290 UNITY AND DIVERSITY IN BIOCHEMISTRY 



absorption depending on the greater or lesser activity of the intestinal caro- 

 tenase which converts carotene to vitamin A and as a result the reserve fats be- 

 come more or less saturated w^ith carotene. Man and other Primates absorb 

 carotenoids in general, as does the frog also ; other animals exercise a selective 

 adsorption. For example, the horse and the cow selectively absorb caro- 

 tenoids and store them without alteration, whilst birds and fish show a pre- 

 ference for xanthophylls. However birds and fish modify one of the ingested 

 xanthophylls, lutein, by oxidation, and the products of the oxidation are 

 deposited in the feathers in the case of birds, and in the skin in the case of fish. 



The carotenoid structure appears to be connected in a general way with 

 the function of photoreception. The most primitive type of photoreception, 

 lacking the presence of any differentiated photoreceptors, is the type called 

 dermatoptic, which is found in primitive types of organisms up to the 

 amphibians and also in plants. The maximum sensitivity of this derma- 

 toptic function is in the ultra-violet part of the spectrum around 365m/u-, 

 and it is the receptor in photokinetic processes involving tropisms towards 

 light. Now, in a number of cases, photoreceptors have evolved secondarily 

 and developed new kinds of receptor molecules adapted to the light from 

 the sun and the sky. All these substances belong to the carotenoid group. 

 In plants, phototropic bending depends on the properties of carotenoids 

 such as xanthophyll in Avena, or /3-carotene in the sporangiophores of 

 Phycomyces. The orientation of an animal depends on visual photoreception 

 and requires the presence of other carotenoids showing the same kind 

 of adaptation to sunlight and having an absorption maximum at around 

 500m/Li. This development is due to the ability, mentioned above, to change 

 some of the plant carotenoids into vitamin A. There are two types of 

 vitamin A : vitamin A^ and vitamin Ag. 



Let us first consider the system in the eye of land vertebrates such as 

 mammals or birds. The pigment of their retina is rhodospin, a rose- 

 coloured carotenoid-protein complex. In aqueous solution its absorption 

 spectrum consists of a single broad band with a maximum at 500m/x. In 

 light it is bleached to orange and yellow pigments and in the process the 

 carotenoid retinene I is liberated. The latter substance has never been 

 found anywhere except in the retina. Its spectrum in chloroform consists of 

 a single band with a maximum at about 387m)Lt. In the retina the mxture of 

 retinene I and protein reverts to rhodopsin and in addition retinene I is 

 converted to vitamin Ai, which, in the intact eye, also reunites with protein 

 to form rhodopsin. This system is not only present in the eyes of mammals 

 and birds but also in the eyes of Invertebrates such as the squid, Loligo, 

 and the crayfish Camharus. 



If we consider the system in the eye of a marine fish we find the rhodopsin 

 system as in birds and mammals and Invertebrates, but this is not the system 

 to be found in the eyes of freshwater fish which contain another system, 



