44 D. M. NEEDHAM VOL. 4 (1950) 



of being an active process. For example, during onset of fatigue, it is the relaxation 

 phase which becomes slowed rather than the contraction phase. This would suggest an 

 answer in the affirmative to the last question. 



THE EFFECT OF ATP ON MYOSIN SOLS 



The experiments, during 1941 and 1942 of J. Needham and his collaborators in 

 Cambridge^" and of Szent-Gyorgyi and his collaborators in Szeged^^ showed the highly 

 specific reversible effect of ATP in diminishing the double refraction of flow and the vis- 

 cosity of solutions of myosin (prepared in the classical way) in 0.5 M KCl. The decrease 

 in the length to breadth ratio of the micelles thus indicated was traced by the Szent- 

 Gyorgyi school to the splitting of actomyosin; and the isolation of the new muscle 

 protein, actin, by Straub^^ followed. 



In a recent publication, Jordan and Oster^^ have described experiments on the 

 light-scattering properties of solutions of classical myosin in 0.5 M KCl before and after 

 addition of ATP ; they interpret their data on the change in ratio of forward to backward 

 scattering as showing an increase in coiling of the protein particles, these being present, 

 before ATP addition, in the form of slightly coiled rods. 



The validity of this interpretation depends upon the presence of the actomyosin 

 in the solution in the form of discrete rods and not in the form of the branching network 

 to be seen in electron micrographs. It is very possible that the dilute solution used did 

 contain rod-shaped particles especially as it had been subjected to ultra centrifugation, 

 which might be expected to carry down the network. 



An increased coiling of such actomyosin particles (or of myosin particles formed 

 from them) under the influence of ATP would obviously be of importance in consider- 

 ations of muscle contraction and further work along these lines, including observations 

 on pure myosin (myosin A), will be of much interest. 



It is a matter too, for future experiment to decide whether evidence for the in- 

 creased coiling of the rods (after they are set free from the network) can be obtained 

 from electron micrographs. So far attention has been concentrated on the behaviour 

 of the network with ATP and the appearance of the resulting debris has not been closely 

 studied. 



Another step forward in our knowledge of the interaction of myosin, actin and ATP 

 was gained by the observations of Bailey and Perry^^ on the effect of -SH reagents. 

 They showed a close correlation between the effect of reagents which oxidize or combine 

 with -SH groups in inhibiting ATPase activity of myosin on the one hand and its power 

 to combine with altin on the other. Thus certain -SH groups of myosin are necessary 

 for its combination with ATP (and this is in line with much other information about 

 enzymes concerned with ATP). These same -SH groups are necessary for combination 

 of myosin with actin, and if ATP is added to actomyosin it displaces actin from these 

 groups and itself combines. These results are important, not only in throwing light on 

 the mechanism of the dissociation effect. The earlier experiments of the Needham and 

 Szent-Gyorgyi groups had indicated an interaction between ATP and the protein 

 responsible for the double refraction of flow and the high viscosity ; that is to say, they 

 made it unlikely that ATPase activity of the myosin preparations was to be put down 

 to presence of small amounts of some other protein. This line of argument is strongly 

 re-inforced by the work of Bailey and Perry which forms the best evidence so far 

 References p. 4g. 



