Cytosolic and mitochondrial l-malate dehydrogenases from ribbed mussel ( Modiolus demissus) gill tissue

Abstract

1. 1. The cytosolic and mitochondrial malate dehydrogenases (cMDH and mMDH) from ribbed mussel gill tissue are dimeric (native M r = 60,000–62,000) and electrophoretically and immunologically distinct. 2. 2. The kinetics of the purified mMDH and the electrophoretically slow-moving cMDH form (C1) were studied. 3. 3. At pH 8, the apparent K ms for NADH and oxaloacetate (OAA) were low (5–20 μM). 4. 4. At pH 9.2, the apparent K ms for malate were high (0.4–1 mM) and the apparent K ms for NAD (40–150 μM) were higher then the K ms for NADH. 5. 5. Inclusion of 10 mM MgCl 2 in assay mixtures with both MDHs increased the apparent K ms of all substrates under all conditions studied. 6. 6. Hydroxymalonate was a competitive inhibitor of OAA with the mMDH and a mixed, non-competitive inhibitor of OAA with the cMDH at pH 8; these apparent K i values (0.25–1.2 mM) were larger (15–70x) than the apparent K i for hydroxymalonate inhibition of the malic enzyme from this tissue (Bishop and Brodey, 1992). 7. 7. ATP was a weak non-competitive inhibitor of the mMDH ( K i = 5.38 mM) and a strong competitive inhibitor of the cMDH ( K i = 0.77 mM).