54 W. F. H. M. MOMMAERTS VOL. 4 (1950) 



then separated by centrifugation. Considerable quantities of P were found in the pre- 

 cipitate. Eventually it was found out, however, that the apparent phosphorylation was 

 proportional to the amount of calcium in the system, and what appeared to be a phos- 

 phorylation turned out to be nothing else than a coprecipitation of actomyosin, Ca+ and 

 inorganic phosphate, the latter being formed by enzymatic splitting of the ATP*. It is 

 true that without Ca+ very small amounts of P were found in the sediment, but those 

 were neglected at that time. 



Meanwhile, however, Buchthal, Deutsch et al.^ conducted their study of just this 

 small effect. They find amounts of about or above 15 fi gram P per 100 mg myosin 

 (Professor Buchthal kindly provided me with additional data not given in the prelimi- 

 nary paper), which would correspond to 5-10"'' mole or more of P transferred to 100 mg 

 myosin (i gram muscle). This is the same order of magnitude as that of the combination 

 between ATP and myosin. Indeed, Buchthal, Deutsch et al. also measured an uptake 

 of nucleotide. It thus seems likely that the primary reaction between ATP and myosin 

 does not remain restricted to a mere combination, but is followed by more intricate 

 interactions as well. 



In spite of the insulBcient information available, some further quantitative aspects 

 of the (acto-) myosin- ATP dissociation curve just referred to may be discussed. We 

 indicate the molar concentrations of the myosin (taking the relative weight of the unit 

 combining with one ATP), the complex, and the ATP with 0^, Cma and c^. From viscosity 

 measurements, as described above, it would be possible to determine the value of K, 

 most easily by measuring the c^ at which half the maximal viscosity response is obtained 

 (for Cma = ^u> K = c^~^). This problem is now under investigation, but previously no 

 values for c^ have been obtained due to experimental difficulties. Naturally, only the 

 concentration of free ATP is relevant here; Engelhardt^° (page i8g), who attempted 

 to calculate an equilibrium constant from my earlier measurements^^ erroneously took 

 the total ATP amount present in the system. If the total ATP concentration is below 

 io~^, (see Fig. 2) c^ is very much smaller, perhaps around io~^. Thus, K will be of the 

 order of 10'' or more, and the value of RT In K will be in the range of 10 000 calories, 

 a very considerable free energy effect. 



There is an independent way of estimating the quantitative relationships between 

 ATP and myosin in a single elementary contractile event. As is well known (comp. 

 Lundsgaard, I.e.), in iodoacetate poisoning, where the muscle uses up its stores of 

 '^ P, some seventy contractions are possible. Such a muscle, before beginning its activity, 

 contains some 2.5-10"^ moles of . — - P per gram, counting only the terminal P of the 

 ATP. One can look upon every twitch as one elementary event involving a fraction of 

 this --^^ P in the form of ATP, which first combines with myosin, and is thereupon decom- 

 posed. For simplicity of argument, it will be assumed that the poisoned muscle performs 

 some 50 full, rather than 70 decreasing twitches. Since 2.5-10"^ moles z^- P enable to 

 50 full twitches, one elementary event involves the reaction of 5-io~' moles of • — ' P 

 with the contractile structure, followed by direct or indirect degradation into inorganic 

 phosphate. Since this same amount of muscle contains nearly 100 mg myosin, it is found 

 that in every complete elementary process i mole of '--' P reacts with 200000 gram 

 myosin. This value is so close to the proportion of i ATP to 1-3 hundred thousands 

 myosin which I regularly found in vitro that it would be hard to consider it as a mere 

 coincidence. 



* The critical attitude of Dr. Gerhard Schmidt is gratefully acknowledged. 

 References p. 56J57. 



