680 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOGY I 



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0.30 - 



0.20 



c 



o 



o ._ 



c 



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1 r 



T 



neor&iinene a 



o //-trans reiinene 



S'^nthes/s of iodops/ns 

 from sfGreo/somers 

 of retinenQ 



/so- lodops/ n 



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/oolops/n 



^ '• ^' necreiinene h 



300 



400 SOO 



VMcxvelencjth-^ mjj 



600 



700 



FIG. II. Synthesis of iodopsin and iso-iodopsin. In a chicken retinal extract, the iodopsin alone 

 was bleached with deep red light to a mi.xture of all-(ra;?.s retincne and photopsin. This product 

 was incubated in the dark with four geometrical isomers of retinene. The absorption spectra were 

 then measured against the red-bleached solution as blank. .\\\-trans and neo-a retinene synthesized 

 no photosensitive pigments, hence remained almost as added. Neo-i retinene formed iodopsin 

 C^inax 562 m^); iso-a retinene, iso-iodopsin (Xmax 510 m/i)- Both photosensitive pigments are ac- 

 companied by residues of unchanged retinene, primarily responsible for the absorption bands at 

 about 370 my.. [From Wald et al. (72).] 



of reactions initiated by light (viz fig. 3 and text 

 above), whereas the nervous response, e\en in a 

 cold-blooded animal, appears within a fraction of a 

 second. Changes in the opsins therefore would seem 

 to offer richer possibilities. 



Rhodopsin has been studied most in this regard. 

 Cattle rhodopsin has a molecular weight of about 

 40,000 and contains one molecule of retinene (30). 

 It has a molar extinction of 40,600 (69). The iso- 

 electric point of frog rhodopsin is at pH 4.47 and 

 goes to pH 4.57 on bleaching (7); cattle rhodopsin 

 is isoionic at pH 5.4 and goes to pH 5.5 on bleaching 

 (45). Neither cattle rhodopsin nor opsin contains 

 available N- or C-terminal amino acids (i). 



The synthesis of rhodopsin from retinene and 

 opsin requires the presence of free sulfhydryl ( — SH) 

 groups on opsin. Conversely, the bleaching of rho- 



dopsin liberates 2 or 3 — SH groups per molecule. 

 This is true equally for rhodopsins from cattle, frogs 

 and squid (68, 69). Exposure of rhodopsin to light 

 also immediately exposes an acid-binding group with a 

 pK of about 6.6, close to the pK of the imidazole 

 group of histidine (45). Furthermore, opsin is much 

 more readily denatured by acid and alkali, or heat, 

 than rhodopsin (31a, 46). 



All of this means that the action of light on rho- 

 dopsin, in addition to splitting off carotenoid, pro- 

 foundly affects the reactivity of the opsin. In the 

 structural context of a rod outer limb, these or like 

 changes are probabh' the ultimate source of excita- 

 tion. 



It is important to realize that rhodopsin is one of 

 the principal structural components of a rod. It 

 accounts for about 40 per cent of the dry weight of 



