Lactate dehydrogenase: electrophoretic behaviour, electron microscopy and structure

Tetrameric and octameric lactate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima. Structure and stability of the two active forms

Lactate dehydrogenase from the hyperthermophilic bacterium Thermotoga maritima has been functionally expressed in Escherichia coli. As shown by gel-permeation chromatography, dynamic light scattering, and ultracentrifugation, the recombinant protein forms homotetrameric and homooctameric assemblies with identical spectral properties and a common subunit molecular mass (35 kDa). Dynamic light scattering and sedimentation equilibrium experiments proved that both species are monodisperse, thus excluding their interconversion in the given ranges of concentration (0.02-50 mg/ml) and temperature (20-80 degrees C). Rechromatography confirms this finding: the octamer does not dissociate at low enzyme concentrations, nor do tetramers dimerize at the given upper limit of concentration. Renaturation of pure tetramers or octamers after preceding guanidine denaturation leads to redistribution of the two species; increased temperature favors octamer formation. Thermal analysis and denaturation by chaotropic agents do not allow the free energies of stabilization of the two forms to be quantified, because heat coagulation and kinetic partitioning between reconstitution and aggregation causes irreversible side reactions. Guanidine denaturation of the octamer leads to a highly cooperative dissociation to tetramers which subsequently dissociate and unfold to yield metastable dimers and, finally, fully unfolded monomers. Evidently, there is no tight coupling of the two tetramers within the stable octameric quaternary structure. Electron microscopy clearly corroborates this conclusion: image processing shows that the dumb-bell-shaped octamer is made up of two tetramers connected via surface contacts without significant changes in the dimensions of the constituent parts.

Epigenetic formation of lactate dehydrogenase isozymes in the house mouse, Mus musculus

The origin of mouse lactate dehydrogenase (LDH) sub-bands was investigated by using our miniaturized polyacrylamide gel electrophoretic apparatus. Mouse LDH isozymes are generated by combinations of three types of A subunit, the primary type and two epigenetically modified forms. These are designated A1, A2, and A3 in the order of their electrophoretic mobilities towards the anode. The A1 subunit arises from the covalent binding of molecules of glutathione through disulfide bonds to the original subunit, A3. The A2 subunit arises from the covalent binding of molecules of cysteine through disulfide bonds to the A3 subunit. All isozymes can be explained as tetramers composed of the three kinds of A subunit (A1, A2, or A3) in combination with B subunits to yield a total of 35 isozymes. The kinetic properties of these sub-bands were also examined. There was no difference between A24 and A34 in the Km for pyruvate and for lactate. Thermostability at 56 degrees C was greater for A34 than for A24. The activities of tetramers at the electrophoretic position of A3B1 and A4 in extracts containing all five isozymes were increased by treatment of the extracts with high concentrations of reduced glutathione or cysteine with the concomitant disappearance or decrease in activity of tetramers at the position of B4 and A3B1. These results suggest that, in the presence of reduced glutathione or cysteine, LDH isozymes containing the B subunit are first dissociated and then the A subunits are preferentaially recombined.

Gel isoelectric focusing of mouse lactate dehydrogenase: heterogeneity of the isoenzymes A4 and X4

LDH of mouse organs (including testis) was investigated by isoelectric focusing in polyacrylamide gels. The number of LDH bands in this pattern considerably exceeds the five (six in testis) of the standard electrophoretic pattern. An attempt was made to identify these bands as tetrameric isoenzymes formed by random association of different subunits. This included isoelectric focusing of purified LDH A, B, and X, two-dimensional separation of LDH, urea treatment of LDH, staining with specific substrates, and comparison of different organs. Further experiments were performed to exclude artifacts possibly produced by the isoelectric focusing technique. Different strains of mice were also investigated. The results demonstrate that in addition to the common five LDH bands (A4-B4) one set of five bands is formed by LDH A (A41-A42) and another one by LDH X (X 41-X42). Moreover, an unusual band was found which has a lower molecular weight and no affinity to the other isoenzymes. The data suggest that the heterogeneity of the LDH pattern revealed by isoelectric focusing arises from post-transcriptional events rather than from a number of additional genes.