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 Crassulacea n 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, wh ile 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 lea ves in the C-3 and CAM modes could be distinguished by differences in electrophoretic mobility during electrophoresis on a non-denaturing po lyacrylamide gel. NADP-malic enzyme extracted from roots in the C-3 mo de and in the CAM mode migrated as fast as the enzyme extracted from l eaves 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 immun oprecipitation 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 leas t two isoforms of NADP-malic enzyme that differ in their levels of exp ression during induction of CAM.