In maize, two plastidic NADP-malic enzyme (C4-NADP-ME) isoforms are present: C4-NADP-ME and nonC4-NADP-ME, each with distinct kinetic and structural features. To investigate the differences in oligomeric states of C4- and nonC4-NADP-ME in plant tissues, we analyzed extracts of maize from etiolated leaves and roots using native PAGE coupled with an in-gel NADP-ME activity assay. All protein bands exhibiting NADP-ME activity were excised and analyzed by mass spectrometry.