90 The Chemistry of the Injured Cell 



Energy Utilisation 



Adenosine triphosphate (ATP) carrying the high-energy phos- 

 phate bond is directed into the contractile mechanism of the muscle 

 cell and there it gives up its energy for the performance of mechani- 

 cal work. It is still not known for certain how this happens but the 

 following account is generally accepted (Szent-Gyorgyi, 1953; 

 Bailey, 1956). 



At least two contractile proteins are present in muscle. They are 

 known as actin and myosin. In the resting state, actin and a 

 myosin-ATP complex exist independently in a dissociated form 

 separated by the electro-positive ionic environment created by the 

 surrounding K ions. Part of the myosin molecule (L-meromyosin) 

 is thin and straight and is kept from folding by repulsive forces be- 

 tween K ions attracted by ionised groups in close proximity along 

 the molecule. When a wave of excitation passes this equilibrium is 

 disturbed. Actin then combines with the myosin-ATP complex 

 to give the contractile protein actomyosin. The characteristics of the 

 myosin molecule are changed and in the presence of ATP the thin 

 L-meromyosin segment becomes more pliable. This segment col- 

 lapses, the myosin molecule is shortened and the muscle cell con- 

 tracts. ATP is converted to ADP and inorganic phosphate during 

 this reaction. For relaxation, the heart requires ATP, creatine phos- 

 phate (CP) and Mg ions. A heat-stable protein, identified as 

 myokinase, is also needed for the renewal of energy expended dur- 

 ing contraction. Szent-Gyorgyi (1956) postulates in relaxation a 

 dissociation of actin and myosin and the regeneration of ATP, 

 presumably in part at least from transfer of high-energy phosphate 

 from CP to ADP. But it is the intracellular ionic environment that 

 determines the changes, by influencing the balance of repulsive and 

 attractive forces, and the tendency of actin and myosin to part or 

 to join. The ionic environment, in turn, is regulated by factors 

 governing the permeability of the cell membranes. (The work 

 efficiency of the phase may be as high as 60 per cent with an over-all 

 work efficiency in the heart muscle of 35 to 40 per cent.) 



It is thought that actin filaments arise from either end of the 

 sarcomere or muscle unit, myosin filaments from the middle of the 



