ANALOGS OF NICOTINAMIDE 497 



NAD to replace NAD as the coenzyme for the various dehydrogenases. 

 3-AcPyr-NAD can function in most NAD-dependent dehydrogenase reac- 

 tions. In some cases it can be reduced more rapidly than NAD (horse liver 

 alcohol dehydrogenase, beef liver glutamic dehydrogenase, Lactobacillvs d- 

 and L-lactate dehydrogenases) and in other cases proceeds more slowly 

 (yeast alcohol dehydrogenase, beef heart lactate dehydrogenase, yeast gly- 

 ceraldehyde-3-P dehydrogenase), while in a few instances the rates are ap- 

 proximately equivalent (rabbit muscle lactate dehydrogenase) (N. 0. Kap- 

 lan et al, 1956; van Eys et al, 1958; N. 0. Kaplan, 1959; Stockell, 1959). 

 3-AcPyr-NADP is reduced about one fifth as fast as NADP in the pig 

 heart isocitrate dehydrogenase system and is inactive in erythrocyte glu- 

 cose-6-P dehydrogenase (N. 0. Kaplan et al., 1956; Marks et al., 1961). The 

 relative rates do not necessarily reflect the relative bindings to the dehy- 

 drogenases. In those cas^s where coenzyme activity is low but binding is 

 appreciable, the NAD or NADP analogs can inhibit the dehydrogenases; 

 thus 3-AcPyr-NAD inhibits glucose-6-P dehydrogenase quite strongly {K,= 

 0.03 mM) and this is competitive. NAD analogs other than 3-AcPyr-NAD 

 are usually less active and tend to be more inhibitory. Thionicotinamide- 

 NAD, nicotinyl-hydroxamate-NAD, and nicotinyl-hydrazide-NAD com- 

 petitively inhibit lactate and alcohol dehydrogenases, whereas 3-benzoyl- 

 pyridine-NAD inhibits beef heart lactate dehydrogenase uncompetitively 

 (Anderson and Kaplan, 1959). The introduction of 3-acetylpyridine, or 

 other pyridine analogs, can thus produce several effects on tissue dehydro- 

 genase activity, and in the general case will bring about an imbalance of 

 the normal relative substrate oxidations, due to altering the rates of the 

 various dehydrogenases in different ways. Unfortunately there has not yet 

 been sufficient study of the oxidative abilities of tissues isolated from ani- 

 mals treated with 3-acetylpyridine. However, it is probably safe to assume 

 that at least a major cause of the toxic effects is the inhibition of certain 

 dehydrogenases by the 3-AcPyr-NAD formed. 



The various NAD analogs have been very useful in demonstrating differ- 

 ences between dehydrogenases from different tissues or species. For exam- 

 ple, beef heart and rabbit muscle lactate dehydrogenases react better with 

 NAD than with 3-AcPyr-NAD, but the lactate dehydrogenases from lob- 

 ster heart and thorax muscle react better with the analog (N. 0. Kaplan, 

 1959). Kaplan et al. (1960) have pointed out that the molecular heterogeneity 

 of enzyme active centers has phylogenetic significance. It is possible to 

 classify animals with respect to the affinities of their dehydrogenases for 

 the coenzymes or their analogs, and it is hoped that further investigation 

 along these lines will elucidade some of the evohitionary problems relative 

 to the changes in the active center configurations. 



We must now examine the evidence for other sites of action for 3-acetyl- 

 pyridine and related analogs, Mcllwain (1950) reported that 3-acetylpyri- 



