496 2. ANALOGS OF ENZYME REACTION COMPONENTS 



analogs. Most of this work has been done by Kaplan and his associates at 

 Johns Hopkins, and a summary of their most important results will be 

 given. It was first demonstrated that the incubation of NAD, brain NADase, 

 and isonicotinyl hydrazide (isoniazid, IHN) leads to the formation of the 

 INH analog of NAD, which was isolated in good yield, and it was postulated 

 that the antitubercular activity of isoniazid may be related to the appear- 

 ance of this nonfunctional or inhibitory analog (Zatman et al., 1954 b). 

 It was soon shown that a variety of pyridine derivatives can exchange with 

 nicotinamide in the presence of certain NADases to form NAD analogs; 

 these include isonicotinamide, iproniazid, ethylnicotinate, and S-acetylpyri- 

 dine (N. 0. Kaplan c^oi., 1954). The formation of 3-AcPyr-NAD* in tissue 

 homogenates and whole animals is inhibited by nicotinamide. The exchange 

 reaction and hydrolysis may be represented as: 



where NRPPRA is NAD, XRPPRA is the NAD analog, E-RPPRA is the 

 relatively stable ribosyl enzyme complex, and X is the pyridine derivative 

 exchangeable with nicotinamide. The over-all exchange reaction would be: 



NRPPRA + X ^ XRPPRA + N 



Injection of 3-acetylpyridine leads to a rise in total pyridine nucleotides in 

 most tissues; in the liver this is NAD and none of the analog is demon- 

 strable, due presumably to the oxidation of 3-acetylpyridine to nicotinate, 

 whereas in brain, spleen, and tumors 3-AcPyr-NAD appears. In tumors the 

 NAD content actually decreases as 3-AcPyr-NAD increases. The equilibrium 

 between NAD and any of its analogs, and the ratio of their concentrations 

 in a particular tissue, will depend on (1) the AF between NAD and the 

 analog, (2) the concentrations of N and X, (3) the rate of transformation 

 of X to nicotinate, if it occurs, and (4) the relative bindings of NAD and 

 its analogs to the dehydrogenases. The time courses for the formation of 

 3-AcPyr-NAD from 3-acetylpyridine and the toxic reactions led to the sug- 

 gestion that the toxic and lethal actions are related to the NAD analog; 

 whether the toxicity depends on a reduction of NAD or a rise in 3-AcPyr- 

 NAD was undecided. 



In order to determine the nature of the effects of 3-acetylpyridine on 

 tissue metabolism, it will be necessary to consider the ability of 3-AcPyr- 



* The analogs of NAD will be designated by prefixes of this type, following Kaplan, 

 since this is convenient if not exactly accurate. 



