500 2. ANALOGS OF ENZYME REACTION COMPONENTS 



duction rate are either not affected or slightly decreased. The effects are 

 somewhat greater at 5 niM, contractile tension increasing 25%. 2-Acetyl- 

 pyridine and 4-acetylpyridine produce very much the same effects, and 

 nicotinamide itself quite potently stimulates atrial contractility, although 

 in this case the resting and action potentials tend to decrease. In view of 

 these effects by substances not giving rise to NAD analogs, the acute ac- 

 tion of 3-acetylpyridine on the atria must be attributed to some other me- 

 chanism. The lack of depressant activity at these concentrations is also 

 interesting, since metabolic disturbances invariably produce certain char- 

 acteristic changes. The contractile stimulation by nicotinamide increases 

 with concentration and at 100 mM is around 100%. The mechanism is not 

 understood but seems to be unrelated to membrane potential changes. 



Inhibition of Dehydrogenases by Nicotinamide and Related Compounds 



The inhibition of NADases and inhibitions dependent on the NADase- 

 catalyzed exchange reactions have been discussed. We now turn to the 

 inhibitions of NADP-dependent dehydrogenases by nicotinamide and other 

 substituted pyridines. The groups and interactions involved in the binding 

 of the pyridine coenzymes to the dehydrogenases have been discussed by 

 Shifrin and Kaplan (1960). Sulfhydryl groups are often important and have 

 frequently been thought to react with the pyridine N of the coenzymes, 

 while in some dehydrogenases Zn++ is involved and perhaps reacts with 

 the phosphate or adenine residues. It is apparent that the coenzymes are 

 bound in different ways to different dehydrogenases and this will determine 

 to some extent the ability of analogs to inhibit. The nature of the very 

 tightly bound intramitochondrial NAD in unknown, but possibly it is much 

 more difficult to inhibit intact mitochondrial dehydrogenases than the iso- 

 lated and often reconstituted enzymes usually studied. 



The first report of an inhibition of metabolism by nicotinamide was made 

 by Baker et al. (1938) in connection with a study of the action of nicotine 

 on cerebral respiration. However, the inhibition of brain slice respiration is 

 slight, 30 mM depressing the oxidation of glucose 8% and lactate 15%. 

 A greater differential effect on glucose and lactate oxidation is exerted by 

 nicotinate, the inhibitions at 30 mM being 9% and 57%, respectively. Von 

 Euler (1942) made the initial investigation of dehydrogenase inhibition and 

 found inhibitions of both beef liver glucose dehydrogenase and heart lactate 

 dehydrogenase (see accompanying tabulation). The generally weak inhi- 

 bitory activity and the greater potencies of nicotinate and pyridine-3-sulf- 

 onate, compared to nicotinamide and pyridine-3-sulfonamide, may indicate 

 that these substances do not actually combine with the pyridine-binding 

 site on the enzymes, and it is even doubtful if they may be classed as NAD 

 or NADP analogs. However, the inhibition by the pyridine-3-sulfonate ap- 

 pears to be competitive. Brink (1953 b) continued von Euler's work in 



