Loading...
(1/99) Roles of his205, his296, his303 and Asp259 in catalysis by NAD+-specific D-lactate dehydrogenase.

The role of three histidine residues (His205, His296 and His303) and Asp259, important for the catalysis of NAD+-specific D-lactate dehydrogenase, was investigated using site-directed mutagenesis. None of these residues is presumed to be involved in coenzyme binding because Km for NADH remained essentially unchanged for all the mutant enzymes. Replacement of His205 with lysine resulted in a 125-fold reduction in kcat and a slight lowering of the Km value for pyruvate. D259N mutant showed a 56-fold reduction in kcat and a fivefold lowering of Km. The enzymatic activity profile shifted towards acidic pH by approximately 2 units. The H303K mutation produced no significant change in kcat values, although Km for pyruvate increased fourfold. Substitution of His296 with lysine produced no significant change in kcat values or in Km for substrate. The results obtained suggest that His205 and Asp259 play an important role in catalysis, whereas His303 does not. This corroborates structural information available for some members of the D-specific dehydrogenases family. The catalytic His296, proposed from structural studies to be the active site acid/base catalyst, is not invariant. Its function can be accomplished by lysine and this has significant implications for the enzymatic mechanism.  (+info)

(2/99) Identification of an archaeal 2-hydroxy acid dehydrogenase catalyzing reactions involved in coenzyme biosynthesis in methanoarchaea.

Two putative malate dehydrogenase genes, MJ1425 and MJ0490, from Methanococcus jannaschii and one from Methanothermus fervidus were cloned and overexpressed in Escherichia coli, and their gene products were tested for the ability to catalyze pyridine nucleotide-dependent oxidation and reduction reactions of the following alpha-hydroxy-alpha-keto acid pairs: (S)-sulfolactic acid and sulfopyruvic acid; (S)-alpha-hydroxyglutaric acid and alpha-ketoglutaric acid; (S)-lactic acid and pyruvic acid; and 1-hydroxy-1,3,4,6-hexanetetracarboxylic acid and 1-oxo-1,3,4, 6-hexanetetracarboxylic acid. Each of these reactions is involved in the formation of coenzyme M, methanopterin, coenzyme F(420), and methanofuran, respectively. Both the MJ1425-encoded enzyme and the MJ0490-encoded enzyme were found to function to different degrees as malate dehydrogenases, reducing oxalacetate to (S)-malate using either NADH or NADPH as a reductant. Both enzymes were found to use either NADH or NADPH to reduce sulfopyruvate to (S)-sulfolactate, but the V(max)/K(m) value for the reduction of sulfopyruvate by NADH using the MJ1425-encoded enzyme was 20 times greater than any other combination of enzymes and pyridine nucleotides. Both the M. fervidus and the MJ1425-encoded enzyme catalyzed the NAD(+)-dependent oxidation of (S)-sulfolactate to sulfopyruvate. The MJ1425-encoded enzyme also catalyzed the NADH-dependent reduction of alpha-ketoglutaric acid to (S)-hydroxyglutaric acid, a component of methanopterin. Neither of the enzymes reduced pyruvate to (S)-lactate, a component of coenzyme F(420). Only the MJ1425-encoded enzyme was found to reduce 1-oxo-1,3,4,6-hexanetetracarboxylic acid, and this reduction occurred only to a small extent and produced an isomer of 1-hydroxy-1,3,4,6-hexanetetracarboxylic acid that is not involved in the biosynthesis of methanofuran c. We conclude that the MJ1425-encoded enzyme is likely to be involved in the biosynthesis of both coenzyme M and methanopterin.  (+info)

(3/99) Maitotoxin-induced membrane blebbing and cell death in bovine aortic endothelial cells.

