Malate dehydrogenase isoforms from ribbed mussel ( Modiolus demissus) gill tissue

Abstract

1. 1. The malate dehydrogenase (MHD) activity from the ribbed mussel gill is polymorphic with two distinct mitochondrial forms (M1 and M2) and five forms that could be resolved from cytosolic extracts (C1 to C5) by DEAE-cellulose chromatography and starch gel electrophoresis. 2. 2. Two of the cytosolic forms (C3 and C4) may represent interchangeable conformational states. 3. 3. With kinetic analysis there appear to be three distinct cytosolic forms (C1, C2 and C3–C4), with C2 possibly behaving as a heterodimer. 4. 4. The identity of C5 is uncertain. 5. 5. The forms isolated from the mitochondria (M1 and M2) exhibited lower apparent K ms for oxaloacetate (OAA) than the cytosolic forms. 6. 6. For all isozymic forms, the apparent K ms for OAA increased as the pH increased between pH 6 and 9 7. 7. Increasing the salt concentration raised the K m for OAA for all forms. 8. 8. The mMDHs were more sensitive to inhibition by NaCl than the cMDHs. 9. 9. Representative cMDH (C1) and mMDH (M2) isozymes exhibited substrate inhibition by high concentrations of OAA with the mMDH possessing lower K is for substrate inhibition than the cMDH at each pH tested. 10. 10. Differences and similarities in K m app. for OAA at the different pHs and salt concentrations indicated that C1, C2 and C3–C4 and C5 were distinct forms, that M1 and M2 were distinct but very similar to each other, and that C1, C2, C3–C4 and C5 were distinct from M1 and M2.