234 J- TIBBS 



phase is a more passive one of relaxation in which the cell regains its 

 original form. This relaxation phase is also characterized by a low rate 

 of ATP splitting. In the case of the sperm tail during motility no 

 such ready division into, or observation of, two such phases of activ- 

 ity is possible. If such phases exist, some components must be con- 

 tracting and others relaxing at the same time. Moreover, a particular 

 point on the tail must be involved in a continuous cycle of contraction 

 and relaxation. 



A high rate of ATP hydrolysis is characteristic of the contraction 

 process in muscle but Marsh (1952) pointed out that, in contrast with 

 this, relaxation is accompanied by a low rate of ATP splitting. Indeed, 

 it seems that enzyme inhibition and a resulting low rate of hydrolysis 

 is a prerequisite for relaxation. High concentrations of ATP relax 

 muscle and have been shown to inhibit the enzyme when these con- 

 centrations exceed those of the magnesium activator (Perry and Grey, 

 1956). In vivo the inhibition is brought about through the agency of 

 a factor discovered by Marsh (1952) and widely known as the "Marsh 

 factor" or "relaxing factor." The mode of action of the factor is un- 

 certain but it seems likely that it acts by chelating some essential metal 

 activator (Baird and Perry, 1960), whereas inhibition by excess sub- 

 strate may be explained by the substrate playing a similar role. 



Sperm tails and protozoan flagella which have a similar morphology 

 possess magnesium-activated adenosine triphosphatase (ATPase) ac- 

 tivity (Nelson, 1954; Tibbs, 1957); estimates of the amount of energy 

 needed to propel the cell through water show that the measured 

 rates of ATP breakdown by flagella in vitro are quite high enough to 

 provide the energy needed for propulsion (Tibbs, 1957; Nelson, 

 1958). It is then relevant to ask if the movements of the flagellum are 

 controlled and governed by the activation and inhibition of the ATP- 

 ase. Or, to put the question in terms which can be investigated ex- 

 perimentally, will excess ATP cause enzyme inhibition and is there 

 any evidence to suggest the presence of a factor in sperm which may 

 inhibit the ATPase of the tail? 



Some information is available on these questions. When the influ- 

 ence of substrate to activator ratio was investigated on sperm tads 

 obtained from the common perch (Perca fluviatilis) results such as 

 those shown in Fig. 1 were obtained (Tibbs, 1959). It is apparent that 

 although enzyme activity decreases with increasing substrate concen- 



