62 The Physiology of Sense Organs 



isomerizes the retinal from an ii-cis to an all-trans configuration, 

 and the chemical bonds necessary for the retinal-protein 

 complex are thereby sterically hindered; thus, the pigment 

 integrity is destroyed. Rhodopsin can be resynthesized following 

 the initial photochemical reaction by alternative mechanisms 

 (fig. 27). The most direct of these involves an enzymatic iso- 

 merization of all-trans retinene to the neo-^ configuration, and 



Rhodopsin 



Nco-b retinene -f opsin 7 All-imnj retinene + opsin 



^ Calcohol dehydrogenase, DPN) 



r 



Neo-i^ vitamin A '^ ■ , ' Ali-trans vitamin A 



CH3 y <?"3 H 



".<^ ^ g ft I 



H*C .C-CH3 /^"^^'H 



Hz 



CH^OH 



11-cis fneo-b) 

 (B) 



Fig. 27. (A) Scheme indicating the reactions involved in the 

 breakdown and synthesis of rhodopsin. (B) Chemical structure 

 of neo-6 retinene. (B from Wald,®^ Fig. 5.) 



this substance can spontaneously combine with available opsin. 

 Alternatively, all-trans retinene is reduced to all-trans vitamin A 

 by the enzyme, alcohol dehydrogenase. The vitamin A isomer is 

 then isomerized to the neo-6 (ii-cis) configuration, and the 

 product of this reaction is oxidized to the active form of retinene. ^^ 

 The loss in visual acuity following exposure to a very strong 

 light has been attributed to the photic breakdown of available 



