Re-examination of Mg-dechelation reaction in the degradation of chlorophylls using chlorophyllin a as a substrate |
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Authors: | Suzuki Toshiyuki Shioi Yuzo |
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Institution: | (1) Department of Biology and Geoscience, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan |
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Abstract: | The Mg-dechelating activity of extracts of Chenopodium album (goosefoot) was investigated using an artificial substrate, chlorophyllin a. The activity was measured spectrophotometrically by the formation of a reaction product, pheophorbin a (Mg-free chlorin), after release of the central Mg. The Mg-releasing protein was highly purified by successive DEAE, Butyl
and HW-55 chromatographies. The molecular weight of the purified protein was 20 k by gel filtration. The protein showed a
broad, but single, pH optimum at 7.5. The K
m value for chlorophyllin a was 95.1 nM at pH 7.5. The Mg-releasing protein was not active with chlorophyllide a, a native substrate, although it was active with Zn-chlorophyllin a. Similar results were obtained from horseradish peroxidase. Only a small molecular weight, metal-chelating substance (MCS)
had Mg-dechelating activity for the native substrate. An inhibitor study showed involvement of radicals in the Mg-dechelation
of the Mg-releasing protein. The purified Mg-releasing protein showed neither peroxidase activity nor absorption bands in
the visible region, and this indicates that the Mg-releasing protein is clearly distinct from horseradish peroxidase, which
is a heme-containing protein. A likely conclusion is that the Mg-releasing protein and horseradish peroxidase are not involved
in the Mg-dechelation in the degradation pathway of chlorophylls. The relevance of the participation of MCS in Mg-dechelation
in the breakdown of chlorophylls (Chls) is also discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | Chenopodium album chlorophyllin chlorophyll degradation Mg-dechelatase Mg-releasing protein pheophorbide formation |
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