| Abstract: | The reversible reaction NAD + CN(-)----NAD-CN was examined for remote secondary 15N isotope effects caused by isotopic substitution at the ring nitrogen of the nicotinamide group. These were compared with analogous effects for dehydrogenase-catalyzed reactions, since both cyanide and the hydride ion add at the N-4 position of the nicotinamide ring. The 15N effects on the rate constants for the forward and reverse processes were examined directly by conducting both the normal and isotopic reactions simultaneously under carefully controlled conditions in the sample and reference cells of a dual-beam spectrophotometer. In both cases, the 15N kinetic isotope effect differed from 1.00 by considerably less than 0.01. The 15N equilibrium isotope effect, 15K, was obtained as the ratio of equilibrium constants measured separately with natural-abundance and labeled NAD by using a concentration jump procedure 1.004 +/- 0.002 (cyanide addition)]. A similar value for 15K of 1.010 +/- 0.008 was obtained in an analogous manner for the reaction catalyzed by lactate dehydrogenase: NAD + lactate----pyruvate + NADH + H+. The latter value is significantly smaller than a previously reported value obtained from kinetic studies 1.044 +/- 0.012; Cook, P. F., Oppenheimer, N. J., & Cleland, W. W. (1981) Biochemistry 20, 1817]. The present value also is smaller than might be expected for a change in bond order from 4 to 3 Cleland, W. W. (1980) Methods Enzymol. 64, 104-125] on the basis of the canonical resonance structures for NAD and NADH.(ABSTRACT TRUNCATED AT 250 WORDS) |
