42 BIOCHIMICA ET BIOPHYSICA ACTA VOL. 4 (1950) 



^lYOSIN AND ADENOSINETRIPHOSPHATE IN RELATION TO MUSCLE 



CONTRACTION 



by 



D. M. NEEDHAM 

 Biochemical Laboratory, Cambridge [England) 



The conception of energy provision by the spHtting off of the terminal phosphate 

 group of ATP, under the influence of myosin or actomyosin acting as ATPase, is central 

 in current hypotheses of muscle contraction. Indeed, in many aspects of metabolism 

 we find evidence that ATP serves as a readily expended store of energy and that much 

 of the free energy of oxidation and glycolysis goes to its resynthesis. In these circum- 

 stances, it is strange to reflect that we are still without accurate knowledge of the 

 amount of free energy available in this reaction ; we do know, however, that it is sur- 

 prisingly small, only of the order of 12000 g cals per g/mol H3PO4 set free. Still more 

 surprising is the small difference in free energy content (only about 6000-8000 g cals) 

 which separates the "energy-rich" phosphate bonds from the "energy-poor" phosphate 

 bonds. It is probably because of its ability to deal in these small stages of energy transfer 

 that the living cell achieves its high efficiency. Thus even normal aerobic contraction is 

 about 20% efficient when tension production or work performance is compared with 

 heat production; and anaerobic contraction about 40% efficient. The anaerobic recovery 

 phase (when creatinephosphate formation is going on at the expense of carbohydrate 

 breakdown to lactic acid) is over 90% efficient: there is little heat production during 

 this period and the formation of the energy-rich phosphate bonds goes on with scarcely 

 any waste in the form of heat. We shall return to this point later. 



The fact that no breakdown of ATP has been demonstrated in normal contraction, 

 but only becomes observable in fargoing fatigue, has recently been emphasized by 

 A. V. HiLL^. By the use of the new micro-methods, for example those of Kalckar^, 

 it should now be possible to estimate ADP in amounts of the order to be expected during 

 a single twitch or a very short series of twitches. Although rephosphorylation by means 

 of creatine phosphate probably follows with great rapidity, by using slow-moving muscle 

 at low temperature it might thus be possible to detect a period of ATP breakdown 

 unobscured or only partly obscured by resynthesis. 



ATPase activity in vivo and in vitro 



The close connection of ATP breakdown with energy provision for contraction once 

 conceded, two very important questions arise — the exact conditions of the ATPase 

 activity and its timing. 



That myosin can act as ATPase is wellknown^ but, as Bailey has shown*, the 

 optimal conditions for the activity of myosin prepared in the classical manner and 

 containing little actomyosin, are not those to be expected within the muscle fibre. The 

 References p. 4g. 



