Damage of the Heart Muscle Cell 89 



the body are the oxidative enzymes and coenzymes, e.g. cytochrome 

 c, cytochrome oxidase and succinic dehydrogenase, so concentrated 

 as in the heart. Finally, energy production and utilisation are more 

 closely adjusted here than in other muscles which means that the 

 myocardium does not need to carry big stores of creatine phosphate 

 as a provision for reserve energy (Lipmann, 1941) nor does it run 

 into oxygen debt when carrying a normal work load. 



The fuels that we have mentioned are broken down to a com- 

 mon intermediate, acetyl-co-enzyme A; the acetyl group then con- 

 denses with oxalacetate to yield citrate, and the familiar tricarbo- 

 xylic acid (Krebs) cycle (Fig. 6) is embarked upon to yield 8 H 

 atoms or electrons (H = H + -J- e) . These represent the energy 

 content of the acetyl fragments and are equivalent to the 4 H atoms 

 associated with the acetyl group together with 4 H atoms derived 

 from water that is added to cycle intermediates in the course of 

 the oxidation of the fragments. Each oxidative step of the cycle is 

 catalysed by glycolytic, lipolytic and citric acid cycle enzymes, many 

 of which reside within the numerous sarcosomes or mitochondria 

 of the heart muscle cells. With each step, too, there is esterification 

 of inorganic phosphate, leading to the creation of high energy 

 phosphate bonds. In this way the bond energy of fuel brought to the 

 myocardial cells is converted into electrons for transport to oxygen. 



Energy Conservation 



Energy set free during these various oxidative processes is con- 

 served through the mechanism of oxidative phosphorylation which 

 has already been considered in Chapter 5. There the business of 

 electron flow along the H transport chain is described. In this way 

 phosphorylation of adenosine diphosphate (ADP) is coupled to the 

 oxidation of the cytochrome enzymes. Creatine phosphate stores, 

 although relatively small in the heart, also provide some high energy 

 phosphate capacity by virtue of the enzyme transphosphorylase. 

 In either case, ATP is the great energy transmitter. The mito- 

 chondria are important sites for these processes. Cardiac mito- 

 chondria are less susceptible to inhibitors than are mitochondria 

 from other organs, but when damaged they seriously disturb the 

 functioning of the muscle cells. 



