218 THE BIOLOGY OF HYDRA : 1961 



a half-maximum response. For the g\utathione-H ydm system this 

 value, ca. 10"^ M, probably closely approximates a true dissociation 

 constant because of the apparent absence of glutathione metabolic 

 products. A rough mass law treatment using the method of Scatch- 

 ard (21) indicates that this constant can be measured within a 

 factor of 2. The significance of this constant is threefold: First, its 

 smallness indicates that the receptor has a high affinity for gluta- 

 thione. Second the value of 10~^ M is within the physiologically 

 active range expected to occur under natural conditions of feeding. 

 And, third, this number provides a means of characterizing the 

 receptor; that is, the glutathione receptor can be said to have a 

 constant of 10~^ M. This constant has been found to be a charac- 

 teristic of the receptor and to be nearly the same no matter what 

 the nutritional state of the Hydra. For example, Figure 3 demon- 

 strates that Hydra starved for two days respond to higher concen- 

 trations of glutathione for a greater period of time than do Hydra 

 starved for one day (lower curve). Nonetheless, the concentration 

 of glutathione eliciting a half-maximal response on both sets of 

 Hydra was 10~^ M. 



The difference in the maximum response observed in Hydra 

 starved one or two days ( Fig. 3 ) become understandable if another 

 comparison is made with enzyme systems. Just as the maximum 

 activity of an enzyme reaction is dependent on the quantity of 

 enzyme present and is not a specific property of the enzyme, in a 

 similar manner the duration of the reflex at concentrations eliciting 

 the maximum response is dependent upon the quantity of com- 

 pleted^ receptor-effector systems of the Hydra. The maximum 

 response is not an intrinsic property of the receptor or of Hydra 

 as is the Km. Thus, Hydra starved for one day are interpreted to 

 have fewer completed receptor-effector systems than Hydra starved 

 for two days. 



As emphasized in the above comparison, just as the total enzyme 

 activity at saturating concentrations of substrate is proportional to 

 the amount of enzyme, so the total maximum response of Hydra to 



lA completed receptor-effector system is defined as one containing all of the com- 

 ponents necessary for it to function when in combination with glutathione. When 

 all the receptor-effector systems are completed, the Hydra is capable of carrying 

 out a maximum response. 



