Activation of phosphodiesterase in frog rod outer segment by rhodopsin analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35) in frog rod outer segment membrane by rhodopsin analogues has been investigated. A rhodopsin analogue modified at the Schiff-base linkage (N-retinyl-opsin) or the beta-ionone ring (3-dehydro-rhodopsin) in the retinylidene chromophore of rhodopsin has some ability in activation of the enzyme. In consideration of our previous observation that opsin including a retinal-oxime can activate the enzyme, it seems likely that the Schiff-base linkage is not always necessary for the phosphodiesterase activation. On the other hand, a change in the length of the side chain of retinal (complex of opsin and beta-ionone, beta-ionylideneacetaldehyde or retinylideneacetaldehyde) or dissection of the conjugate double-bond system of the side chain (retro-gamma-rhodopsin) remarkably reduces the activation ability. However, 5,8-epoxy-rhodopsin having a similar dissected conjugate double-bond system induces some enzyme activation because of its rigid conformation around C7-C8-C9 single bonds. Consequently, it is suggested that the necessary portion of rhodopsin chromophore for the activation of the enzyme is the rigid conjugate double-bond system between the beta-ionone ring and the Schiff-base linkage in its all-trans form.