No Easy Way Out (Or in)

In retrospect, when Jardetzky (1966) published a " simple allosteric model for membrane pumps, " he initiated a conceptual movement away from the (then still) prevailing anthropomorphic view of transporters and pumps as " carriers " in the literal sense, membrane-bound entities that somehow made their way across the membrane, loaded with cargo. The idea of an " alternating access " model (as it is now called) as a way to explain transmembrane transport had already been proposed by Patlak (1957) and by Vidaver (1966), but Jardetzky (1966) and especially Läuger (1979) popularized it. A trio of papers in this volume (Lu and Hilgemann, 1999a,b; Hilgemann and Lu, 1999) presents an alternating access model for a GABA transporter (GAT1, the first to be cloned, by Guastella et al., 1990). The experimental kinetic database already accumulated on this transporter expressed at high levels in Xenopus oo-cytes, obtained both by two-electrode voltage clamp (Mager et al., 1993, 1996) and by the giant excised patch (Hilgemann, 1989) technique, is one of the most extensive on any carrier (certainly in a single expression system). The proposed model (Hilgemann and Lu, 1999) is probably the most complete " fitted " model extant for any transporter, active or passive. The term " fitted " is in opposition to those exercises where an entire kinetic model is presented, and " plausible " or literature values are assigned to its numerous rate coefficients to generate model behavior that mimics a variety of known experimental results. In the present case, no more rate coefficients, charge movements, Eyring barriers , stoichiometries, etc. are extracted than the data are capable of yielding. (The authors here clearly identify two inaccessible rate coefficients, and " fix " them arbitrarily .) The results make for a sobering read, even by those without specific interest in GABA transport, because they illustrate the magnitude of the modeling enterprise. There is simply no easy way. The database must be vast, with multiple approaches and extensive coverage of both steady state and pre–steady state experiments , before an attempt at global modeling can realistically be undertaken. " True " kinetic effects of the substrates must be distinguished from possible modulating ones. The starting model itself must then be ruthlessly pruned of all experimentally inaccessible parameters to yield the minimal model, and the minimal set of adjustable parameters, that will account for all accumulated observations. Several kinetic steps will …