Glutamate dehydrogenase isoforms in lupine roots and root nodules. Immunological studies

Abstract

In roots of the lupine (Lupinus luteus L. cv. Ventus) glutamate dehydrogenase (EC 1.4.1.2–4) was present in a single, electrophoretically homogeneous form (GDHR), while in root nodules eight forms of the enzyme (GDHN) were detected. Highly purified fractions of glutamate dehydrogenase from roots and nodules were used to prepare antisera in rabbits. The two antisera recognized extracts of glutamate dehydrogenase from both roots and nodules. Low concentrations of the antiserum raised against nodule glutamate dehydrogenase precipitated GDH from the nodule extract (the homologous antigen) effectively but decreased the activity of GDH from the root extract only slightly. At high concentrations, however, the antiserum totally precipitated both enzyme extracts. Ouchterlony's double diffusion test showed that seven forms of root nodule glutamate dehydrogenase arose as the result of random association of two subunits in a hexameric complex, while the eighth form probably consisted of totally distinct subunits. The question of whether the new glutamate dehydrogenase forms, present exclusively in root nodules, can be classified as nodulins, is discussed.