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PubMed1975

Maltose chemoreceptor of Escherichia coli.

Strains carrying mutations in the maltose system of Escherichia coli were assayed for maltose taxis, maltose uptake at 1 and 10 muM maltose, and maltose-binding activity released by osmotic shock. An earlier conclusion that the metabolism of maltose is not necessary for chemoreception is extended to include the functioning of maltodextrin phosphorylase, the product of malP, and the genetic control of the maltose receptor by the product of malT is confirmed. Mutants in malF and malK are defective in maltose transport at low concentrations as well as high concentrations, as previously shown, but are essentially normal in maltose taxis. The product of malE has been previously shown to be the maltose-binding protein and was implicated in maltose transport. Most malE mutants are defective in maltose taxis, and all those tested are defective in maltose transport at low concentrations. Thus, as previously suggested, the maltose-binding protein probably serves as the recognition component of the maltose receptor, as well as a component of the transport system. tsome malE mutants release maltose-binding activity and are tactic toward maltose, although defective in maltose transport, implying that the binding protein has separate sites for interaction with the chemotaxis and transport systems. Some mutations in lamB, whose product is the receptor for the bacteriophage lamba, cause defects in maltose taxis, indicating some involvement of that product in maltose reception.

Aspartic Acid↗
PubMed2002

Intracellular maltose is sufficient to induce MAL gene expression in Saccharomyces cerevisiae.

The presence of maltose induces M4L gene expression in Saccharomyces cells, but little is known abouthow maltose is sensed. Strains with all maltose permease genes deleted are unable to induce MAL geneexpression. In this study, we examined the role of maltose permease in maltose sensing by substituting a heterologous transporter for the native maltose permease. PmSUC2 encodes a sucrose transporter from the dicot plant Plantago major that exhibits no significant sequence homology to maltose permease. When expressed in Saccharomyces cerevisiae, PmSUC2 is capable of transporting maltose, albeit at a reduced rate. We showed that introduction of PmSUC2 restores maltose-inducible MAL gene expression to a maltose permease-null mutant and that this induction requires the MAL activator. These data indicate that intracellular maltose is sufficient to induce MAL gene expression independently of the mechanism of maltose transport. By usingstrains expressing defective mal61 mutant alleles, we demonstrated a correlation between the rate of maltose transport and the level of the induction, which is particularly evident in medium containing very limiting concentrations of maltose. Moreover, our results indicate that a rather low concentration of intracellular maltose is needed to trigger MAL gene expression. We also showed that constitutive overexpression of either MAL61 maltose permease or PmSUC2 suppresses the noninducible phenotype of a defective mal13 MAL-activator allele, suggesting that this suppression is solely a function of maltose transport activity and is not specific to the sequence of the permease. Our studies indicate that maltose permease does not function as the maltose sensor in S. cerevisiae.

Gene Expression Regulation, Fungal↗
PubMed1976

[Animal experiment studies on parenteral utilization of maltose].

The utilisation of parenterally administered maltose was investigated in the anaesthetized rat, and in rats fixed in metabolic cages. Additionally, the metabolism of maltose was measured with the isolated perfused rat liver. During intravenous infusion of 0.3 g, 0.6 g or 1.2 g maltose (corresponding to 0.9 g, 1.8 g or 3.6 g/kg bodyweight) per hour a steady-state for maltose in blood was attained. Blood glucose concentration rose during the maltose infusions. The utilisation of parenterally administered maltose was established by a high rate of glycogen storage in the liver and by a decrease in concentration of free fatty acids in serum. During the 72 hour infusion of maltose at a rate of 0.23 g/hour (corresponding to 0.70 g/kg bodyweight) a constant blood maltose concentration of 60 mg/100 ml was measured. Simultaneously, the blood glucose concentration increased. The excretion of maltose and of glucose was approximately 5% of the total amount administered intravenously. The nitrogen sparing effect of maltose was equal to that of glucose (or glucose substitutes). In the streptozotocindiabetic rats, the renal excretion of maltose and glucose was 20-30% of the total amount. Moreover, blood glucose concentration was elevated significantly during maltose infusion. In the isolated perfused rat liver maltose hydrolysis was established. However, the glucose obtained by this hydrolysis was not metabolized by the isolated organ as was observed for glucose. On the other hand, the glucose substitutes (fructose, xylitol, sorbitol) are also partially transformed to glucose by the isolated liver. These substances, however, were additionally utilized by this organ in other ways. On the basis of these results it is concluded that maltose is metabolized following its intravenous application. Therefore, maltose should have advantages compared to glucose because of the lower osmotic pressure. The metabolism of maltose, however, is similar to that of glucose. The advantages of the glucose substitutes (fructose, sorbitol, xylitol) are not shared by maltose.

Animals↗
PubMed2000

Protein phosphatase type-1 regulatory subunits Reg1p and Reg2p act as signal transducers in the glucose-induced inactivation of maltose permease in Saccharomyces cerevisiae.

