ISOFORMS OF NADP-MALIC ENZYME FROM MESEMBRYANTHEMUM-CRYSTALLINUM L THAT ARE INVOLVED IN C-3 PHOTOSYNTHESIS AND CRASSULACEAN ACID METABOLISM

Exposure of the facultative halophyte Mesembryanthemum crystallinum L. to salt stress induces a shift from C-3 photosynthesis to Crassulacean acid metabolism (CAM). During induction of CAM, the activity of NADP-malic enzyme (EC 1.1.1.40) increased as much as 12-fold in leaves, while the enzymatic activity in roots fell to half of the original level. These changes in the activity of the enzyme corresponded to changes in levels of the enzyme protein. NADP-malic enzymes extracted from leaves in the C-3 and CAM modes could be distinguished by differences in electrophoretic mobility during electrophoresis on a non-denaturing polyacrylamide gel. NADP-malic enzyme extracted from roots in the C-3 mode and in the CAM mode migrated as fast as the enzyme extracted from leaves in the CAM mode on the same gel. Although the pattern of peptide fragments from NADP-malic enzyme from CAM-mode leaves was similar to that from C-3-mode leaves, as indicated by peptide mapping, both immunoprecipitation and an enzyme-linked immunosorbent assay revealed some antigenic differences between the enzymes extracted from leaves in the C-3 and the CAM modes. These results suggest the existence of at least two isoforms of NADP-malic enzyme that differ in their levels of expression during induction of CAM.