title: Product dependence of deuterium isotope effects in enzyme-catalyzed reactions.
abstract: Theory for enzyme-catalyzed reactions is developed for the dependence on product concentration of deuterium isotope effects on V and V/K. Generally, a product that decreases the off-rate for a second product to zero causes the isotope effect on V/K to decrease to DKeq and that on V to decrease to a value between 1 and DKeq. If the second product off-rate is decreased to a finite value, DV and D(V/K) will decrease to a value greater than DKeq, while if there is no effect on the off-rate for the second product, DV and D(V/K) will not change. Interestingly, for a ping-pong mechanism, the presence of the product that provides a reversible connection between the isotope-sensitive step and the isotope-insensitive half-reaction will give an isotope effect on V/K for the latter. (In the absence of the product, the isotope effect on V/K for the isotope-insensitive half-reaction will be unity.) Theory is supported with data for alcohol and lactate dehydrogenases. For lactate dehydrogenase, D(V/Kpyruvate) decreases from 1.93 +/- 0.02 at zero to 1.16 +/- 0.02 at infinite lactate concentration, while DV decreases from a value of 1.75 +/- 0.03 at zero to a value of 0.93 +/- 0.05 at infinite lactate concentration. Thus, it appears that the pathway in which lactate is released first is greatly preferred, but the pathway in which NAD+ is released before lactate is observed at high lactate concentration.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Microbiology and Immunology, Texas College of Osteopathic Medicine, Fort Worth 76107.
authors.forename: P F
authors.initials: PF
authors.lastname: Cook
authors.forename: M Y
authors.initials: MY
authors.lastname: Yoon
authors.forename: S
authors.initials: S
authors.lastname: Hara
authors.forename: G D
authors.initials: GD
authors.lastname: McClure
mesh_terms: D000426:Alcohol Dehydrogenase / Q000378:metabolism*; D000818:Animals; D002384:Catalysis; D003903:Deuterium / Q000494:pharmacology*; D000431:Ethanol / Q000378:metabolism / Q000494:pharmacology; D007700:Kinetics; D007770:L-Lactate Dehydrogenase / Q000378:metabolism*; D008433:Mathematics; D009243:NAD / Q000378:metabolism; D011817:Rabbits; D012441:Saccharomyces cerevisiae / Q000201:enzymology
chemical_list: D009243:NAD; D000431:Ethanol; D003903:Deuterium; D000426:Alcohol Dehydrogenase; D007770:L-Lactate Dehydrogenase