BACKGROUND: Maitotoxin, a potent cytolytic agent, causes an increase in cytosolic free Ca2+ concentration ([Ca2+]i) via activation of Ca2+-permeable, non-selective cation channels (CaNSC). Channel activation is followed by formation of large endogenous pores that allow ethidium and propidium-based vital dyes to enter the cell. Although activation of these cytolytic/oncotic pores, or COP, precedes release of lactate dehydrogenase, an indication of oncotic cell death, the relationship between CaNSC, COP, membrane lysis, and the associated changes in cell morphology has not been clearly defined. In the present study, the effect maitotoxin on [Ca2+]i, vital dye uptake, lactate dehydrogenase release, and membrane blebbing was examined in bovine aortic endothelial cells. RESULTS: Maitotoxin produced a concentration-dependent increase in [Ca2+]i followed by a biphasic uptake of ethidium. Comparison of ethidium (Mw 314 Da), YO-PRO-1 (Mw 375 Da), and POPO-3 (Mw 715 Da) showed that the rate of dye uptake during the first phase was inversely proportional to molecular weight, whereas the second phase appeared to be all-or-nothing. The second phase of dye uptake correlated in time with the release of lactate dehydrogenase. Uptake of vital dyes at the single cell level, determined by time-lapse videomicroscopy, was also biphasic. The first phase was associated with formation of small membrane blebs, whereas the second phase was associated with dramatic bleb dilation. CONCLUSIONS: These results suggest that maitotoxin-induced Ca2+ influx in bovine aortic endothelial cells is followed by activation of COP. COP formation is associated with controlled membrane blebbing which ultimately gives rise to uncontrolled bleb dilation, lactate dehydrogenase release, and oncotic cell death.  (+info)

(4/99) Functional replacement of the Escherichia coli D-(-)-lactate dehydrogenase gene (ldhA) with the L-(+)-lactate dehydrogenase gene (ldhL) from Pediococcus acidilactici.

The microbial production of L-(+)-lactic acid is rapidly expanding to allow increased production of polylactic acid (PLA), a renewable, biodegradable plastic. The physical properties of PLA can be tailored for specific applications by controlling the ratio of L-(+) and D-(-) isomers. For most uses of PLA, the L-(+) isomer is more abundant. As an approach to reduce costs associated with biocatalysis (complex nutrients, antibiotics, aeration, product purification, and waste disposal), a recombinant derivative of Escherichia coli W3110 was developed that contains five chromosomal deletions (focA-pflB frdBC adhE ackA ldhA). This strain was constructed from a D-(-)-lactic acid-producing strain, SZ63 (focA-pflB frdBC adhE ackA), by replacing part of the chromosomal ldhA coding region with Pediococcus acidilactici ldhL encoding an L-lactate dehydrogenase. Although the initial strain (SZ79) grew and fermented poorly, a mutant (SZ85) was readily isolated by selecting for improved growth. SZ85 exhibited a 30-fold increase in L-lactate dehydrogenase activity in comparison to SZ79, functionally replacing the native D-lactate dehydrogenase activity. Sequencing revealed mutations in the upstream, coding, and terminator regions of ldhL in SZ85, which are presumed to be responsible for increased L-lactate dehydrogenase activity. SZ85 produced L-lactic acid in M9 mineral salts medium containing glucose or xylose with a yield of 93 to 95%, a purity of 98% (based on total fermentation products), and an optical purity greater than 99%. Unlike other recombinant biocatalysts for L-lactic acid, SZ85 remained prototrophic and is devoid of plasmids and antibiotic resistance genes.  (+info)

(5/99) Glutamate 264 modulates the pH dependence of the NAD(+)-dependent D-lactate dehydrogenase.

Recently, we amplified the Lactobacillus bulgaricus NAD(+)-dependent D-lactate dehydrogenase gene by the polymerase chain reaction, cloned and overexpressed it in Escherichia coli (Kochhar, S., Chuard, N., and Hottinger, H. (1992) Biochem. Biophys. Res. Commun. 185, 705-712). Polymerase chain reaction-amplified DNA fragments may contain base changes resulting in mutant gene products. A comparison of specific activities of D-lactate dehydrogenase in the crude extracts of 50 recombinant clones indicated that one of the clones had drastically reduced enzyme activity. Nucleotide sequence analysis of the insert DNA showed an exchange of A to G at position 795 resulting in substitution of Glu264 to Gly in the D-lactate dehydrogenase. The purified mutant D-lactate dehydrogenase showed a shift of 2 units in its optimum pH toward the acidic range. The dependence of kcat/Km on the pH of the mutant enzyme showed that the pKa of the free enzyme was around 4, at least 2 pH units lower than that of the wild-type enzyme. Both the wild-type and the mutant enzyme at their respective optimum pH values showed similar kcat and Km values. The data suggest that the highly conserved Glu264 is not critical for enzyme catalysis, but it must be situated within hydrogen bonding distance to amino acid residue(s) involved in substrate binding as well as in catalysis.  (+info)

