Kinetic features of cotransport mechanisms under isotope exchange conditions

The mechanisms of carrier-mediated cotransport across biological membranes can be divided into those with ordered and those with random sequences of substrate binding to and debinding from the carrier. When the membrane is considered a symmetry plane, the sequence of substrate binding and debinding of ordered mechanisms can exhibited either of two symmetries: mirror or glide. The carrier-mediated cotransport of solutes is kinetically analyzed for equilibrium exchange in membrane vesicles to obtain criteria for distinguishing the different transport mechanisms. Three types of kinetic measurement are necessary to determine the type of cotransport mechanism: (1) isotope exchange of either substrate as a function of its concentration (Km determination); (2) isotope exchange of either substrate as a function of cosubstrate concentration (activation curves); and (3) ratios of mass exchange rates of both substrates as a function of various substrate concentration ratios. These criteria are applied to and discussed in terms of Na+-dependent D-glucose cotransport and putative Na+-Cl- cotransport.