhodopsin II to metarhodopsin III.