Transport of monosaccharides in Rhodotorula gracilis in the absence of metabolic energy

The mechanism of sugar uptake by sugarcane suspension cells

Sugarcane cell suspensions took up sugar from the medium at rates comparable to or greater than sugarcane tissue slices or plants in the field. This system offers an opportunity for the study of kinetic and energetic mechanisms of sugar transport in storage parenchyma-like cells in the absence of heterogeneity introduced by tissues. The following results were obtained: (a) The sugar uptake system was specific for hexoses; as previously proposed, sucrose was hydrolyzed by an extracellular invertase before the sugar moieties were taken up; no evidence for multiple sugar uptake systems was obtained. - (b) Uptake of the glucose-analog 3-O-methylglucose (3-OMG) reached a plateau value with an intracellular concentration higher than in the medium (approximately 15-fold). - (c) There was a balance of influx and efflux during steady state; the rate of exchange influx was lower than the rate of net influx; the Km value was higher (70 μM) than for net influx (24 μM); the exchange efflux is proposed to be mediated by the same transport system with a Km value of approximately 2.6 mM for internal 3-OMG; the rate of net efflux of hexoses was less than a third of the rate of exchange efflux. - (d) The uptake of hexoses proceeded as proton-symport with a stoichiometry of 0.87 H(+) per sugar; during the onset of hexose transport there was a K(+) exit of 0.94 K(+) per sugar for charge compensation. (It was assumed that the "real" stoichiometries are 1 H(+) and 1 K(+) per sugar.) The Km values for sugar transport and sugar-induced proton uptake were identical. Sucrose induced proton uptake only in the presence of cell wall invertase. - (e) There was no net proton uptake with 3-OMG by cells which were preloaded with glucose though there was significant sugar uptake. It is assumed, therefore, that the exit of hexose occurs together with protons. - (f) The protonmotive potential of sugarcane cells corresponded to about 120 mV: pH-gradient 1.1 units, membrane potential of-60 mV (these values increased if vacuolar pH and membrane potential were also considered). It was abolished by uncouplers, and the magnitude of the components depended on the external pH value. We present evidence for the operation of a proton-coupled sugar transport system in cell suspensions that were derived from, and have characteristics of, storage parenchyma. The quantitative rates of sugar transport suggest that the role of this transport system is not limiting for sugar storage.

A kinetic analysis of D-xylose transport in Rhodotorula glutinis

The kinetics of D-xylose transport were studied in Rhodotorula glutinis. Analysis of the saturation isotherm revealed the presence of at least two carriers for D-xylose in the Rhodotorula plasma membrane. These two carriers exhibited Km values differing by more than an order of magnitude. The low-Km carrier was repressed in rapidly growing cells and derepressed by starvation of the cells. Several hexoses were observed to inhibit D-xylose transport. In the studies reported here, the inhibitions produced by D-galactose and 2-deoxy-D-glucose were examined in some detail in order to define the interactions of these sugars with the D-xylose carriers. 2-Deoxy-D-glucose competitively inhibited both of the D-xylose carriers. In contrast, only the low-Km carrier was competitively inhibited by D-galactose.

Some physiological observations on the uptake of D-glucose and 2-deoxy-D-glucose by starving and exponentially-growing yeasts

Some methods for measuring the uptake of sugars by yeasts were investigated critically. A study was made of the effects of starvation of Pichia pinus, Candida utilis, Saccharomyces cerevisiae and Rhodosporidium toruloides on their uptake of D-glucose and 2-deoxy-D-glucose. Marked changes in the rates of uptake of these sugars occured during 10 h of starvation, including (a) an immediate increase of up to 75% above that for growing cells and (b) a continuous decline to as little as 4%. Each yeast behaved differently. The rates did not remain constant during the periods of starvation often used for studies on the transport of sugars into yeasts. For Pichia pinus, there were striking differences, associated with starvation, between the transport of 2-deoxy-D-glucose and D-glucose, despite evidence that the two sugars enter this yeast by means of the same carrier. Some physiological explanations for these findings are discussed.