354 ANIMAL BIOCHEMISTRY 



is endergonic and occurs without an energy-coupling mechanism. 

 Decomposition of rhodopsin requires energy and is driven by absorp- 

 tion of light with the formation of certain poorly characterized inter- 

 mediates. 



The mechanism of nerve stimulation is still in doubt. One prelim- 

 inary theory suggests that r?5-retinene combines with and blocks — SH 

 groups of the opsin. When these groups are freed during the photo- 

 chemical reaction, it is postulated that they trigger nerve impulses. 

 Whatever the mechanism, the process is remarkably sensitive and 

 efficient. A single quantum of light dissociates a molecule of retinene 

 from rhodopsin, and himian beings can detect light in an amount 

 just sufficient to split one molecule of rhodopsin in each of only 

 15 to 20 per cent of the rods present in the eye. Apparently there is a 

 large magnifying effect between the photochemical event and the 

 nerve impulse. 



Vision based on the rods is without the sensation of color which is 

 experienced only when a higher threshold is exceeded and the cones 

 come into play. These sensitive elements are believed to contain pig- 

 ments of three different colors responding to light of different colors. 

 One has been isolated from chicken retinas and shown to be a protein 

 complex with retinenei based on a protein different from that of 

 rhodopsin. The change in protein alters the absorption spectrum of 

 the complex and yields a pigment of different color. Perhaps three 

 such complexes are sufficient to produce the nerve stimulations we 

 know as color. 



Requirements 



For nutritional purposes vitamin A is still expressed in international 

 units equal in effect to 0.6 ^.g. of ^S-carotene. Diets should contain 

 one or the other or a combination equivalent to the recommended 

 daily allowances. Vitamin A is stored extensively in the liver, from 

 which it is withdrawn at need. However, the presence of a large 

 reserve complicates the estimation of the supply regularly required. 



Various studies suggest that 1,300 units daily may be adequate for 

 human beings, but it is customary to increase the figure to 3,000 to 

 5,000 for adults and 8,000 units during lactation in order to provide 

 a safety factor. Allowances for beef cattle range from 25 to 55 mg. 

 of carotene, depending on body weight, with 2 to 40 mg. for swine. 

 Poultry feeds should contain 2,000 to 3,300 units of vitamin A, depend- 

 ing on age, type, and physiological activity, with 4,000 units for breed- 

 ing turkeys. 



Carrots, sweet potatoes, outer leaves of lettuce and cabbage, apricots, 



