VOL. 4 (1950) MYOSIN, ATP, AND MUSCLE CONTRACTION 45 



for the ATPase activity of myosin itself. The knowledge gained from the study of inter- 

 action between ATP and myosin sols must clearly play a useful part in our progress 

 towards understanding of muscle contraction. But it does not seem that any deductions 

 having a direct bearing on the question we have raised can be drawn from it at the 

 moment. Certainly a deduction recently made from the results of Dainty et al.'^^ by 

 MoRALES^^ that "the catalytic activity of ATPase, that is of acto-myosin, rises exponen- 

 tially with disorientation of the protein" is not justified. 



THE EFFECT OF ATP ON MYOSIN THREADS 



The similarity in rod and intrinsic birefringence and in the X-ray diagram between 

 artifical myosin threads and muscle fibres led to hope that important progress might 

 be made by study of the effect of ATP upon such threads ; especially since it was found 

 that they still retain ATPase activity and could withstand a certain amount of tension 

 (Engelhardt^'^) without breaking. 



Engelhardt used threads made from classical myosin and containing about 2% 

 protein. Subjected to loads of about 200 mg such threads show a reversible extension. 

 If the threads are tested, immersed not in KCl solution but in 0.005 ^I ATP, this exten- 

 sibility is increased by 50-100%. 



This effect of rise in extensibility with loaded threads is in contrast to the striking 

 shortening effect obtained by Szent-Gyorgyi^^ with unloaded actomyosin threads 

 (myosin B), suspended in dilute (0.05 M) KCl. Addition of ATP (0.002 M) led to isodi- 

 mensional contraction, with shortening up to 66%. This shrinkage of the actomyosin 

 thread is accompanied by great loss of water, the percentage falling from about 97 to 50. 



The observations of Buchthal et al}^ form a link between these two sets of obser- 

 vations. Using actomyosin threads (which had been dried to a N content of 16.15% and 

 then allowed to imbibe water for 30 minutes from 0.9% NaCl solution) they found that 

 addition of 0.002 M ATP caused a 30% shortening of the unloaded thread; while with 

 a load of no mg there was an increase in length of 30%. Even so small a load as 5 mg 

 caused a slight lengthening. 



Perry et alP have contributed an instructive electron microscope and X-ray study 

 of the synaeretic effect of ATP on actomyosin gel in 0.05% KCl. The photographs of 

 the control gel show a dense tangled network. After ATP addition, the network has 

 opened out; it would appear that small linear fibres are first formed (as might be ex- 

 pected on a splitting to actin and myosin) and that these aggregate side by side to form 

 denser fibres. The X-ray diagrams from the same material show no ^fundamental differ- 

 ence between actomyosin before and after synaeresis. These observEftions were taken to 

 indicate that there is no increased intramolecular folding with intramolecular synaeresis, 

 but rather that the water loss is intermolecular accompanied by lateral aggregation. 



When all these facts are considered together it seems that the discrepancy originally 

 felt between the results of Engelhardt and of Szent-Gyorgyi disappears. The effect 

 of the x\TP in both cases is to cause breakdown of the actomyosin network followed by 

 aggregation of the particles and squeezing out of water. When the thread is loaded, the 

 fall in elasticity consequent upon the disappearance of the network is the obvious 

 aspect; when the thread is unloaded, this aspect is not noticeable but the shortening 

 due to S5maeresis can manifest itself. 



Buchthal et al}^ have reported that treatment of fresh actomyosin threads (3% 

 References p. 4g. 



