128 A SYMPOSIUM ON RESPIRATORY ENZYMES 



to more recent findings the vitamin nature of nicotinamide for the 

 rat is doubtful (81, 82). Dann and Handler recently have shown 

 that nicotinic acid is formed by the chick embryo (83). 



An important observation made in these experiments was that the 

 organism does not synthesize coenzyme beyond the normal level of 

 coenzyme content under favorable dietary conditions. Even an ad- 

 ministration of nicotinamide far in excess of the normal requirement 

 results in no significant synthesis beyond the normal level. In 

 erythrocytes, however, an increase in coenzyme content upon ad- 

 ministration of an excess of nicotinamide was observed by Kohn and 

 Klein, Vilter and Spies, Axelrod and others (59, 60). 



Numerous experiments with isolated enzyme preparations from 

 yeast, liver, and muscle have thus far failed to give a noteworthy 

 synthesis of the coenzymes from the structural units. The destruc- 

 tive tendency of these preparations has always been found to be 

 remarkable. It was observed by Euler and co-workers (64—66) that in 

 tissues the coenzyme content after death, and especially upon 

 destruction of the cell structure, decreases rapidly. The significance 

 of this finding for the methods of determination and the precautions 

 necessary have been pointed out repeatedly. 



Mann and Quastel have recently confirmed the findings of Euler. 

 Of special interest is their finding that free nicotinamide in great 

 excess prevents the postmortem decomposition of cozymase (67), a 

 result which the authors explain by assuming that nicotinamide and 

 cozymase compete for the active center of the nucleotidase which 

 destroys the coenzyme. 



Lennerstrand found some years ago (68, 69) that a destruction 

 of cozymase by washed dried yeast (apozymase) takes place, but is 

 inhibited by phosphate and hexosediphosphate. Apparently the 

 substrate protects the coenzyme from destruction. It is not yet pos- 

 sible to say what products are formed by the inactivation of cozy- 

 mase by apozymase. If glucose and phosphate are added to the 

 inactivated cozymase plus apozymase, a resynthesis takes place 

 after several hours, and as much as 50 per cent of the cozymase 

 originally present is restored. 



A similar effect was observed recently with cocarboxylase. When 

 cocarboxylase is incubated with aetiozymase, it is destroyed. In the 

 presence of pyruvate a much slower inactivation takes place (33). 

 It is possible that underlying these findings is an important principle 

 of regulation of the coenzyme level. 



It seemed possible that a regeneration of cozymase from the nico- 



