Biomedical subjects

K Ring

Publications and source records attributed to K Ring.

At least 37 records · Page 2Linked to original sources

PubMed1977

Characterization of L-aspartate uptake by Streptomyces hydrogenans.

Multiple transport systems for L-aspartic acid exist in Steptomyces hydrogenans. The intracellular accumulation of L-aspartate against a concentration gradient was immediately inhibited by proton conductors, such as carbonyl cyanide p-trifluoromethoxyphenylhydrazone, 2,4-dinitrophenol or nigericin. Transport activity was gradually lost when inhibitors of protein synthesis were added. L-Aspartate transport had two pH optima at 6.5 and 4.5. At pH 6.5, two saturable transport components with different Km and Vmax values could be resolved by kinetic studies. A high-affinity system (system I) preferred the L-isomers of the anionic forms of aspartic and glutamic acid. At the same pH, a second, low-affinity system (system II) operated, which was presumably less specific than system I and also able to accept, at high concentrations, neutral amino acids. At pH 4.5, the Lineweaver-Burk plot revealed only a single catalytic component, with Km and Vmax values similar to those of system II. Again, in contrast to system I, this component showed high affinity for neutral amino acids. The data suggest that L-aspartic acid and L-glutamic acid are transported by this system as neutral zwitterionic molecules.

Amino Acids↗

PubMed1976

Regulation of amino acid transport in growing cells of Streptomyces hydrogenans. II. Correlation between transport capacity and growth rate in chemostat cultures.

The uptake of various amino acids into Streptomyces hydrogenans grown in chemostatically and turbidostatically controlled steady state cultures has been investigated. A close correlation between transport capacity and the growth rates of the cells was found. As shown by kinetic analysis, the increased transport is due to elevated maximum uptake rates, the apparent Michaelis constants remaining unchanged. Analysis of the unidirectional fluxes of cycloleucine revealed that not only the influx is raised as the growth rate is increased but also the efflux. Hence, the conclusion is drawn that the growth-rate dependent modulation of transport capacity is, at least, partially due to the variation of the concentration of active transport components. Since the cells were grown in the absence of external amino acids the results suggest that amino acid transport into S. hydrogenans is under control of endogenous effectors.

Amino Acids↗

PubMed1976

Regulation of amino acid transport in growing cells of Streptomyces hydrogenans. I. Modulation of transport capacity and amino acid pool composition during the growth cycle.

(1) The active uptake of different amino acids by growing cells of Streptomyces hydrogenans was shown to be correlated with the physiological age of the cells. During the lag phase of growth the transport capacity increased and attained its highest level when the growth rate was maximum. During further growth the transport capacity declined progressively. The lowest transport activity was observed when the culture shifted into the stationary growth phase. (2) Such modulation of transport capacity was independent on the presence or absence of amino acids in the growth medium of the cells. (3) The size and the composition of the pool of free intracellular amino acids was also undergoing substantial variations during the growth cycle of the culture. In the lag phase, the levels of all amino acids decreased markedly and attained their lowest values at the end of this phase. During further growth the pool size was slowly replenished. (4) Removal of the pool resulted in a considerable gain of transport capacity. Therefore, it was concluded that active amino acid transport in growing Streptomyces hydrogenans is under feedback control by intracellular amino acids. (5) Quantitatively, the modulation of the pool size could not fully account for the variation of the transport capacity. Since a pool-independent stimulation of transport was found to be correlated with the increase of the growth rate of the cells, the possibility is discussed that the stimulation of transport is either due to increased levels of distinct RNA species, which might provide positive feedback signals for transport, or by increased rates of de novo synthesis of transport limiting proteins.

Amino Acids↗

PubMed1976

Effect of alkali ions on the active transport of neutral amino acids into Streptomyces hydrogenans.

