38 BIOCHIMICA ET BIOPHYSICA ACTA VOL. 4 (1950) 



ACTOMYOSIN AND MUSCULAR CONTRACTION 



by 



A. SZENT-GYORGYI* 



Marine Biological Laboratory Woods Hole, Massachusetts and 

 Laboratory of Physical Biology, Experimental Biology and Medicine Institute, Bethesda 14, 



Maryland (U.S.A.) 



It has been shown in the author's laboratory^" that two structural proteins can be 

 extracted from the muscle fibril, actin (F. B. Straub) and myosin. The two, if mixed 

 at a proper ionic concentration, unite to a complex, actomyosin, which has the remark- 

 able property of contractility. Actomyosin threads contract under influence of ATP. 

 This contraction, though imitating in many ways contraction of muscle, differs from 

 it also in several respects. Two of these differences are rather striking and led Buchthal, 

 Deutsch, Knappeis, and Petersen, as well as Astbury, Perry, Reed, and Spark 

 to the conclusion that "contraction" of actomyosin has little to do with muscular 

 contraction. According to Astbury, "contraction" of actomyosin is simply a colloidal 

 synaeresis while muscular contraction is an entirely different phenomenon. The two 

 observations, on which this conclusion was based, were the following: muscle contracts 

 anisodiametrically, becoming shorter and thicker without changing volume, while 

 "contracting" actomyosin threads become shorter and proportionately thinner, thus 

 simply shrinking. The second objection is based on Buchthal's observation: while an 

 unloaded actomyosin thread shortens in ATP, a loaded thread lengthens in the same 

 solution, thus behaving contrary to muscle which shortens whether loaded or unloaded. 



In this paper, the author, after pointing out certain analogies between the con- 

 traction of muscle and actomyosin, hopes to show that the objections raised by Buch- 

 thal and Astbury can easily be explained and do not plead for a basic dissimilaritj^ 

 of the two processes. 



If a washed fibre bundle of the musculus psoas of the rabbit is suspended in a 

 Ringer solution, containing o.ooi M Mg and 0.2% ATP, it contracts and develops the 

 same tension as the muscle developed maximally i}i vivo, showing that it was the normal 

 mechanism of contraction. which has been put into motion by ATP. This reaction is 

 very specific, and all attempts to produce it with any other substance than ATP have 

 hitherto failed. The same muscle fibre can be made to shorten also by other means, as 

 for instance, by heat. At 70° shortening may be extensive, but no appreciable tension 

 will be produced. 



If the same washed psoas muscle is suspended in water and decomposed in the 

 Waring blender into a suspension, on addition of the salts of the Ringer solution, a 

 moderate fiocculation will be observed. On addition of ATP an excessive precipitation 

 occurs which has been termed "superprecipitation". Evidently, this superprecipitation 



* Special Fellow, U. S. Public Health Service. 

 References p. 41. 



