Purification of malate dehydrogenase from chicken liver mitochondria. Existence of a small quantity of cytosolic isoenzyme

1. A new purification method for chicken liver mitochondrial malate dehydrogenase is described. The application of affinity chromatography through 5'AMP-Sepharose and Blue-Sepharose permits to obtain homogeneous preparations, with good yields (47%), in a short time (48 hr). 2. The 5'AMP-Sepharose chromatography reveals the presence of two malate dehydrogenase species in the mitochondrial extracts. 3. A comparative study of these forms point out the cytosolic nature of the minority form and suggests that its presence could be due to a slight interaction of the cytosolic malate dehydrogenase with mitochondrial membranes.

Simultaneous purification and characterization of aspartate aminotransferase isoenzymes from chicken liver

Cytosolic and mitochondrial isoenzymes of aspartate aminotransferase (EC 2.6.1.1) were purified to homogeneity from chicken liver, without previous fractionation of the subcellular components. The procedure includes initial heat treatment and ammonium sulfate fractionation. The two isoenzymes can then be separated by a DEAE-Sepharose chromatography using a linear gradient of L-aspartate (reaction substrate). The separated fractions can be further purified by a parallel step with HA-Ultrogel prior to octyl-Sepharose (c-AAT) and CM-Sepharose (m-AAT) chromatographies. Michaelis constants, pI values, inhibition by adipate and subforms generation with time were studied for both isoenzymes.

Purification and comparative studies of several mitochondrial aspartate aminotransferases from avian liver

A new purification method has been developed which only exploits the chromatographic behaviour of avian liver mitochondrial aspartate aminotransferase enzymes (m-AAT), and permits a rapid isolation of the protein (4 days) in large quantities with high yield and low cost. m-AAT from turkey, chicken and quail livers have been isolated by chromatography on CM-Sepharose, Sephadex G-100 and 5' AMP-Sepharose using TEA-acetate buffer (pH 7.4), and specific activities (A.E.) of 311.6, 318.9, 320.1 I.U./mg respectively were obtained. Preparations were homogeneous as judged by various electrophoretic techniques and by size exclusion HPLC. The amino acid composition, Stokes Radius, subunit molecular weight and pI values have been determined and compared, finding no appreciable differences among them. In contrast, the absorption spectrum of the turkey enzyme differed from those of chicken and quail at both pH 7.4 and pH 5.0.

A kinetic method for quantification of aspartate aminotransferase isoenzymes

A spectrophotometric assay is proposed to determine the levels of aspartate aminotransferase (AAT) isoenzymes from chicken liver by a steady-state kinetic method which depends on the differential inhibition of these isoenzyme forms by high concentrations of substrate 2-oxoglutarate at pH 6.2. The use of a standard curve permits the determination of the percentage of chicken liver c-AAT and m-AAT isoenzymes. This method yields results in good correlation with those achieved by different extent adipate inhibition and by differential centrifugation.

Microheterogeneity of the malate dehydrogenase from several sources

Different homogeneously purified cytosolic malate dehydrogenases gave, on isoelectric focusing, several active bands. The phenomenon could not be assigned to differences in their molecular weights or to alterations in the enzyme preparations during the purification procedure. Resolution of the multiple malate dehydrogenase active bands was achieved by chromatofocusing. The aged isolated subforms always yielded the original electrofocusing pattern. This fact suggests that conformational isomerism is a likely explanation for the charge heterogeneity of the enzymes studied.

Intramitochondrial location and some characteristics of chicken liver aspartate aminotransferase

Chicken liver mitochondrial aspartate aminotransferase was found to be located in the intermembrane space and bound to the inner mitochondrial membrane. Purification of two mitochondrial fractions containing aspartate aminotransferase activity was performed. Both fractions showed similar chromatographic behaviour and identical isoelectric point and molecular weight values. There were no significant differences in the general kinetic mechanism, Km values, substrates inhibition and effect of various anions on the activity of mitochondrial aspartate aminotransferase purified from both fractions.

Factors affecting malate dehydrogenase activity in freezing-thawing processes

Malate dehydrogenases from several sources show different behaviour when frozen-thawed in 100 mM sodium phosphate buffer, pH 7.4, containing chaotropic ions. The effects produced by the addition of various metabolites, protein concentration and buffer medium used on the loss of activity induced by the freezing-thawing process are reported. The major part of the loss of activity is caused by the formation of "wrong" aggregates of high mol. wt.

