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

Abstract

The origin of mouse lactate dehydrogenase (LDH) subbands 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 subbands were also examined. There was no difference between A²₄ and A³₄ in the Km for pyruvate and for lactate. Thermostability at 56°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 preferentially recombined.