The active transport of neutral amino acids into Streptomyces hydrogenans is inhibited by external Na+. There is no indication that in these cells amino acid accumulation is driven by an inward gradient of Na+. The extent of transport inhibition by Na+ depends on the nature of the amino acid. It decreases with increasing chain length of the amino acid molecules i.e. with increasing non-polar properties of the side chain. Kinetic studies show that Na+ competes with the amino acid for a binding site at the amino acid carrier. There is a close relation between the Ki values for Na+ and the number of C atoms of the amino acids. Other cations also inhibit neutral amino acid uptake competitively; the effectiveness decreases in the order Li+ greater than Na+ greater than K+ greater than Rb+ greater than Cs+. Anions do not have a significant effect on the uptake of neutral amino acids. After prolonged incubation of the cells with 150 mM Na+, in addition to the competitive inhibition of transport Na+ induces an increase in membrane permeability for amino acids.

Amino Acids↗

PubMed1976

Influence of the flavonoid (+)-catechin on the permeability of Ehrlich mouse ascites tumour cell membranes.

The effect of the flavonoid (+)-catechin and its hydrophilic derivative epicatechinsulphonate on the permeability of Ehrlich mouse ascites tumour cells (EMAT) was investigated. As a sensitive assay, the transmembrane fluxes of two different solutes were measured, the inwardly directed free diffusion of 14C-thiourea, and the carrier-mediated efflux of intracellularly accumulated 12C-1-aminocyclopentane-1-carboxylic acid. (+)-Catechn was found to reduce the permeability of EMAT membranes for both solutes. The primary target of the drug appears to be the membrane itself. The effectiveness of the drug was dependent on its concentration. Inhibition of fluxes was observed at 0.86 mM; the inhibition gradually increased as the concentration was increased. In contrast to (+)-catechin, epicatechinsulphonate was rather ineffective, even at a concentration as large as 10 mM. The effect of (+)-catechin was seen within a few minutes after its addition. However, it was considerably intensified as the incubation was prolonged. The effectivity of (+)-catechin decreased with increasing cell density. Thus, the drug appears to be absorbed by the cells. From the various data and the observation that incorporation of a strong lipophobic sulphonate residue into the moderately lipophilic catechin molecule markedly lowers the effectivity of the flavonoid, it is concluded that (+)-catechin, as a membrane stabilizing drug, interacts directly with certain constituents of the cell envelope, presumably membrane lipids.

Adsorption↗

PubMed1976

[Interrelationship between the transport of L-aspartate and potassium ions into the cell (author's transl)].

The mechanisms involved in active transport and intracellular accumulation of amino acids have been reviewed. In particular, the frequently observed interrelationship between the transport of acidic amino acids and potassium ions was discussed. Kinetic studies on the uptake of radioactive L-aspartate and K+ in the microorganism Streptomyces hydrogenans were performed. The following results were obtained:1. L-Aspartate was actively transported into the cells. However, only a part of the aspartate taken up from the medium remained in the pool as free amino acid. Within 60 min, up to 35% of the label was incorporated into protein. By thin-layer chromatography of cell extracts several radioactive metabolites of aspartate were detected. 2. Aspartate was transported by a t least two different uptake systems exhibiting moderate specificity. At neutral pH , the amino acid was transported as anion; its uptake was inhibited by L-glutamate as well as by dicarboxylic acids, whereas neutral amino acids did not have a significant effect. 3. The influx of aspartate into K+-rich cells was stimulated specifically by extracellular Rb+ and K+, whereas Ki+ and Na+ inhibited aspartate transport. 4. Kinetic analysis of the aspartate influx showed that extracellular K+ increased the affinity of the transport systems for aspartate by a factor of three. These results suggest that K+ is bound by the aspartate carrier and is cotransported together with the amino acid across the membrane. 5. Kinetic measurements of the uptake of 42K+ revealed that the influx of K+ as well was stimulated by extracellular aspartate. Likewise the rate of 28Mg2+ uptake was increased by aspartate.

Amino Acids↗

PubMed1974