Comparative analysis of the malate dehydrogenases isolated from the cytosolic fraction of several tissues of guinea-pig Cavia porcellus

The specific activities of the malate dehydrogenase and lactate dehydrogenase present in the soluble fraction of several guinea-pig tissues are reported. The electrophoretic patterns showed always two forms (A and B) with malate dehydrogenase activity and the five isoenzymes of lactate dehydrogenase. Chromatography of the different soluble fractions through 5' AMP-Sepharose allowed both molecular forms of malate dehydrogenase to be separated and obtained free from lactate dehydrogenase. Comparative studies of the two forms of malate dehydrogenase evidenced that the A and B forms exhibited cytosolic and mitochondrial characteristics, respectively.

Malate dehydrogenase species in the cytosolic fraction of chicken liver

The malate dehydrogenase activity in the cytosolic fraction isolated from chicken hepatocytes is resolved by DEAE-Sephacel chromatography in three active, electrophoretically distinct, species obtained in homogeneous form by affinity chromatography on 5'-AMP-Sepharose and Blue-Sepharose. Two of those species, according to the results obtained, might represent different conformational isomers of the enzyme molecule. Their purified preparations show identical amino-acid compositions and physico-chemical properties very similar to those of the cytosolic isoenzyme of other sources. The third one corresponds to a slight contamination of the mitochondrial isoenzyme.

Separation and kinetic properties of the molecular forms of chicken liver cytoplasmic aspartate aminotransferase

A method is proposed for the separation of the five molecular forms, alpha, beta, gamma, delta and epsilon, of chicken liver cytoplasmic aspartate aminotransferase free from lactate dehydrogenase activity. These molecular forms varied in isoelectric point, but no differences were observed either in their Michaelis constants or in the degree of their inhibition by excess of 2-oxoglutarate or L-aspartate.

Effect of phosphate and other inorganic anions on the activity of chicken liver cytosolic aspartate aminotransferase

Chicken liver aspartate aminotransferase was inhibited by several inorganic anions. The inhibitory effect of the anions was related to their chaotropic character. Apparent Km (2-oxoglutarate) and Km (L-aspartate) values depended on the molarity of the buffer. The profile of the curves obtained did not depend on the nature of the enzyme sample assayed. Phosphate slightly inhibited the holoaspartate aminotransferase and was a strong inhibitor of apoaspartate aminotransferase with respect to pyridoxal phosphate.

Modification of the kinetic parameters of chicken liver cytoplasmic aspartate aminotransferase by lactate dehydrogenase

A method for the purification of chicken liver soluble aspartate aminotransferase, lactate dehydrogenase free, is proposed. The preparation, which contained a mixture of the five molecular forms of the enzyme, showed a 120-fold increase in specific activity, with respect to the initial homogenates. Differences in Km(2-oxoglutarate) and saturating concentrations among solutions of purified enzyme and soluble fraction were due to the 2-oxoglutarate reductase activity of lactate dehydrogenase.

Chromatographic behaviour of the molecular forms of guinea-pig skeletal muscle cytoplasmic malate dehydrogenase

The isolated molecular forms of guinea-pig skeletal muscle cytoplasmic malate dehydrogenase have a different chromatographic behaviour through affinity or hydrophobic interaction gels; in all cases the retention of the B form is more noticeable. Chromatography of a partly purified preparation through 5' AMP-Sepharose allows both molecular forms of malate dehydrogenase to be separated and obtained free from lactate dehydrogenase.

Kinetic mechanism of the molecular forms of chicken liver mitochondrial malate dehydrogenase

1. The reaction kinetic mechanism (pH 7.4) of the molecular forms of chicken liver m-MDH is of the order bi-bi ternary complex type with the existence of the E-oxaloacetate, E-L-malate, E-NAD+-oxaloacetate, E-NADH-L-malate, E-NAD+-NADH, E-NAD+-NAD+, E-NADH-NAD+ and E-NAD-NADH abortive complexes. 2. The saturating concentration values of the substrates are notably modified, in certain cases, in the presence of the reaction products.

Intramitochondrial location of the molecular forms of chicken liver mitochondrial malate dehydrogenase

1. The two molecular forms of mitochondrial malate dehydrogenase are partly bound to the mitochondrial membranes. 2. The A form is located on the outer surface of the inner mitochondrial membrane and also in the intermembrane space. 3. The B form of the enzyme appears in the matrix and bound in part, probably, to the inner surface of the inner mitochondrial membrane. 4. Glutamate dehydrogenase, glutamate oxaloacetate transaminase, fumarase and lactate dehydrogenase are bound, to a greater or lesser extent, to the mitochondrial membranes, the fumarase having the highest degree of binding.