Anomalous kinetics of sucrose uptake in maize scutellum slices1

Abstract

Abstract The kinetics of sucrose uptake into maize scutellum slices showed that the uptake mechanism had a saturable component with a Km of l.5mol m−3 sucrose. Nevertheless, uptake rate was constant (zero order) over extended periods of time until the bathing solution was nearly depleted of sucrose. It is concluded that these anomalous uptake kinetics reflect sucrose influx across the plasmalemma because of the following results: (a) Efflux of sucrose into buffer was negligible compared with uptake rate, (b) When slices were incubated in fructose, sucrose was synthesized and there was a net release of sucrose to the bathing solution until a steady‐state was reached when influx and efflux were equal in magnitude. After the steady‐state was reached, efflux of sucrose from the slices was nearly the same in magnitude as the estimated rate of uptake that would have occurred from bathing solutions initially containing the steady‐state sucrose concentration, (c) Exchange of sucrose between bathing solution and slices was negligible compared with uptake rate, (d) Pretreatment of slices with uranyl nitrate abolished sucrose uptake, but uptake rate was re‐established in these slices after treatment with HCl (pH 2). Uptake rate was set by the initial sucrose concentration of the bathing solution, and was not influenced by the level of endogenous sucrose or by the rate at which the sucrose concentration of the bathing solution declined. Abrupt increases in sucrose concentration during the uptake period increased the rate of uptake only if the concentration was increased above that at the start of the uptake period. Following abrupt decreases in sucrose concentration, there was a lag of about 30 min before uptake rate decreased greatly. If slices were washed and replaced in a fresh sucrose solution during the uptake period, a new uptake rate was set to correspond to the new initial sucrose concentration. It is suggested that the sucrose carrier has a transport site with a relatively low Km (much below 1.5mol m−3) and that the measured Km (1.5mol m−3) is that of a site that binds sucrose and thereby controls the rate of uptake. The low Km suggested for the transport site would explain the zero order kinetics but a model of the uptake mechanism that includes the control site cannot, as yet, be constructed from the data.