192 C. REID 



the — SH group of the opsin were first blocked with /)-chloromercuribenzoate. 

 It seems very probable therefore that after Hght absorption, which occurs in the 

 retinene part of the molecule, there is an excited electron in the immediate vicinity 

 of an SH group or a sulphur bond to the retinene itself. On the exposure of the 

 eye to strong Hght, changes in retinal pH have been observed which may well 

 result from a shift in the equihbriimi 



RSH > RS- + H+ 



which occurs when RSH is electronically excited. It seems clear that here again 

 sulphur plays a key role at a point in the mechanism of vision very close to the 

 photoabsorption step. 



Phototropism 



The struggle towards the light must have been one of the earUest survival 

 mechanisms incorporated into living systems and phototropism is therefore 

 relevant to our considerations. 



The known 'auxins' are not sulphur-containing compounds. However, it has 

 recently been shown [6] that the auxin naphthaleneacetic acid undergoes a 

 specific reaction with the protein of an enzyme extract obtained from rapidly 

 growing pea seedlings, but that the reaction is prevented if — SH groups in the 

 enzyme are blocked. This work depends upon the interpretation of absorption 

 spectnmi changes of materials in vitro. If this intimate association between the 

 auxin and — SH-containing molecules is confirmed, it provides more evidence 

 that a general pattern of sulphur involvement in photoprocesses can be traced. 



Bioluminescence 



Only a very small proportion of Hving organisms show bioluminescence. In the 

 cases so far investigated (bacteria such as Photobacterium, Crustacea such as 

 Cypridina, and the firefly, Photinus pyralis) the processes appear completely 

 different from each other. These two facts suggest that bioluminescence is not 

 part of the common evolutionary pattern, but has arisen accidentally in various 

 quite unrelated species. If this view is correct, it is of much later origin than are 

 the processes involving light absorption. This is consistent with the fact that no 

 special role has yet been assigned to sulphur compotmds in bioluminescence 

 reactions, although in one case (firefly luciferin [7]) the presence of an — SH 

 group has been suggested. 



RADIATION DAMAGE 

 For several years it has been recognized that a number of substances can act 

 as partial 'protectors' against radiation damage. Since radiation damage is usually 

 of an oxidative nature, any added reducing material, and thus all organic matter, 

 has some protective effect. Much the most efficient protectors are sulphur 

 compounds [8]. It is of interest that free sulphur ranks with thiourea as one of 

 the most efficient protective agents known. Protection measured in terms of 

 retained activity of extracted enzymes, or of suspended living organisms show 

 essentially the same results. 