The REG1 gene encodes a regulatory subunit of the type-1 protein phosphatase (PP1) G1c7 in Saccharomyces cerevisiae, which directs the catalytic subunit to substrates involved in glucose repression. Loss of REG1 relieves glucose repression of many genes, including the MAL structural genes that encode the maltose fermentation enzymes. In this report, we explore the role of Reglp and its homolog Reg2p in glucose-induced inactivation of maltose permease. Glucose stimulates the proteolysis of maltose permease and very rapid loss of maltose transport activity - more rapid than can be explained by loss of the permease protein alone. In a reg1delta strain we observe a significantly reduced rate of glucose-induced proteolysis of maltose permease, and the rapid loss of maltose transport activity does not occur. Instead, surprisingly, the slow rate of proteolysis of maltose permease is accompanied by an increase in maltose transport activity. Loss of Reg2p modestly reduces the rates of both glucose-induced proteolysis of maltose permease and inactivation of maltose transport activity. Overexpression of Reg2p in a reg1delta strain suppresses the effect on maltose permease proteolysis and partially restores the inactivation of maltose transport activity, but does not affect the insensitivity of MAL gene expression to repression by glucose observed in this strain. Thus, protein phosphatase type-1 (Glc7p-Reglp and Glc7p-Reg2p) plays a role in transduction of the glucose signal during glucose-induced proteolysis of maltose permease, but only Glc7p-Reglp is involved in glucose-induced inactivation of maltose transport activity and glucose repression of MAL gene expression. Overexpression of REG1 partially restores proteolysis of maltose permease in a grr1delta strain, which lacks glucose signaling, but does not rescue rapid inactivation of maltose transport activity or sensitivity to glucose repression. A model for the role of Reglp and Reg2p in glucose signaling pathways is discussed. We also uncovered a previously unrecognized G2/M delay in the grr1delta but not the reg1delta strains, and this delay is suppressed by REG1 overexpression. The G1/S delay seen in grr1delta mutants is slightly suppressed as well, but REG1 overexpression does not suppress other grr1delta phenotypes such as insensitivity to glucose repression.

Blotting, Western↗
PubMed1981

Formation and excretion of acetylmaltose after accumulation of maltose in Escherichia coli.

malB(+)malQ strains accumulate maltose via the maltose-binding-protein-dependent transport system but are unable to metabolize it. Nevertheless, some of the maltose is modified after entering the cell. This newly formed compound exhibited a higher R(f) value than did maltose upon thin-layer and paper chromatography with the usual sugar-separating solvents. Treatment of this compound with acid and alkali reformed maltose. The identity of this compound with acetylmaltose was derived from mass spectrometry. Nuclear magnetic resonance spectra of the compound confirmed the presence of the acetyl group but did not allow its precise location on the maltose moiety. However, linkage to the 1-position of maltose could be excluded. Analysis of the mass spectra indicated that the nonreducing end of maltose was acetylated. Other substrates of the maltose transport system, such as maltotetraose, maltopentaose, and maltohexaose, were also modified after accumulation into the cell. Several products were formed; the heterogeneity of these products was probably caused by different degrees of acetylation. The enzymatic activity responsible for maltose and maltodextrin acetylation is unknown. However, it is clear that the lacA-dependent thiogalactoside transacetylase was not necessary for the acetylation of maltose. Strains that accumulate maltose via a bypass of the normal malB-dependent transport system also acetylated maltose even in the absence of any malB gene products. Thus, the acetylating activity was not connected to the malB system. Acetylmaltose as well as acetylated maltodextrins was excreted into the medium. Acetylmaltose is not a substrate of the maltose transport system. Thus, maltose acetylation may be an effective detoxification mechanism.

ATP-Binding Cassette Transporters↗
PubMed1988

Source of the urinary maltose and maltotriose excreted during intravenous infusion of oligosaccharide solutions in young pigs.

Pigs infused with preparations of glucose oligosaccharides excrete sizeable quantities of maltose in urine despite the absence of maltose in the infused solution. Maltose infused without other oligosaccharides is well utilized. Our studies examined possible sources of excreted maltose. We first examined whether simultaneous infusion of larger oligosaccharides with maltose inhibits maltose utilization. Four young pigs were infused for four days with 20 g/d of a maltose-free oligosaccharide preparation to which tracer quantities of U-14C-maltose were added. Urinary excretion of maltose-plus-maltotriose increased significantly (P less than .05) from a mean +/- SD baseline value of .01 +/- .01 g/d to an overall four-day mean value of 1.33 +/- 0.47 g/d. However, only 10.7 +/- 0.78% of infused 14C-maltose was excreted, indicating that simultaneous infusion of larger oligosaccharides did not inhibit maltose utilization. The second study examined whether maltotriose present in the oligosaccharide mixture was the source of excreted maltose. Four young pigs were infused for three days with 20 g/d of a special preparation of oligosaccharides containing only trace quantities of maltose and maltotriose. Urinary maltose plus maltotriose excretion increased significantly (P less than .05) from a mean +/- SD baseline value of .01 +/- .01 g/d to 1.65 +/- 0.41 g/d during oligosaccharide infusion. The data suggest that excreted maltose and maltotriose arise from the catabolism of larger oligosaccharides.

Animals↗
PubMed1993

Energetics and kinetics of maltose transport in Saccharomyces cerevisiae: a continuous culture study.

In Saccharomyces cerevisiae, maltose is transported by a proton symport mechanism, whereas glucose transport occurs via facilitated diffusion. The energy requirement for maltose transport was evaluated with a metabolic model based on an experimental value of YATP for growth on glucose and an ATP requirement for maltose transport of 1 mol.mol-1. The predictions of the model were verified experimentally with anaerobic, sugar-limited chemostat cultures growing on a range of maltose-glucose mixtures at a fixed dilution rate of 0.1 h-1. The biomass yield (grams of cells.gram of sugar-1) decreased linearly with increasing amounts of maltose in the mixture. The yield was 25% lower during growth on maltose than during that on glucose, in agreement with the model predictions. During sugar-limited growth, the residual concentrations of maltose and glucose in the culture increased in proportion to their relative concentrations in the medium feed. From the residual maltose concentration, the in situ rates of maltose consumption by cultures, and the Km of the maltose carrier for maltose, it was calculated that the amount of this carrier was proportional to the in situ maltose consumption rate. This was also found for the amount of intracellular maltose. These two maltose-specific enzymes therefore exert high control over the maltose flux in S. cerevisiae in anaerobic, sugar-limited, steady-state cultures.

Adenosine Triphosphate↗