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Plant and Cell Physiology, 1977, Vol. 18, No. 5 1109-1120
© 1977

Article

Coupling of malate decarboxylation to CO2 fixation and the reduction of 3-phosphoglyceric acid in corn bundle sheath cells1,2

Hideaki Usuda3 and Shigetoh Miyachi

Institute of Applied Microbiology, University of Tokyo Bunkyo-ku, Tokyo 113, Japan

2 Requests for reprints should be addressed to S. Miyachi, Institute of Applied Microbiology, University of Tokyo, Bunkyo-Ku, Tokyo 113, Japan

  1. In the presence of NADP+ and Mg++, the bundle sheath strands isolated from corn (Zea mays) leaves by cellulase treatments decarboxylated malate in the light at an initial rate (200 µmoles/mg chl.hr), which was sufficient to account for photosynthetic CO2 fixation in intact leaves. This rate gradually slowed down and then stopped. The final level of the malate decarboxylated was approximately equal to the amount of NADP+ added.
  2. Rapid and continued decarboxylation of malate was observed when NADP+, 3-phosphoglyceric acid and ATP (and Mg++) were added together. The addition of ADP instead of ATP showed a similar effect. Light did not show any effect on the malate decarboxylation in the presence of ATP or ADP.
  3. When malate was added to the bundle sheath strands in the presence of exogenous NADP+ NADP+ was rapidly reduced. The reduction stopped after 2 min when, 73% of the added NADP+ was reduced. The further addition of 3-phosphoglyceric acid and ATP brought about a decrease in the NADPH-level, which rose again to attain a new steady level.
  4. The transfer of radioactivity from (1-14C-3-phosphoglyceric acid to dihydroxyacetone phosphate in the bundle sheath strands in the presence of ATP and NADP+ was greatly enhanced by the addition of malate.
  5. In the presence of ribose 5-phosphate and ATP, the rate of 14C-transfer from (4-14C)-malate to the intermediates of the reductive pentose phosphate cycle was equal to that of 14CO2 fixation in the light.

All these results support the current view that in the bundle sheath cells of C4 plants belonging to the NADP-malic enzyme-group, the decarboxylation of malate is coupled to the fixation of the released CO2 and the reduction of 3-phosphoglyceric acid formed as a result of CO2 fixation.

1 Part of this research was reported at the 40th Annual Meeting of the Botanical Society of Japan Osaka, December, 1975.

3 Present address: Laboratory of Chemistry, Faculty of Medicine, Teikyo University, 359 Otsuka, Hachioji-City, Tokyo 173, Japan.

(Received April 30, 1977; )
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