Cyclic Pathway of Rhodopsin and its Relationship to Hyperpolarization of the Rod

The Rhodopsin Cycle

The visual pigment rhodopsin (sometimes called "visual purple") is bound to the plasma membrane of the rod and forms transmembrane protein complexes within it. Rhodopsin undergoes a cyclic decomposition and reconstitution in response to the presence of light. This rather complicated cycle is the basis for absorption of light and its transduction into a nervous signal.

Rhodopsin has two components: scotopsin, a protein moiety; and 11-cis-retinal, a carotene derivative. When combined, these two subunits create the conjugated rhodopsin molecule.

Energy from impinging light excites the electrons in the 11-cis-retinal subunit and converts it to a different configuration, 11-trans-retinal. Because this is conformationally incompatible with the scotopsin moiety, it begins to detach from it, and the rhodopsin conjugate begins to break up into its component parts.

The disintegration of rhodopsin into retinal and scotopsin is progressive, with a series of short-lived intermediate compounds formed, as shown in the diagram to the right. The eventual result is release of the two components of rhodopsin from each other completely. One of the breakdown products, metarhodopsin II, is the agent that ultimately effects the change in the rod membrane's charge.

Metarhodopsin II is an enzyme. It acts to activate a second membrane-bound protein in the rod, transducin. Transducin is in its turn an enzyme activating rod-resident phosphodiesterase, a third enzyme in the cascade, capable of hydrolyzing cyclic GMP.

Cyclic GMP's role is to keep sodium channels in the membrane of the rod open, so that sodium flux is facilitated. In dark conditions, this is the normal situation: channels are open, sodium flux from the extracellular space is approximately equal to sodium loss via the pump system of the inner rod segment, and the rod membrane is not hyperpolarized.

Under conditions of impinging light, when the metarhodopsin II—transducin—phosphodiesterase cascade is initated, however, cGMP is destroyed, sodium channels are closed, and the flow of sodium ions into the rod outer segment is slowed or stopped. This causes it to become more negative, i.e., hyperpolarized in the presence of light. The cascade is reversed by the presence of rhodopsin kinase, another enzyme present in the rod outer segment, and the sodium channels are re-opened.

Reconstitution of Rhodopsin

Obviously, the rhodopsin has to be reconstituted, or the ability to respond to light will be lost completely in a few seconds at most. This takes place by two side pathways. First, the 11-trans-retinal is re-converted to the 11-cis-retinal form via an isomerase enzyme. Since the scotopsin moiety is present (having been removed from the rhodopsin) it immediately will combine with this to regenerate new rhodopsin.

Role of Vitamin A

New 11-cis-retinal can also be generated from 11-trans-retinol, or vitamin A. Vitamin A is a derivative form of 11-trans-retinal, enzymatically convertible to it. The isomerase reaction can in turn convert the trans form to the cis isomer, making new 11-cis-retinal available to recombine with scotopsin. By this pathway additional rhodopsin is manufactured to adapt to continuously dark conditions. The pigment epithelium layer of the retina is a storage site for vitamin A.

Implications

The vast amplification of the plasma membrane in the rod outer segment, and the catalytic properties of the enzymes in the cascade from metarhodopsin II—transducin—phosphodiesterase—cGMP breakdown mean that a single photon of light has a great effect on the rod membrane.

One photon, the minimum quantity of light possible, will cause the movement of millions of sodium ions, because of the catalytic nature of the enzymes and the large surface area provided for them to work. Hence the rods are extremely sensitive to light. Animals that live in dark environments (such as deep-water fishes and cave-dwelling creatures) always have far more rods than cones, because it is important to them to be able to see in the minimum amount of light. Other adaptations of the eye structure, such as a large pupil diameter, are also important; but the sensitivity of the chemical cascade is the most important factor in determining the ability of an animal to respond to dim light.

Adaptation to dark in most animals is a matter of generating more 11-cis-retinal from vitamin A, and conjugating it to scotopsin to make more rhodopsin. Similarly, reduction of sensitivity to light means a reduction of the availability of rhodopsin, and hence a conversion of the 11-cis-retinal to the inactive trans form, and conversion of trans retinal back to vitamin A, making it unavailable for conjugation to scotopsin.

Anything which interferes with the rhodopsin cycle or the enzyme cascade it triggers will obviously affect vision, especially in the dark. An individual on a diet deficient in vitamin A can have reserves so low as to make him incapable of producing enough 11-cis-retinal to see effectively in dim light. Drugs or chemicals that affect vitamin A metabolism may lead to similar problems. Hereditary deficiencies in these pathways are also possible sources of this "night blindness."