352 Molecular Basis of Vision /1 9 : 2 



none indicate less than three. In spite of this, only two photosensitive 

 pigments have been isolated from human retinas. One of these, 

 rhodopsin, is found in the rods. It has been studied in detail; its struc- 

 ture and action form the basis for the next section (and indeed most of 

 this chapter). Other mammalian pigments active in vision are intro- 

 duced in Section 3 of this chapter. 



The molecular basis for mammalian vision is far from being com- 

 pletely understood; this contrasts sharply with the knowledge of the 

 geometrical optics of the eye. Invertebrate vision has been even less 

 well studied on a molecular level. Although it is evident that many 

 insects possess highly developed color senses which extend far into the 

 ultraviolet, few of the photopigments responsible for insect vision have 

 been isolated. 



As stated earlier, molecular biophysics is regarded by some bio- 

 physicists as the most fundamental part of biophysics. From this point 

 of view, the most significant aspects of the biophysics of vision are the 

 least well understood. 



2. Rhodopsin 



It had been known for many years that pigments which might be 

 associated with vision existed in the retina. In 1876, Boll named a 

 pigment "Sehrot" which he saw in frog retinas. It was described as a 

 brilliant red pigment. In 1878, Kuhne observed the bleaching of a 

 pigment, rhodopsin, which he extracted with bile salts. Rhodopsin has 

 also been called visual purple because of its characteristic color. It is 

 by far the most studied of the photosensitive pigments. 



Today, rhodopsin has been purified and studied in many laboratories. 

 In order to extract it, fresh, dark-adapted retinas are mashed in a dim 

 red light. The mashed retinas are then subjected to differential centri- 

 fugation until a fairly pure suspension of rods is obtained. The rods are 

 hardened with alum, which makes most proteins insoluble. Then the 

 hardened rods are extracted exhaustively with buffers to remove all 

 water-soluble material, after which they are dried. The rods are next 

 extracted exhaustively with petroleum ether to remove all fat-soluble 

 substances. This leaves insoluble particles which can be suspended 

 only with suitable detergents. The particles containing the pigment 

 rhodopsin are suspended in a 2 per cent solution of digitonin, or in bile 

 salts. The entire purification must be carried out in a deep red light in 

 order to have an appreciable yield of rhodopsin. 



Is the rhodopsin one ends up with anything like the original ? Tests 

 show that it has almost the same absorption spectrum, although at 



