Re-examination of Mg-dechelation reaction in the degradation of chlorophylls using chlorophyllin a as a substrate.
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.
Article: The stay-green revolution: Recent progress in deciphering the mechanisms of chlorophyll degradation in higher plants[show abstract] [hide abstract]
ABSTRACT: Plants control the abundance of pigments within their cells depending on the developmental stage of particular tissues or organs and in response to environmental cues. Chlorophyll is degraded in plants as a result of natural or induced senescence, at the onset of fruit ripening, and in response to pathogen infection. Many of the steps that contribute to chlorophyll degradation, including the identity of breakdown intermediates and the enzymes that catalyze their conversions have been identified during the last decade. In particular, knowledge at the molecular level has advanced considerably over the past two years with the identity of the long sought STAY-GREEN (SGR/SID) gene revealed, together with the realization that several stay-green mutants of various species are also defective in SGR orthologs. Genetic analysis of additional stay-green mutants of rice has also led to the discovery of chlorophyll b reductase. Furthermore, recent functional analysis of putative chlorophyllase knockout mutants of Arabidopsis found that they were only very slightly compromised in their ability to degrade chlorophyll and did not have reduced chlorophyllase activity. These data suggest the need to re-examine the initial step of the chlorophyll degradation pathway. This review focuses on these discoveries and other recent research that has helped to shape our current understanding of chlorophyll degradation.Plant Science.