Abstract: Primary kinetic hydrogen isotope effects (KIE's) for hydride transfer between NAD'analogues are between 6 and 4, for reactions with equlibrium constants ranging from 0.7 to 10l6. There is no sign of the sharp change in KIE which would signal the change of a rate-limiting step in a multistep mechanism, and other evidence also indicates a one-step mechanism. 2 The secondary KIE produced by changing the hydrogen already attached ...