NICOTINIC ACID (NIACIN) 



butylicum produced from 30 to 180 molecules of nicotinic acid per cell 

 per second, and that each contained from 96,000 to 120,000 molecules 

 of nicotinic acid per cell. The " turnover number " of three different 

 non-bacterial enzyme systems containing cozymase ranged from 300 

 to 450 molecules per molecule of enzjnne per second, that is, between 

 300 and 450 molecules of substrate reacted with each molecule of 

 enzyme per second. 



The rate of inactivation of nicotinamide by Proteus vulgaris was 

 estimated to be 5 molecules per cell per second, the rate of inactivation 

 of cozymase by H. parainfluenzae 11 molecules per cell per second, 

 the rate of inactivation and reactivation of cozymase by yeast 12 to 

 18 molecules per cell per second and the rate of interconversion of 

 coenzymes I and II by yeast 8 to 10 molecules per cell per second ; in 

 each instance, the rate given is for the non-proliferating organism. 

 Thus, the reactions of S5mthesis, inactivation and interconversion 

 occurred in nicotinic acid derivatives with velocities not far removed 

 from 10 molecules per cell per second, suggesting the existence of a 

 group of well-defined reactions of low velocity, which Mcllwain terms 

 " reactions of m/t mol-order ", because their velocity is some m/x 

 mols. per gram of dry weight of the organism per second. The ordinary 

 reactions of the cell proceed much faster, being reactions of jli mol. 

 order ; there is therefore a sharp distinction between the two types. 



Actually the rate at which coenzyme is synthesised is not constant. 

 L. arahinosus, for instance, synthesised cozymase at a rate of 24 m^Lt 

 mol. per mg. of dry weight per hour in a nicotinic acid-deficient 

 medium and at a rate of less than 3 mju, mol. per mg. per hour in 

 presence of nicotinic acid. The rate of synthesis appears to be con- 

 trolled by a mechanism that inhibits synthesis as the cells become 

 saturated. ^^ 



If an enzyme that effects an m/x mol. reaction is operating with a 

 turnover number of say 50 molecules per molecule per second, then in 

 the twenty minutes of a bacterial generation it could have controlled 

 the production of 6 x 10* molecules, possibly of a coenzyme capable 

 of acting with a similar turnover number. In this event, the effect 

 of the initial enzyme molecule would extend to i-8 x 10^ molecules ; 

 that is, if the substrate were glucose, 5-4 X lo-^^ g., or about five 

 times the bacterial mass, would have been metabolised. 



When bacteria are irradiated, the genes may be altered and the 

 biochemical reactions of the cell modified accordingly. It has been 

 suggested that each gene controls one biochemical reaction. Some 

 enzymes occur in the bacterial cell in large numbers and, in such 

 instances, one gene must influence the production of large numbers of 

 enzyme molecules. 



Other enzymes, however, occur in much smaller numbers. The 



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