Tight coupling of monosaccharide transport and metabolism in Rhodotorula gracilis

Enantioselective uptake and degradation of the chiral herbicide dichlorprop [(RS)-2-(2,4-dichlorophenoxy)propanoic acid] by Sphingomonas herbicidovorans MH

Sphingomonas herbicidovorans MH was able to completely degrade both enantiomers of the chiral herbicide dichlorprop [(RS)-2-(2,4-dichlorophenoxy)propanoic acid], with preferential degradation of the (S) enantiomer over the (R) enantiomer. These results are in agreement with the recently reported enantioselective degradation of mecoprop [(RS)-2-(4-chloro-2-methylphenoxy)propanoic acid] by this bacterium (C. Zipper, K. Nickel, W. Angst, and H.-P. E. Kohler, Appl. Environ. Microbiol. 62:4318-4322, 1996). Uptake of (R)-dichlorprop, (S)-dichlorporp, and 2,4-D (2,4-dichlorophenoxyacetic acid) was inducible. Initial uptake rates of cells grown on the respective substrate showed substrate saturation kinetics with apparent affinity constants (Kt) of 108, 93, and 117 microM and maximal velocities (Vmax) of 19, 10, and 21 nmol min-1 mg of protein-1 for (R)-dichlorprop, (S)-dichlorprop, and 2,4-D, respectively. Transport of (R)-dichlorprop, (S)-dichlorprop, and 2,4-D was completely inhibited by various uncouplers and by nigericin but was only marginally inhibited by valinomycin and by the ATPase inhibitor N,N'-dicyclohexylcarbodiimine. Experiments on the substrate specificity of the putative transport systems revealed that (R)-dichlorprop uptake was inhibited by (R)-mecoprop but not by (S)-mecoprop, (S)-dichlorprop, or 2,4-D. On the other hand, the (S)-dichlorprop transport was inhibited by (S)-mecoprop but not by (R)-mecoprop, (R)-dichlorprop, or 2,4-D. These results provide evidence that the first step in the degradation of dichlorprop, mecoprop, and 2,4-D by S. herbicidovorans is active transport and that three inducible, proton gradient-driven uptake systems exist: one for (R)-dichlorprop and (R)-mecoprop, another for (S)-dichlorprop and (S)-mecoprop, and a third for 2,4-D.

The glucose-dependent transport of L-malate in Zygosaccharomyces bailii

Zygosaccharomyces bailii possesses a constitutive malic enzyme, but only small amounts of malate are decomposed when the cells ferment fructose. Cells growing anaerobically on glucose (glucose cells) decompose malate, whereas fructose cells do not. Only glucose cells show an increase in the intracellular concentration of malate when suspended in a malate-containing solution. The transport system for malate is induced by glucose, but it is repressed by fructose. The synthesis of this transport system is inhibited by cycloheximide. Of the two enantiomers L-malate is transported preferentially. The transport of malate by induced cells is not only inhibited by addition of fructose but also inactivated. This inactivation is independent of the presence of cycloheximide. The transport of malate is inhibited by uranyl ions; various other inhibitors of transport and phosphorylation were of little influence. It is assumed that the inducible protein carrier for malate operates by facilitated diffusion. Fructose cells of Z. bailii and cells of Saccharomyces cerevisiae do not contain a transport system for malate.

Effect of endosulfan on plasma membrane function of the yeast Rhodotorula gracilis

The effects of endosulfan on Rhodotorula gracilis cells are varied and concentration-dependent. They are exhibited as increased rate of respiration, retardation in pH-recovering activity of cell suspensions and loss in the rate or D-xylose uptake. The temperature dependence of sugar uptake indicated that endosulfan reacts with some membrane component(s).

The effect of intracellular pH on the rate of hexose uptake in Chlorella

The rate of hexose uptake by Chlorella is reduced by uncouplers such as carbonyl cyanide p-trifluoromethoxyphenyl hydrazone or dinitrophenol even before concentration equilibrium is reached. The addition of uncouplers changes the membrane potential and the intracellular pH. The membrane potential does not influence the initial velocity of net sugar uptake, whereas manipulation of the cell pH by means of dimethyloxazolidinedione or by butyric acid uncovered a dramatic influence of cell pH on the rate of hexose uptake: at pH values of 7.5--6.8 maximal rate of uptake is observed but at more acid pH a strong inhibition takes place with virtually total blockage of uptake at pH 6.1. The decrease of cell pH to 6.1 in the presence of carbonyl cyanide p-trifluoromethoxyphenyl hydrazone could therefore account for the decrease in hexose transport rate. It was shown that the intracellular pH as such determines the rate of uptake and not the pH difference between inside and outside; the transport rate did not correlate with delta pH.

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.

Energy requirements for maltose transport in yeast

Maltose transport in yeast (Saccharomyces cerevisiae) is inhibited by uncouplers under conditions where the intracellular concentration of the sugar is lower than in the medium. The uncouplers did not deplete the ATP content of the yeast cells and a 50--100-fold reduction in ATP caused by antimycin and 2-deoxyglucose had no effect on maltose transport. In ATP-depleted cells, the maltose transported is partially hydrolyzed to glucose but not further metabolized and therefore a mechanism of transport involving phosphorylation can be discarded. One proton is cotransported with every maltose molecule. The fact that maltose transport is inhibited by KCl but not by NaCl, Tris-Cl or KSCN suggest that the electroneutrality during maltose and proton uptake can be maintained by the exit of K+ from the cells or by the entry of a permeable anion as SCN-. These results indicate that the translocation of maltose across the yeast plasma membrane is not dependent on ATP and is coupled to the electrochemical gradient of protons in this membrane. When this gradient is abolished by uncouplers, the transport system is not able to function even in favour of a concentration gradient of the sugar.

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.

Temperature dependence of the energy-linked monosaccharide transport across the cell membrane of Rhodotorula gracilis

The temperature dependence of the active monosaccharide transport across the cell membrane of the yeast Rhodotorula gracilis has been studied between 0 and 55 degrees C with D-xylose as the transported substrate: (i) Between 0 and 10 degrees C there is virtually no transport. (ii) The initial velocity of transport increases exponentially from 15 to 30 degrees C (deltaE equal to 32 plus or minus 2 kcal/mol). (iii) At 30 degrees C a sharp "break" occurs in the Arrhenius plot and with increasing temperature the transport becomes inactivated, with a positive slope of the corresponding straight line ("deltaE equal to minus 15 kcal/mol"). (iv) In the temperature range of 50-55 degrees C, both the transport and the metabolic activity cease. In order to account for the abrupt changes of the membrane permeability, we attempted to ascribe them to phase transitions in the membrane structure: the first one, between 10 and 15 degrees C, to the crystalline: liquid-crystalline phase change; the second one, around 30 degrees C, to a change from highly ordered (low entropy) to less ordered (high entropy) membrane structure. Whereas the former phase transition is reversible, the latter appears to be irreversible. Arrhenius plots of the cell respiration exhibit a "break" at 30 degrees C, as well. However, at higher temperatures there is no thermal inactivation of the respiratory activity. The importance of a proper organization of the cell membrane constituents for the efficient transport function is discussed.