ENERGY 511 



enough to allow the electrostatic attraction suggested. Contraction is 

 accompanied by liberation of potassium ions which then can be 

 dialyzed out. In addition, ATP is split but whether before or after 

 the contraction is not yet known. The cleavage of ATP is attributed 

 tentatively to the myosin portion of the protein complex, since myosin 

 is an active phosphatase for this substrate. Some workers view the 

 cleavage as following contraction and supplying energy to charge tlie 

 system preparatory to another contraction. Others feel that ATP is 

 split just before contraction, and the resulting energy causes the 

 changes in the distribution of ions followed by contraction of the 

 protein complex. In any case, the energy is efficiently transferred. 



Because the concentration of ATP in muscle is low at any given 

 time and even the reserve of creatine phosphate is limited, prolonged 

 muscular activity depends upon metabolism to provide a continuing 

 supply of energy. Oxygen is transported by the hemoglobin of the 

 blood and transferred to a related protein in muscle called myoglobin. 

 Then a demand for ATP leads to the metabolic breakdown of gly- 

 cogen to carbon dioxide and water with the energy passed to ATP for 

 use in muscle contraction. The reactions include those of glycolysis 

 (page 168) or the pentosephosphate pathway (page 223), the tricar- 

 boxylic acid cycle (page 171), and the electron-transfer system (page 

 172). 



Sometimes, however, emergency requirements for energy temporar- 

 ily exceed the rates of respiration and circulation, and the supply of 

 oxygen becomes inadequate. In this event, glycogen is metabolized via 

 glycolysis only. For this purpose DPN+ must be available, but elec- 

 trons cannot be transferred through flavoproteins and cytochromes 

 without oxygen. Therefore, some other oxidation of DPNH must oc- 

 cur. In muscle this reaction involves pyruvate, which is reduced to 

 lactate to give the DPN+ back again. 



CH3COCOO- + DPNH -f H3O+ <r ^^'''"' 



dehydrogenase 



CH3CHOHCOO- + DPN+ + H2O 



Lactate accumulates in working muscle until increased respiration 

 and circulation provide an adequate supply of oxygen for complete 

 metabolic oxidation. Some lactate dialyzes into the blood and is metab- 

 olized in other parts of the body. When physical activity ceases, part 

 of the lactate reverts through the glycolytic system to glycogen. The 

 proportion thus returning to storage depends upon the magnitude 

 and extent of the exertion undergone. These processes are sum- 

 marized in Figure 21-1. The creatine phosphate thus acts to stabilize 



