Energetic coupling of Na-glucose cotransport.

(1) Energetic coupling in Na-linked glucose transport in renal brush border membrane vesicles has been studied in terms of various carrier models differing with respect to reaction order (random vs. ordered), and to rate limitation of steps within the routes of carrier-mediated solute transfer (translation across the membrane barrier vs. binding/release between carrier and bulk solution). (2) By computer simulation it was found that effective energetic coupling requires the leakage routes to be significantly, if not predominantly, rate-limited by their (barrier-crossing) translatory steps. This does not apply to the transfer route of the ternary complex, as coupling is possible whether or not this route is rate-limited by the translatory step. (3) The system transports glucose in the absence of Na+ (uniport) and the unidirectional flux is stimulated by unlabeled glucose on the trans side (negative tracer coupling). It is concluded that glucose binds to the carrier on either side without Na, as would be consistent with either a random system or one mode of ordered system with mirror symmetry (glucose binds before Na) but inconsistent with either mode of glide symmetry. The tracer coupling appears to indicate that the rate coefficient of carrier-mediated glucose transfer exceeds that of the empty carrier. (4) The Na-linked zero-trans flow of glucose in either direction is strongly trans-inhibited by Na. This consistent with a random system in which Na blocks or retards the translocation of the glucose-free carrier, thereby reducing 'slipping' through an internal leakage route. It is also consistent with the above mentioned ordered system, (i.e., in the absence of Na-transport without D-glucose) if it is assumed that trans Na interferes with the dissociation of the ternary complex, thereby slowing the release of glucose. (5) Minimum equilibrium exchange of glucose is stimulated in the presence of Na. This appears to indicate that Na expands the flow density of carrier-mediated glucose transfer. This expansion does not result from a 'velocity effect' (the ternary complex moving faster than the binary glucose carrier complex), as Na fails to stimulate maximum equilibrium exchange. It can instead be accounted for by an 'affinity effect' (the affinity of the carrier for glucose being increased by Na) as Na depresses the Michaelis constant of equilibrium exchange.(ABSTRACT TRUNCATED AT 400 WORDS)