(6/99) The level of lactic dehydrogenase activity as an indicator of the grwoth of influenza virus in the embryonate egg.

The relation, between time and the levels of lactic dehydrogenase activity of chorioallantoic fluids from embryonate eggs infected with the PR8 strain of influenza virus were determined quantitatively. The mean values, based on 10 determinations for each time interval, followed a sigmoid curve, with the greatest rates of change occurring between 48 and 72 hours after the inoculation of virus. The activities of the fluids from infected eggs at the 72nd hour or later were approximately 18 times higher than those from non-infected eggs. Based on the data above, a qualitative test for the presence of infection with influenza virus was developed.  (+info)

(7/99) Molecular heterogeneity of lactic dehydrogenase in avian malaria (Plasmodium lophurae).

Lactic dehydrogenase activity increased in direct proportion to the degree of parasitization in synchronous infections of duck erythrocytes. Deviations from this linearity could be accounted for on the basis of the developmental stage of the parasite. Erythrocyte-free P. lophurae showed activities which averaged 3 times that of uninfected erythrocytes, whereas infected erythrocytes had intermediate values. In addition, a patent infection was generally reflected by an increase in the lactic dehydrogenase activity in the plasma, but no direct correlation with parasitemia was established. Molecular heterogeneity of the enzyme was determined on the basis of kinetic data and electrophoretic isolation on a starch block. The uninfected red blood cell showed a major anodal and a minor cathodal peak of lactic dehydrogenase activity, and was further characterized by a kinetic constant representing a high pH optimum with low concentrations of substrate. Isolated P. lophurae had a single, cathodal peak of activity dissimilar from that of the uninfected erythrocyte, and a kinetic constant describing a low pH optimum with a high concentration of substrate. Infected erythrocytes showed a combination of these electrophoretic entities and an intermediate range of kinetic constants. The data indicate that the avian malaria parasite P. lophurae contains a lactic dehydrogenase qualitatively dissimilar from that of its host cell, and the increased enzymatic activity of infected erythrocytes is a result of the enzyme content of the growing parasite added to that of the red blood cell. It is suggested that the LDH of the parasite has a physiological advantage under those conditions which prevail inside the red blood cell.  (+info)

(8/99) Variations in the lactic dehydrogenase of vertebrate erythrocytes.

Erythrocytes from representatives of the 5 classes of vertebrates revealed a marked species variation in the number of LDH isozymes, in the distribution of the total LDH activity among these isozymes, and in their electrophoretic mobilities. Starch gel electrophoresis of hemolysates followed by direct histochemical demonstration of LDH activity with nitro blue tetrazolium as dye and phenazine methosulfate as electron transporter showed that closely related species exhibited similar LDH patterns. The rhesus monkey had LDH isozymes of similar pattern to those of human hemolysate but slightly slower in electrophoretic mobility. The goat and sheep each had 1 band of LDH activity in their erythrocytes of identical electrophoretic mobility, whereas the single band in steer hemolysate migrated slightly faster. The 5 bands of chicken hemolysate were quite similar in pattern to the 5 bands of duck hemolysate but migrated slightly faster and exhibited a different distribution of the total LDH activity. The 2 species of snake each had 1 band of LDH activity with identical mobility. Staining occurred with the levorotatory form of the substrate and not with the dextrorotatory form. Examination of more than 380 human hemolysates failed to reveal any differences among individuals in the main LDH bands. The genetic basis for the species differences in erythrocyte lactic dehydrogenases is discussed.  (+info)