HOWARD M. LENHOFF 221 



dure of Boyer (2), parallel experiments were run in which the 

 respective solutions were assayed for sulfhydryl groups after the 

 Hydra had closed their mouths. No perceptible decrease in the 

 sulfhydryl content of the solution occurred. 



Similar experiments were carried out using the glutathione ana- 

 logs ophthalmic acid and S-methyl glutathione, compounds that 

 activate the feeding reflex and are not auto-oxidizable. As shown 

 in Table 6, these analogs like glutathione, retain much of their activ- 

 ity after several exposures to Hydra. It can be concluded from all 

 the experiments summarized in Table 6 that the feeding reflex nor- 

 mally ends as a result of some other cause than the oxidation, disap- 

 pearance, or alteration of the glutathione molecule; also it does not 

 end because of the accumulation of inhibitors in the culture solution. 



Further examination of Table 6, however, does indicate some 

 shortening of tf after using the same glutathione solution on three 

 successive groups of Hydra. Thus, it appears that either the gluta- 

 thione concentration was in some manner slightly lowered, or that 

 some inhibitory factor gradually accumulated in the environment. 



It is not necessary to assume that the glutathione or glutathione 

 analogs are altered or destroyed when combining with the receptor. 

 There are known instances in which a biological response is initi- 

 ated by a molecule (non-coenzymic in function) combining with 

 a specific site without being metabolized. For example, thiogalacto- 

 side induces the adaptive fonnation of the enzyme /?-galactosidase 

 without being hydrolyzed ( 19 ) . 



Thus, from the data in Table 6, we might postulate as one result 

 of glutathione activation, the consumption of some substance in 

 the receptor-effector system, the concentration of which limits the 

 duration of the feeding reflex to 25-35 minutes. If this postulate is 

 true, then one might expect that after the Hydra have carried out 

 a maximum response, there will be a period during which they 

 give no further response to a fresh solution of glutathione. Secondly, 

 there will be another period in which they regain their ability to 

 respond maximally. This proved to be the case as shown in Figure 

 4. In this experiment large numbers of Hydra were exposed to gluta- 

 thione for forty minutes. The animals were then washed with and 

 placed into the glutathione-free culture solution, and, at intervals, 

 exposed to a fresh solution of glutathione. The results show that 



