THE CHEMISTRY OF VISION
Authors: COLLINS F. D. 
Published: January 2008

An account is given of the isolation of the outer segments of retinal rods, their internal structure as revealed by the electron microscope, and their chemical composition.

The relationship between action spectra (e.g. the scotopic luminosity curve) and absorption spectra is discussed. Provided certain conditions are satisfied, the chemist is able to compare the absorption spectra of pigments isolated from the retina with action spectra which have been recorded for the same type of retina by the physiologist. Thus the absorption spectrum of rhodopsin corresponds to the scotopic luminosity curve. If there was general agreement on a theory of colour vision, this would give valuable information concerning the number of photosensitive pigments to be expected.

At present the only photosensitive pigments whose existence is unequivocal are rhodopsin (found in mammals, birds, reptiles, Amphibia, most fish and the squid), porphyropsin (found in some fish and Amphibia) and iodopsin (found so far only in the fowl). Recent claims to have extracted other pigments are discussed.

The extraction and examination of rhodopsin is described. Until recently the various products obtained from rhodopsin by the action of light were characterized by their absoprtion spectra only. These consisted of retinene, indicator yellow, lumirhodopsin, meta‐rhodopsin and iso‐rhodopsin.

The retina has an enzyme, believed to be identical with alcohol dehydrogenase, which catalyses the interconversion of vitamin A and retinene in the presence of coenzyme 1.

It has been found possible to effect the regeneration of rhodopsin in vitro. Wald and his co‐workers have shown that a system containing retinene (or vitamin A, coenzyme i and alcohol dehydrogenase) and the specific protein of rhodopsin, ‘opsin’, will synthesize rhodopsin. However, these workers found that synthetic vitamin A or retinene was not effective, but could be made so by exposure to light. Collins and his co‐workers have obtained regeneration in vitro using synthetic vitamin A or retinene; their systems appear to be nearly complete. They have also shown that pyridoxal phosphate augments the amount of regeneration, although no explanation can be given of this result. The rhodopsin system is outlined in a diagram and table.

In the next section an account is given of the results of chemical work aimed at identifying the pigments known only by their absorption spectra. The amounts of these pigments are too small to be isolated and indirect methods have to be used. Retinene has now been identified as vitamin A aldehyde. Indicator yellow has been shown not to be an artifact (as suggested by some workers), and to be a Schiffs base of retinene (an N‐substituted retinene imine). However, the structure of the acid form of indicator yellow is still unknown. Because of their possible connection with visual pigments, a brief account is given of vitamin A amine and methylamine. Certain other reactions of retinene are described (e.g. the reaction with sulphydryl groups) in order to assist the discussion of the nature of rhodopsin itself.

The significance of the fact that vitamin A, retinene and retinene methylimine react with antimony trichloride, sulphuric acid and phosphoric acid to give compounds which have absorption spectra resembling the action spectra obtained by physiologists in various retinas, is discussed.

The outstanding problems are: (1) the structure of acid indicator yellow, (2) the structure of active‐vitamin A and active‐retinene, (3) the role of pyridoxal phosphate, and (4) the role of sulphydryl groups. Possible alternative explanations are presented, and the evidence for and against discussed.

I am greatly indebted to Prof. R. A. Morton, F.R.S., for his many helpful criticisms and advice.