494 2. ANALOGS OF ENZYME REACTION COMPONENTS 



protected by providing nicotinate or nicotinamide in the diet. On the other 

 hand, yeast and most bacteria seem to be quite resistant to 3-acetylpyri- 

 dine, although the growth of Lactobacillus casei in nicotinate-free medium 

 is inhibited around 50% at 16.5 mM, a depression that can be reversed by 

 nicotinate but not by nicotinamide. Chick embryos are killed by 450-600 //g 

 3-acetylpyridine injected into the eggs and sublethal doses cause distur- 

 bances in embryogenesis (Ackermann and Taylor, 1948), These effects can 

 be completely reversed by nicotinamide; even 6000 jugjegg of the analog 

 can be counteracted by 380 //g of nicotinamide, indicating a competitive 

 relationship. Changes in the heart, characteristic of nicotinate deficiency, 

 are produced by perfusion of 1.6-8 mM 3-acetylpyridine through the iso- 

 lated rabbit heart, dysrhythmias and a-v block occurring within 30 min 

 (Braun, 1949). Subsequent perfusion with nicotinamide reverses these ef- 

 fects but the concentration must be around 100 times that of the analog. 

 These early observations all point to the interference by 3-acetylpyridine 

 in the metabolism or function of nicotinate or nicotinamide. If it is assumed 

 that the primary role of these metabolites is the formation of the NAD and 

 NADP coenzymes, the following possible mechanisms for inhibition by 3- 

 acetylpyridine might be imagined. (1) Inhibition of some step in the syn- 

 thesis of NAD [especially reactions (1) to (3) in the scheme on page 486], 

 (2) inhibition of the interconversion of nicotinate and nicotinamide, (3) en- 

 trance into one of the pathways of nicotinate metabolism to form inhibitory 

 intermediates, (4) formation of an NAD analog, either through the normal 

 pathway or by the exchange reaction catalyzed by NADase, (5) inhibition 

 of NADases and related enzymes, and (6) direct interference with NAD or 

 NADP to inhibit dehydrogenase activity. 



It will be well to consider certain aspects of the metabolism of 3-acetyl- 

 pyridine before taking up the problem of how this analog induces its inhi- 

 bitory effects. 3-Acetylpyridine at doses around 0.5 g/day increases the urin- 

 ary excretion of iV-methylnicotinamide in both normal and nicotinate-de- 

 ficient dogs (Gaebler and Beher, 1951). The iV-methylnicotinamide could 

 arise either from a disturbance of nicotinamide metabolism, since iV-meth- 

 ylation is an important reaction in the elimination of nicotinamide, or 

 directly from the 3-acetylpyridine. The oxidation of 3-acetylpyridine to 

 nicotinate might be anticipated because benzoate is formed from acetophe- 

 none in the tissues, the entire sequence being 



3-acetylpyridine ->■ nicotinate -> nicotinamide -> iV-methylnicotinamide. 



It was found that the latter explanation is correct by determining labeled 

 iV-methylnicotinamide formed from labeled 3-acetylpyridine (Beher et al., 

 1952). It was pointed out that in the course of its oxidation and methylation 

 the analog might also interfere with nicotinate metabolism. Since iV-meth- 

 ylnicotinamide accounts for only about 10% of the administered 3-acetyl- 



