Kinetic mechanisms of inhibitor binding: relevance to the fast-acting slow-binding paradigm

Although phlorizin inhibition of Na+-glucose cotransport occurs within a few seconds, 3H-phlorizin binding to the sodium-coupled glucose transport protein(s) requires several minutes to reach equilibrium (the fast-acting slow-binding paradigm). Using kinetic models of arbitrary dimension that can be reduced to a two-state diagram according to Cha's formalism, we show that three basic mechanisms of inhibitor binding can be identified whereby the inhibitor binding step either (A) represents, (B) precedes, or (C) follows the rate-limiting step in a binding reaction. We demonstrate that each of mechanisms A-C is associated with a set of unique kinetic properties, and that the time scale over which one may expect to observe mechanism C is conditioned by the turnover number of the catalytic cycle. In contrast, mechanisms A and B may be relevant to either fast-acting or slow-binding inhibitors. However, slow-binding inhibition according to mechanism A may not be compatible with a fast-acting behavior on the steady-state time scale of a few seconds. We conclude that the recruitment hypothesis (mechanism C) cannot account for slow phlorizin binding to the sodium-coupled glucose transport protein(s), and that mechanism B is the only alternative that may explain the fast-acting slow-binding paradigm.