| Budget Amount *help |
¥1,500,000 (Direct Cost: ¥1,500,000)
Fiscal Year 1989: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 1988: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
The activities of pyruvate, Pi dikinase (PPDK) and NADP-malate dehydrogenase (MDH) increase with increasing light intensity in C_4 leaves. This means that C_4 mesophyll stromal system must be able to "sense" changing light intensities. But the question of how this "sensing" is translated into a response remains unanswered. It has been suggested from in vitro studies that light modulation of the two enzyme activities was caused through changes in the status of adenine nucleotides (AdN) or NADP(H). These two enzyme activities and the status of AdN and NADP(H) in situ were detentitied after nonaqueous isolation of mesophyll chloroplasts. Only a weal (no) relationship was found between NADP-MDH (PPDK) activity and NADP(H) (AdN) status in mesophyll chloroplasts of the intact maize leaf under various light intensities. A possible regulatory mechanism in which the "thermodynamically active" levels of the compounds mentioned above play a crucial role in translating changes in light intensity into a response or in regulating the activities of PPDK and NADP-MDH was proposed. This hypothesis was supported the following experiment. Estimation of thermodynamically active ADP level under different light intensities was carried out with a reconstituted system. The system includes spinach thylakoids, ADP, ATP-PFK, FBPase, Pi, MgCl_2, F6P, phenazinemethosulfate, and catalase. Photophosphorylation was carried out for 60-90s. Then the level of thernmynamically active ADP was monitored by adding pyruvate kinase (PK) and phosphoenolpyruvate and then measuring the resulting product pyruvate. Under light intensity of 100 (500) uE/m^2 s, 13% (4%) of total ADP (which levels were similar under two light conditions) was detected by PK. Namely, this result supports the hypothesis that thermodynamically active ADP level decreases with increasing light intensity and the decreased ADP level is sensed by the PPDK regulatory protein which then activates PPDK.
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