The Nature of the Fibril

K. Bailey
1950 Proceedings of the Royal Society of London. Biological Sciences  
70 A. V. Hill and others changes, that there exists a real protein metabolism which must have its reper cussion on many physical properties of the muscle, not only on its elastic and optical properties, but also on impedance changes, pH changes, heat production, double refraction, etc. Before we can correlate physical and chemical events with success, we should know not only the properties of the muscle proteins in solution, the investigation of which has given so many interesting facts by the
more » ... sting facts by the hands of Weber, Edsall, Needham, Bailey, Szent-Gyorgyi and others, but chiefly the changes of the linkages of the proteins in the muscle machine itself during its activity, and there is very hard work to do in this field. One of the chief problems in muscle biochemistry at the present time is to elucidate the way in which the component proteins of the muscle fibril are assembled to give a contractile element. The myosin-actin-ATP system is so unusual th at it is generally thought to contain the essential components for contraction, but it is true to say that in vitro studies of this system are merely suggestiv generally led to speculations on the mechanism of contraction which are either dubious or not amenable to experimental proof. The first difficulty is th at in the molecular sense the nature of the interaction of myosin and actin is not clear. Under the conditions of salt concentration at which actin and myosin separately are dispersed in solution, the effect of ATP on actomyosin is to produce a large decrease in viscosity. This has been interpreted as a dissolution of the links which bind actin and myosin together, liberating the component molecules from an anastomosed network of fibres. More specifically, it seems due to the competition of ATP for certain groupings which not only fink up with actin, but are responsible for the adenosine-triphosphatase activity of myosin. In salt-poor medium, where myosin by itself is no longer soluble, the effect of ATP is to produce an intense synaeresis, which superficially is suggestive of 'contraction'. There is no reason to suppose that the action of ATP in this case is at all different from th at in a salt-rich medium; the breaking of the actin-myosin links is followed, however, by a further interaction which is reminiscent of the synaeresis of a fibrin clot. Jordan & Oster (1948) have concluded from light-scattering studies that, even at high ionic strength, ATP causes not a dissociation of actin and myosin, but an increased coiling of the actomyosin particle. There is no doubt, however, th at the mechanical properties of actomyosin fibres in the presence of ATP are more in keeping with the original explanation th at linkages responsible for anastomosis are broken down. W hat bearing the synaeresis phenomenon has upon the mechanism of contraction is not very clear, since the changes involved are inter-and not intramolecular. I t is possible th at the interaction reflects the means by which these water-holding proteins are tightly packed in the fibril. These comments serve to show th at we are not yet in a position to reconstruct the muscle fibril. Other difficulties arise in considering changes which occur in the
doi:10.1098/rspb.1950.0019 fatcat:slzafs7pcvafhl36etsk5h6tce