Kinetic isotope effects nomenclature

The Northrop-Cleland nomenclature system for isotope effects greatly simplifies their discussion the non-abundant isotope and, in the case of secondary effects, site of substitution are written as superscripts to V or VjK in parentheses, so that °(F/A) refers to an a-deuterium kinetic isotope effect on kcatZ-Klm and (K) refers to an effect on kcat- The effects on individual rate and equilibrium constants are written as superscripts is the p-tritium effect on an equilibrium constant and " k+2 is the effect on the k+2 step. Although in principle potentially ambiguous (e.g. could in principle refer to or the relatively short-lived F), in practice any ambiguity is resolved from the context. 

The nomenclature of solvent isotope effects is the same as the Northrop s nomenclature for kinetic isotope effects described in preceding sections (Cook, 1991). Consider a simple monosubstrate reaction. 

The rate equation for solvent isotope effects with more complex bisubstrate mechanisms may be derived in the same way as was described in Section 173.3 for the kinetic isotope effects, using the corresponding nomenclature. 

In bisubstrate enzyme reactions, an effort must be made to reduce the complexity of kinetic expression. Therefore, and according to the nomenclature of Northrop (1977), we shall write the conventional expression for an isotope effect, as... 

At this point, we may note that the most immediate characteristic of an overall solvent isotope effect on steady-state kinetic parameters, kwonlkoon, Vmoh/ Food, or (F/i0HOH/ F/K)DOD, is its direction. The customary nomenclature attributes a normal solvent isotope effect when the kinetic parameters are larger in HOH, and an inverse solvent isotope effect when the kinetic parameters are larger in DOD. Solvent isotope effects can also be primary and secondary. 

---