Portal de Periódicos da CAPES

Mitochondria as a site of C4 acid decarboxylation in C4-pathway photosynthesis

1975; Elsevier BV; Volume: 167; Issue: 2 Linguagem: Inglês

10.1016/0003-9861(75)90513-5

1096-0384

T. Kagawa, M.D. Hatch,

Metalloenzymes and iron-sulfur proteins

One group of C4, species utilize a NAD-malic enzyme to decarboxylate C4 acids. This enzyme, together with a major isoenzyme of aspartate aminotransferase and a NAD-malate dehydrogenase, is localized in the mitochondria of the bundle sheath cells and the following pathway for C4, acid decarboxylation has been proposed: aspartate → oxaloacetate → malate → CO2 + pyruvate. The present study reports that mitochondria isolated from the bundle sheath cells of one of these species, Atriplex spongiosa, are capable of decarboxylating C4, acids at rates between 5 and 8 μmol/min/mg chlorophyll. For maximum decarboxylating activities, these particles required aspartate, 2-oxoglutarate and phosphate as well as malate; in the absence of any one of these compounds, activity was reduced to 0.3–0.8 μmol/min/mg chlorophyll. Rates for C4 acid decarboxylation were much greater than the respiratory activities of these particles, including the capacity to form citrate or to oxidize malate, succinate, pyruvate or 2-oxoglutarate (0.03–0.6 μmol/min/mg chlorophyll). A comparison of mitochondria prepared from leaves of various C4, and C3, species showed that only the mitochondria from the bundle sheath cells of plants with high NAD-malic enzyme have capacities for rapid C4 acid decarboxylation. The effects of a variety of experimental conditions on C4 acid decarboxylating activities are also reported. The role of these mitochondria in C4 photosynthesis is discussed.