Studies on location of the chromophore in papain-digested rod outer segment membranes

Rhodopsin within rod outer segments is cleaved by papain to three membrane-bound fragments, H, M and L. The apparent molecular weights in dodecyl sulphate polyacrylamide electrophoresis are 27400±1000 (H), 21700±1200 (M) and 7000±3600 (L) daltons. Fragment H is split into segments of apparent molecular weight ≤7000 daltons with either mercaptoethanol or dithiothreitol under vigorous conditions. The number of free amino groups of lysine remaining in fragments H and L following extensive acetimidation of papain-cleaved rod outer segments is determined on the isolated fragments by dansylation. Insufficient residues are found for the retinal prosthetic group to be assigned unequivocally to fragment H, while fragment L is excluded as the location for the chromophore.