VOL. 4 (1950) 



PRIMARY REACTION IN MUSCULAR ACTIVITY 



53 



50 



5 ?0- 



10 ■ 10-^ 



of the ATP. In an attempt to specify the nature of this primary reaction between ATP 

 and actomyosin, the quantitative relation between the amount of xA.TP added and the 

 magnitude of the physical effect 

 has been studied^^' ^^. Because of* 

 particular experimental dithcul-| 

 ties, the results have not yet been^ 

 satisfactory, but an example as 

 that of Fig. 2 shows that one has 

 to assume the formation of a spa- 

 ringly dissociated compound be- 

 tween ATP and (acto)- myosin. 

 Further quantitative researches 

 are in progress. The relationships 

 depend on whether Mg+ or Ca+ are 

 present and the best result in the 

 presence of the promoting Mg+ 

 showed that i mole of ATP causes 

 the maximal change in as mxuch as 

 300000 grams of myosin. The dot- 

 ted line (Fig. 2) represents what 

 would be expected if the ATP- 

 actomyosin complex would be 

 completely undissociated; the de- 

 viation of this from the actual curve 

 is possibly still less than is indi- 

 cated by the results, which are 

 obtained by difficult measurements 

 in a rapidly changing system. The 

 third conclusion reads therefore: 

 the effect of ATP upon a measured 



physical property of actomyosin is due to the formation of a sparingly dissociated 

 ATP-(acto-) myosin complex. One is led to a similar conclusion by studies of the same 

 combination in heterogeneous, contractile actomyosinsystems, but I had insufficient 

 opportunity to study this in full detail. In solution, the measured effect was maximal 

 when I mole of ATP was present for 300000 gram myosin. It is possible that upon 

 addition of more ATP, more is bound and stronger physical changes are induced. This 

 cannot be measured in solution, but may possibly be found in further studies with 

 different methods. Not more can thus be stated than that 300000 gram myosin combine 

 with at least i mole of ATP, or roughly that 100 mg myosin, present in one gram of 

 muscle, combine with 3- io"~' mole ATP or more. 



Naturally, the mere demonstration that ATP, when interacting with actomyosin, 

 actually combines with it (most probably with the myosin component only), is yet no 

 explanation of the mechanism of its action. In this connection, the question arises 

 whether ATP is only bound to myosin, or whether any further reaction takes place 

 between them. More precisely the question may be asked whether myosin is phos- 

 phorylated by ATP. The author spent a summer trying to demonstrate such a phos- 

 phorylation. Actomyosin and ATP were allowed to react in a proper medium, and were 

 References p. 56J5J. 



15-10-' 



Amciini of A TF" added 



Fig. 2. Dissociation curve of the ATP-myosin complex. 

 The effect of varying quantities of ATP upon the magni- 

 tude of the viscosity drop (at 0° ; extrapolated to zero- 

 time) of actomyosin was studied. System: 20 mg 

 actomyosin in 10 ml 0.5 molar KCl, 0.02 molar MgClj. 

 Abscissa: amounts of ATP added to this system. 

 Ordinate: viscosity drop, expressed as percentage of 

 the effect obtained with a large excess of ATP. The 

 dotted line, tentatively drawn -as representing complete 

 absence of dissociation, indicates that the maximal 

 effect is reached when 5.7-10-^ moles ATP combine 

 with 20 mg actomyosin, corresponding to i mol ATP 

 per 300000 g myosin. The difference between the dotted 

 and the experimental line indicates that at half-equi- 

 librium the concentration of free ATP is much less than 

 the total ATP concentration of 3-io— ® moles per liter 

 (see text) 



