44 ALBERT L. LEHNINGER 



causes extrusion of small solute molecules along with water. On the other 

 hand, since swollen mitochondria are still relatively impermeable to large 

 molecular weight compounds such as serum albumin and polyvinylpyrroli- 

 done [29], the soluble proteins and other high molecular weight substances 

 in the intramitochondrial space probably do not leave during contraction. 

 If this is the case, then osmotic work is carried out during ATP-induced 

 contraction, because it leads to a more concentrated solution of the high 

 molecular weight solutes inside the mitochondria. 



Preliminary examination of thyroxine-swoUen and ATP-contracted 

 mitochondria with the electron microscope [45] shows the swollen mito- 

 chondria to be very large and spherical, containing large optically clear 

 vesicles and few or no recognizable cristae. After contraction, they are 

 much smaller, optically dense, contain no vesicles, and show nearly normal 

 cristae. 



Swelling and contraction of mitochondria therefore clearly involve the 

 respiratory chain and the associated energy coupling mechanisms, but the 

 two phases employ or are activated by different segments or portions of 

 this complex enzymic machinery. Swelling requires the action of the 

 respiratory chain, but the contraction does not; however, terminal stages 

 of energy coupling appear to be involved in the latter phase. The swelling 

 and contraction therefore appear not to be reversible in the sense that they 

 employ reversibly the same controlling catalysts. Furthermore, because 

 of the occurrence of two mitochondrial membranes it is possible that 

 swelling may be a function of the properties of the inner membrane, for 

 example, and contraction a function of the outer membrane, since all 

 kinds of mitochondrial swelling can be contracted again by ATP [40]. 



Because sucrose and other polyhydroxylic compounds such as glucose, 

 raffinose, fructose, dextran, xylose, mannitol, and sorbitol in concentrations 

 of o-i M to 0-6 M inhibit both swelling and contraction (the latter more 

 strongly), as well as ATP-ase and the ATP-P^^- exchange in osmotically 

 insensitive digitonin particles, we have suggested that these compounds 

 are efficacious in preserving mitochondrial morphology during isolation 

 more for their ability to act as inhibitors of an intermediate enzymic 

 reaction involved in the swelling-contraction cycle than for their relative 

 slowness of penetration [29, 35]. Simple alcohols or compounds like 

 ethylene glycol and glycerol do not inhibit. It has been suggested that the 

 polyhydroxylic alcohols act as artificial acceptors in "transferase " reactions, 

 displacing the normal group acceptor. 



Biochemistry of the contractile process 



It now seems possible to approach chemical analysis of the mechanism 

 of contraction. A guiding principle for such approaches is the hypothesis 



