NICOTINIC ACID (NIACIN) 



the inhibitory action was reversed owing to competition between the 

 two substances, similar to that between ^-aminobenzoic acid and 

 sulphanilamide (page 546) or between pantothenic acid and pantoyl- 

 tam-ine (page 381). 



The growth of Pr. vulgaris was partially inhibited by pyridine-j8- 

 sulphonic acid ^^ at a concentration of 4 x io~^ mole per 1. and com- 

 pletely at 4 X io~^ mole per 1. The inhibition was counteracted by 

 nicotinic acid and by thiazole-5-carboxylic acid amide, a close analogue 

 of nicotinamide ; the thiazole derivative had no inhibitory action on 

 Pr. vulgaris. 



Pyridine-^-sulphonic acid did not inhibit the growth of Strepto- 

 bacterium plantarum,^^ but nicotinic acid methiodide and pyridine-3- 

 sulphonamide methiodide were more effective than the sulphonic acid, 

 owing to the presence of the iodide ion. Thionicotinamide had some 

 inhibitory effect. Some suppression of growth also occurred with 

 picolinic acid and its amide, and with quinoline-2- and -3-carboxylic 

 acids. Thiazole-5-carboxylic acid amide had a slight inhibitory 

 action on S. aureus,^'' but thiazole-5-sulphonic acid and 2-(thiazole- 

 5 '-carboxylamido-) -pyridine were inert. Thiazole-4-sulphonic acid 

 likewise possessed no inhibitory activity. ^^ 



Pyridine-j8-sulphonic acid inhibited the dehydrogenation of lactic 

 acid and of glucose when the concentration of the coenz3mie was kept 

 constant and that of the sulphonic acid was increased,^^ so that the 

 latter would appear to compete with the coenzyme for the apoenzyme. 

 The inhibitory action decreased as the concentration of the coenzyme 

 or of nicotinic acid or nicotinamide was increased. The affinity of 

 cozymase for apodehydrogenase was two to three times that of pyri- 

 dine-^-sulphonic acid, whilst nicotinic acid, benzoic acid and benzene 

 sulphonic acid had about the same affinity as pyridine-j8-sulphonic 

 acid for apodehydrogenase ; pyridine-^-sulphonamide was consider- 

 ably less active. The inhibition of enzyme activity by these com- 

 pounds was not due solely to the carboxylic or sulphonic acid groups 

 but was rather a function of the whole molecule. 



According to P. Karrer and W. Manz,^^ the antagonistic action of 

 pyridine-3-sulphonamide towards nicotinamide was probably not due 

 to displacement of the latter by the former from codehydrogenase I or 

 II. If, however, displacement actually does occur, then the altered 

 codehydrogenase is probably capable of reversible reduction, since 

 pyridine-3-sulphonamide methiodide and ethiodide were reduced by 

 sodiimi dithionite to i-methyl- and i-ethyl-i : 2-dihydropyridine-3- 

 sulphonamide respectively. 



Pyridine-j8-sulphonic acid exerted an antagonistic effect on the 

 growth-stimulation produced by tetrahydronicotinic acid to the same 

 extent as with nicotinic acid.^s Tetrahydronicotinic acid, but not 



292 



