[Enzymes of the outer segments of the retinal rods: The problem of localization and coupling with rhodopsin]

The problem of the presence of enzymes in retina rod outer segments is summarized using both literature and the author's own data. Difficulties in the solving of this problem are analyzed. It has been shown that some enzymes involved in the primary mechanisms of the photoreceptor process are localized in the inner segment of the photoreceptor cell at a distance from the photoreceptor membranes. A conclusion is drawn that the enzymes necessary for rhodopsin (or its retinal-part) conversions as well as those ensuring synthesis and decay of cyclic nucleotides are concentrated in the outer segments. These data served as a ground for a conclusion about a strictly "biochemial" specialization of particular parts of the photoreceptor cell in which tightly linked chemical transformations proceed in different compartments. Besides, some unusual properties of the plasma membrane of the photoreceptor cell are noticed, in particular, the absence in part of this membrane of some typical marking enzymes. The interrelations between rhodopsin and enzymes as well as a possible participation of Ca-ions in this process are examined. It is supposed that one part of the rhodopsin molecule may act as a Ca2+ binding protein.