226 THE BIOLOGY OF HYDRA : 1961 



somewhere between the receptor and before the reaction involving 

 the consumption of X. 



The activation by proteases has also been useful in determining 

 the relative site at which calcium functions. Since the presence of 

 environmental calcium ion is required for the activation of both 

 glutathione (10) and proteases (9), we feel that calcium plays a 

 role in the effector system rather than in the combination of gluta- 

 thione with the receptor. 



A recent development which places added importance upon the 

 activation of the feeding reflex by f)roteases is the discovery by 

 Fulton (6) that proteases also activate the feeding reflex in Cordy- 

 lophora. His results are striking in that he has also shown that 

 Cordylophora do not carry out the feeding reflex in response to the 

 peptide reduced glutathione, but rather to the single imino acid pro- 

 line (5). Thus, although Hydra and Cordylophora have different 

 specific excitatory compounds, the feeding reflex in both animals can 

 be activated by proteases. In addition, Physalia gastroozoids, which 

 normally respond to glutathione ( 15 ) , also are activated by i)ro- 

 teases ( 12 ) . All of these results suggest that the protease is acting 

 on some step which is common to the feeding reflex of all these 

 organisms irrespective of the excitatory compound involved. 



SUMMARY AND CONCLUSIONS 



With the aid of the simplified scheme shown in Figure 5, I 

 would like to summarize the present state of knowledge concerning 

 the mechanism by which glutathione combines with and activates 

 the glutathione-receptor of Hydra to elicit the feeding reflex. The 

 activity of the glutathione resides in the size and configuration of the 

 7-glutamylalanylglycine backbone of the tripeptide, and not in the 

 reducing properties of the molecule ( 3, 13, 17 ) . Concentrations of 

 glutathione greater than 5 X 10 ~"' M activate all of the receptor- 

 effector systems (Fig. 3), which are probably localized in the area 

 immediately around the mouth and on the tentacles (8). The con- 

 centration of glutathione eliciting a half maximum response is 10~*^ 

 M (Fig. 3). In order for a response to take place, the glutathione 

 must be constantly present at the receptor site (Table 5). The associ- 



