PARENT SESSION
Posters P6C Photosynthesis, respiration and alternative electron sinks. Abstracts (660-680)

Characterization of NAD-dependent malate dehydrogenases from spinach chloroplasts. Tijana Cvetic^*,1, Sonja Veljovic-Jovanovic², Zeljko Vucinic², ¹ Faculty of Biology, Belgrade, Yugoslavia² Center for Multidisciplinary Studies, Belgrade, Yugoslavia

ABSTRACT- Chloroplasts contain both NADP and NAD dependent malate dehydrogenases. We analysed NAD dependent MDH activity in both the intact and broken chloroplasts, isolated stroma and purified chloroplast envelopes, using spectrophotometric and spectrofulorimetric measurements. Neither form of NAD dependent enzyme was redox-regulated, as opposed to the NADP dependent form. The membrane-bound form shows a sharp pH optimum at 8, while the stromal from had a broad optimum at pH 8.5. MDH activity was saturated at 200M NADH and 120M OAA concentrations in envelope fraction, 250M NADH and 140M OAA in stromal fraction and 260M NADH and 200M OAA in leaf soluble extract. Electrophoretic analyses showed that the membrane-bound enzyme form is tightly bound to the envelope, and treatment with 0.1% Triton X-100 released an enzyme of different electrophoretic mobility compared to the soluble forms. Isoelectrofocusing of detergent-treated samples revealed an envelope enzyme with pI different from soluble forms. These findings suggest that membrane-bound MDH activity is not due to contamination from other cell compartments and support the idea of a novel isoform of plastidic NAD-dependent MDH. Such a membrane-bound enzyme could play a role in membrane redox processes and transfer of reducing equivalents to and from the cytosol.

KEY WORDS: spinach chloroplast, NAD-malate dehydrogenase