ANALOGS OF NICOTINAMIDE 503 



tions sufficiently high to interfere with dehydrogenases. Feigelson et al. 

 (1951) investigated this problem in liver homogenates and noted first that 

 nicotinamide reduces endogenous respiration, an effect reversed by NAD. 

 At 50-100 mM, nicotinamide stimulates the endogenous respiration some- 

 what, perhaps due to protection of NAD, but at higher concentrations in- 

 hibits quite potently. Malate dehydrogenase was partially purified and ni- 

 cotinamide inhibited competitively with respect to NAD with K^ — 0.00367 

 mM, and K, = 113 raM, corresponding to about 5 kcal/mole tighter bind- 

 ing for the NAD. Care must thus be used in the choice of nicotinamide con- 

 centration when NADase inhibition is desired. Results with lactate and 

 glucose-6-P dehydrogenases from rabbit erythrocytes are verj' similar (Ali- 

 visatos and Denstedt, 1952). Nicotinamide inhibits competitively with K^ 

 around 100 mM. It was also shown that incubation of the apoenzyme with 

 nicotinamide in the absence of NADP leads to progressive irreversible inac- 

 tivation of the dehydrogenases; this may be related to the possible location 

 of binding sites for the coenzymes on adjacent helices of the apoenzyme, 

 separation of these sites occurring unless they are held together by the 

 coenzymes. Nicotinamide also inhibits 6-phosphogluconate dehydrogenase 

 competitively (Dickens and Glock, 1951). The NADH oxidases from pigeon 

 liver microsomes and mitochondria are inhibited 40% and 23% by 20 mM 

 and 80% and 78% by 200 mM nicotinamide, respectively (Jacobson and 

 Kaplan, 1957 a). 



A unique nicotinamide derivative, A'-piperidinomethylnicotinamide, 

 which is claimed to be a specific dehydrogenase inhibitor has been reported 

 by Matkovics et al. (1961). Inhibition of methylene blue reduction in liver 

 homogenates in the presence of various substrates was studied. Unfortu- 

 nately the inhibitor concentrations are not given (only the milligrams added) 



CONH-CH.— N 



N-Piperidinomethylnicotinamide 



and no control experiments on endogenous activity are included. Inhibition 

 of the oxidation of glucose, malate, lactate, and glutamate was observed. 

 However, succinate oxidation is also inhibited, indicating that this sub- 

 stance is not specific for the NAD(P) dehydrogenases. Furthermore, no 

 evidence for competition with the coenzymes was provided. Much more 

 work must be done before this inhibitor can be accepted as having specific 

 anticoenzyme activity. 



