Malate dehydrogenase species in the cytosolic fraction of chicken liver

Cloning, sequencing and functional expression of a DNA encoding pig cytosolic malate dehydrogenase: purification and characterization of the recombinant enzyme

Using the polymerase chain reaction, DNA encoding cytosolic malate dehydrogenase (cMDH) has been cloned from a pig heart cDNA library. Large amounts of the enzyme (30 mg per litre of original culture) have been produced in Escherichia coli using an inducible expression vector (pKK223-3) in which the 5'-non-coding region of the gene was replaced with the tac promoter. The complete nucleotide sequence of the DNA is reported for the first time. The recombinant cMDH purified was shown to be identical to the native enzyme according to: chromatographic behaviour, isoelectric point, N-terminal amino acid sequence, and physiochemical and catalytic properties.

An improved purification method for cytosolic malate dehydrogenase from several sources

A new purification method for cytosolic malate dehydrogenases from several sources has been developed. The procedure, employing chromatographies on 5'AMP-Sepharose, DEAE-Sephacel and Blue-Sepharose, allows for a rapid isolation of the enzyme (approximately 40 hours), in large quantities, with good yields (45-54%). The specific activity of final preparations were around 1300 I.U./mg and were judged homogeneous by polyacrylamide gradient gel and sodium dodecyl sulfate polyacrylamide gel electrophoresis, high performance size exclusion chromatography and isoelectric focusing.

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.

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.