244 J- TIBB s 



some extent and rather ineffectively, by calcium. Otherwise the fac- 

 tors controlling the change are very specific. Moreover, the produc- 

 tion of the effect in the presence of ATP follows closely those condi- 

 tions governing the activation of the enzyme and there is little doubt 

 that the optical density reduction and ATP splitting are intimately 

 connected. 



Presumably the optical density reduction is the result of a lowering 

 in the refractive index of the sperm tails which take up water and 

 swell. This is the interpretation given to this kind of result in the 

 case of mitochondria by Cleland (1952) and would appear to be the 

 only possible one in this case also. A direct confirmation of this could 

 probably be obtained by comparing the wet weights of flagella ob- 

 tained, on the one hand, from suspensions which have not been al- 

 lowed to split ATP and, on the other, from suspensions which have 

 been allowed to do so. This aspect of the matter is one with which the 

 author is concerned at the present time. 



The complete dependence of the process on ATP splitting makes 

 it most unlikely that the change arises from a hemolysis-like leakage 

 or from fragmentation of the tails. The effect could not arise as the 

 result of the adsorption, with no volume change, of the substrate on 

 the enzyme since, if this had any effect on the refractive index at all, 

 it seems more likely that it would increase it. Moreover, overnight 

 storage resulted in preparations which could still be activated en- 

 zymically and which would therefore still adsorb substrate and show 

 any refractive index changes associated with this. After such storage 

 however, these preparations did not show the optical density effect. 

 This agrees with the swelling hypothesis and indicates that some kind 

 of structural deterioration takes place on standing. This conclusion 

 is also supported by the fact that not all freshly prepared sperm tails 

 showed this effect. Yet in all these preparations the enzyme was capa- 

 ble of normal activation. Similarly, a swelling due merely to the in- 

 troduction of water in the form of adsorbed and hydrated ATP ions 

 should be latent in all preparations as long as the enzyme is capable 

 of normal activation. Thus the swelling does not appear to be an 

 artifact of ATP splitting but to have some special biological signifi- 

 cance. 



When the enzyme was inhibited in sperm tail preparations which 

 had just finished swelling as the result of ATP splitting, no reversal of 



