Overview: Theoretical Aspects of Isotope Effects on the Pattern of Metabolites Formed by Cytochrome P-450

  • James R. Gillette
  • Kenneth Korzekwa
Part of the Advances in Experimental Medicine and Biology book series (AEMB, volume 283)


Measurements of the effects of substitutions of deuterium at specific positions of substrates on the kinetics of metabolism of substrates by enzymes have provided valuable information on the mechanisms of enzymes. Most of the theories that permit interpretation of these measurements are based on the following assumptions: 1) Deuterium is substituted on only one position. 2) The isotope effect is on only one rate constant, namely the one which is associated with the breaking of a C-H Bond. 3) The enzyme forms only one product from the substrate. These assumptions thus imply that isotope effects on the reversible binding of the substrate in the active site of the enzyme are negligible. Secondary isotope effects also are usually assumed to be negligible. Northrop (1977) has pointed out that when these assumptions are valid, the apparent isotope effects manifested by the mechanisms of most enzymes may be shown to fit the following relationships:
$${D_{(V/K)}} = \frac{{({V_{\max }}/{K_m})H}}{{({V_{\max }}/{K_m})D}} = \frac{{C + {k_H}/{k_D}}}{{C + 1}}$$
where C is the “Commitment to Catalysis”.
$${D_V} = \frac{{({V_{\max }})H}}{{({V_{\max }})D}} = \frac{{R + {k_H}/{k_D}}}{{R + 1}}$$
where R is the “Ratio of Catalysis”.


Isotope Effect Protonated Form Water Formation Kinetic Isotope Effect Competitive Experiment 
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  2. Nothrop, D.B. (1975). Steady-state analysis of kinetic isotope effects in enzymic reactions. Biochemistry 14, 2644–2651.CrossRefGoogle Scholar
  3. Northrop, D.B. (1977). Determining the absolute magnitude of hydrogen isotope effects. In Isotope Effects on Enzyme Catalyzed Reactions. ( W.W. Cleland, M.H. O’Leary and D.B. Northrop, Eds.), pp. 122–151, University Park Press, Baltimore.Google Scholar

Copyright information

© Plenum Press, New York 1991

Authors and Affiliations

  • James R. Gillette
    • 1
  • Kenneth Korzekwa
    • 1
  1. 1.Laboratory of Chemical PharmacologyNational Heart, Lung, and Blood InstituteBethesdaUSA

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