BODY TISSUES 303 



cnibccUlc'cl in Liie ni)().sin fibers, exists in two loims, fibrous aciin and 

 globular actin. The fibrous actin forms a complex with myosin, 

 yielding the viscous actomyosin. Fibrous actin is thouglu to be formed 

 by polymerization of the globular precursor through the inlluence of 

 ATP (page 140). 



Globular actin + wATP -^ fibrous actin + wADP + nHPOr 



Actomyosin is thought by some to constitute the adenosine triphos- 

 phatase found in muscle tissue. Others feel that ATPase and myosin 

 are closely related and difficult to separate but are not identical. This 

 concept is supported by inhibitor, activator, and denaturation studies. 

 Myosin is a large molecule, with a molecular weight in the neighbor- 

 hood of 1,000,000, while actin is much smaller, about 70,000 g./mole. 

 Actomyosin has a molectilar weight in excess of 1,000,000. 



Actomyosin is a viscous substance and is birefringent. Threads spun 

 from this protein by extrusion into a dilute salt solution show con- 

 traction in the presence of ATP, potassiimi, and magnesium ions. 



Myogen is a water-soluble albumin type of protein found in muscle, 

 although it is not involved directly in muscle contraction. However, 

 it does contain a number of enzymes involved in carbohydrate me- 

 tabolism. The enzymes detected in myogen include isomerase, aldolase, 

 triosephosphate dehydrogenase, and phosphorylase. 



Muscle Action 



Contraction of muscle is brought about by the shortening or sliding 

 of actomyosin molecules in the myofibril by a mechanism that is not 

 completely understood. X-ray data show that the molecule is denser 

 in the contracted state than in the extended form. Measurements also 

 show a change in electrical potential. Different theories have been 

 developed to explain these observations. One such theory assumes a 

 stretching and compression of the actomyosin helix during relaxation 

 and contraction, respectively. Another theory proposes a sliding fila- 

 ment model, with the actomyosin molecides sliding by one another, 

 thus producing relaxation or contraction of the myofibril, depending 

 on the direction of motion. Ihe first theory suggests that actomyosin, 

 as do all proteins, contains polar groups associated with the constituent 

 amino acids. The interattrac tion of these polar groups, along with 

 hydrogen bonding, influences the rigidity of the helix. If an ATP 

 molecule is introduced, the three associated negative charges cause a 

 reorientation of the interpolar attractions with contraction resulting. 

 The energy for the contraction is supplied by the hydrolysis of ATP. 

 Muscular contraction could be likened to the extension or com- 



