Glyoxysomal malate dehydrogenase and malate synthase from soybean cotyledons (Glycine max L.): enzyme association,antibody production and cDNA cloning

In order to investigate a possible association between soybean malate synthase (MS; l-malate glyoxylate-lyase, CoA-acetylating, EC 4.1.3.2) and glyoxysomal malate dehydrogenase (gMDH; (S)-malate: NAD⁺ oxidoreductase, EC 1.1.1.37), two consecutive enzymes in the glyoxylate cycle, their elution profiles were analyzed on Superdex 200 HR fast protein liquid chromatography columns equilibrated in low- and high-ionicstrength buffers. Starting with soluble proteins extracted from the cotyledons of 5-d-old soybean seedlings and a 45% ammonium sulfate precipitation, MS and gMDH coeluted on Superdex 200 HR (low-ionic-strength buffer) as a complex with an approximate relative molecular mass (M_r) of 670000. Dissociation was achieved in the presence of 50 mM KCl and 5 mM MgCl₂, with the elution of MS as an octamer of M_r 510000 and of gMDH as a dimer of M_r 73 000. Polyclonal antibodies raised to the native copurified enzymes recognized both denatured MS and gMDH on immunoblots, and their native forms after gel filtration. When these antibodies were used to screen a ZAP II expression library containing cDNA from 3-d-old soybean cotyledons, they identified seven clones encoding gMDH, whereas ten clones encoding MS were identified using an antibody to SDS-PAGE-purified MS. Of these cDNA clones a 1.8 kb clone for MS and a 1.3-kb clone for gMDH were fully sequenced. While 88% identity was found between mature soybean gMDH and watermelon gMDH, the N-terminal transit peptides showed only 37% identity. Despite this low identity, the soybean gMDH transit peptide conserves the consensus R(X₆)HL motif also found in plant and mammalian thiolases.The nucleotide sequence data reported in this paper have been submitted to Genbank and assigned the accession numbers LOI628 for gMDH and L01629 for MS.