52 



W. F. H. M. MOMMAERTS 



VOL. 4 (1950) 



its extent can be quantitatively established by determining the volume of the gel pellet 

 after centrifugation (Mommaerts^^). One can thus study contraction at various levels of 

 subcellular and supermolecular organization. 



A still simpler system is a solution of actomyosin in 0.5 M KCl. As Szent-Gyorgyi 

 has discribed^^' ^^, the high viscosity of such a solution is greatly decreased by ATP. 

 The analysis of this effect has shown that it is not due to a contraction of dissolved 

 actomyosin micells^-' '^' ^^' ^^. The true reason, as is well established now, is a disaggre- 

 gation of the actomyosin into its components, myosin and actin. Although the immediate 

 connection between this disaggregation and the contraction at lower ionic strengths is 

 not clear, it may be presumed that the first effect of ATP is identical in both cases. One 

 of the aspects of this first effect apparently is an elimination of certain intermolecular 

 bonds. In the case of dissolved actomyosin, which is on the verge of disaggregation, the 

 complex falls apart. At low salt concentration, where more or other bonds may exist, 

 this dissociation cannot reveal itself, but the contraction can. It appears unlikely that 

 in solutions of actomyosin contraction takes place side by side with the disaggregation. 

 For it is an empirical fact (Szent-Gyorgyi, I.e. ; Erdos^^) that without actin myosin 

 cannot contract ; in 0.5 M KCl solution, ATP separates the actin and myosin so that no 

 contractile complex then exists. Although the relation between the two effects is not 

 understood, the study of the disaggregation in solution is highly useful, for it enables 

 a great variety of experiments to be performed which would not be possible in strongly 

 heterogeneous systems. As a result of the study of this viscosity effect, mainly three 

 h conclusions seem possible: 



3.0i 1 1 — r 1 1 i 1 First, the effect is fast. It appa- 



rently takes a fraction of a second 

 to reach completion. Methods for 

 the exact study of its time course 

 have not yet been available. 



2.o\ 1 1 i \ 1 \ I The second conclusion needs 



more elaborate explanation^^. Fig. i 

 shows a few examples of the visco- 

 simetric measurement of the effect 

 of ATP upon an actomyosin solu- 



i.o\ 1 -/ — \ jf I y^ I I I i tion. It will be seen that, after the 



initial viscosity response a recovery 

 effect sets in, which takes more time 

 the more ATP had been added. It 

 is inhibited by Mg+ and activated 

 by Ca+-ions, and is to be identified 

 with the removal of the ATP by 

 the ATPase associated (Polis and 

 Meyerhof^") with the myosin. The 

 viscosity response itself is not inhi- 

 bited by Mg+ and activated by Ca+ 

 (rather the opposite) and can also 

 take place if no hydrolysis occurs. 

 Hence the second conclusion: the 

 effect of ATP upon the aggregation of actomyosin is not caused by any known breakdown 

 References p. sOj^y. 



20' 30 



Time 



Fig. I. Effect of ATP upon the viscosity of an acto- 

 myosin solution. At zero time, ATP is added. In all 

 experiments, 2.5 mg actomyosin were present per ml, 

 dissolved in 0.5 molar KCl at neutral reaction. Curve i 

 (/\) refers to an experiment in the presence of o.ooi 

 molar CaClg, curve 5 (n) to an experiment with 0.00 1 

 molar MgClj. The amount of ATP added was 25-10-* 

 moles in the experiments i, 2 and 5; 50-10-* in 3; 

 200 • 10—* in 4 (see text). 



