DISCUSSION ON ANIMAL TISSUE RESPIRATION 275 



factors are supplied, the activity of the system is just as great as that 

 of intact tissue. 



More complex are the coenzyme systems in which two additional 

 factors are soluble, namely, the dehydrogenase and the coenzyme. 

 However, by means of fortification with the appropriate coenzyme 

 and inhibition of the coenzyme nucleotidase it should be quite 

 feasible to study these complicated systems in tissue homogenates. 



THE STIMULATORY EFFECT OF CALCIUM UPON THE 

 SUCCINOXIDASE ACTIVITY OF RAT TISSUES 



A. E. AXELROD 



University of Wisconsin 



Variations in the ionic composition of the medium in which the 

 surviving tissue respires are known to exert profound eflFects upon 

 the extent of the respiration. Our attention was drawn to these 

 ionic effects by the observation that calcium (as calcium chloride) 

 stimulates markedly the succinoxidase activity of minced rat liver. 

 This phenomenon was investigated further in the succinoxidase sys- 

 tem of tissue homogenates prepared according to Potter and Elve- 

 hjem. The following results were obtained. In the absence of added 

 cytochrome c the succinoxidase activity of minced liver was in- 

 creased 43 to 80 per cent by the addition of 20 micrograms of cal- 

 cium. With homogenized liver (40 mg. per flask) the addition of 

 20 micrograms of calcium resulted in increases of 93 and 48 per 

 cent in the absence and presence, respectively, of added cytochrome 

 c (3 X 10'* mole per flask). The succinoxidase activity of homoge- 

 nized kidney cortex (20 mg. per flask) was stimulated 40 per cent in the 

 presence of added calcium. Added cytochrome c did not affect the 

 magnitude of the stimulatory effect of calcium in this tissue. The 

 most pronounced effect of calcium was observed in the case of 

 homogenized heart tissue (20 mg. per flask), in which the addition 

 of 20 micrograms of calcium in the presence of 3 X 10"* mole of 

 cytochrome c caused an increase of 200 per cent in the succinoxidase 

 activity. Under our experimental conditions the addition of 20 

 micrograms of calcium always yielded the maximum stimulatory 

 effect. In many cases the addition of smaller amounts of calcium (as 

 little as 1 or 2 micrograms) resulted in a marked acceleration of 

 succinoxidase activity. In only a few isolated cases was a calcium 

 effect observed in brain and skeletal muscle. 



The apphcation of the homogenized tissue technique to the study 



