740 LIGHT AND LIFE 



This is what happens when pepsinogen is converted to pepsin, trypsino- 

 gen to trypsin, chymotrypsinogen to chymotrypsin. In each case the 

 catalytic center on the zymogen molecule is covered by some small 

 fragment of its structure. When this is removed, the enzyme can go 

 to work. 



One hardly need stress the close superficial resemblance between 

 such a process and what we believe to be the mechanism of bleach- 

 ing of a visual pigment (see Fig. 4) . The intact pigment is "corked" 

 by the close attachment of the retinene chromophore to opsin. The 

 absorption of a photon, by opening up this attachment, may expose 

 a catalytic center. That is, the visual pigments may in effect be 

 zymogens, the opsins active enzymes. 



If opsin were indeed an enzyme, what process may it catalyze? If 

 we only knew what we would like to achieve in this regard, we 

 might begin to look for it. Unfortunately, we do not yet know what 

 is needed for an excitation, and hence where to begin to look. 



Yet if either of these suggestions is relevant — whether the puncture 

 of a membrane or the activation of an enzyme — we can be stire that 

 either effect will persist for a time. The puncture will stay open, 

 the enzyme will have a little time in which to act. For the retinene 

 isomerized to ?i\\-trans by light must be re-isomerized back to the 

 W-cis configuration before it can recombine with opsin, bringing the 

 action to an end. This may be the point of the isomerization cycle 

 in vision. 



Photopigment Conceritration and J'isudl Seusitixnty 



The processes that begin with the action of light on a visual pig- 

 ment end in visual sensations. We do not try in general to measure 

 visual sensations; what we usually do instead is measure the light, 

 incident on the eye, needed to evoke a more or less constant visual 

 response. Usually this is a threshold response of one kind or another, 

 and the energy required to stimulate it is the energy threshold. 

 The reciprocal of this quantity is taken as the measure of visual 

 sensitivity. 



Parallel with this external measure of sensitivity must go some in- 

 ternal measine, that characterizes the capacity of the visual system 

 to resj)ond to the stinudus. This is of course the independent vari- 

 able; tlie amount of light required to see merely measures its status. 



The visual pathways are sufficiently complex so that it might seem 

 reasonable to suppose that endless ])ossibilities exist for varying its 

 sensitivity. To a degree, of course, this is so. Normally, however, the 



