Abstract
Linear tetrapyrroles, called phyllobilins, are obtained as major catabolites upon chlorophyll degradation. Primarily, colorless phylloleucobilins featuring four deconjugated pyrrole units were identified. Their yellow counterparts, phylloxanthobilins, were discovered more recently. Although the two catabolites differ only by one double bond, physicochemical properties are very distinct. Moreover, the presence of the double bond seems to enhance physiologically relevant bioactivities: in contrast to phylloleucobilin, we identified a potent anti‐proliferative activity for a phylloxanthobilin, and show that this natural product induces apoptotic cell death and a cell cycle arrest in cancer cells. Interestingly, upon modifying inactive phylloleucobilin by esterification, an anti‐proliferative activity can be observed that increases with the chain lengths of the alkyl esters. We provide first evidence for anti‐cancer activity of phyllobilins, report a novel plant source for a phylloxanthobilin, and by using paper spray MS, show that these bioactive yellow chlorophyll catabolites are more prevalent in Nature than previously assumed.
Highlights
The biochemical degradation of the green plant pigment chlorophyll (Chl) has remained unresolved for a long time, until abundant linear tetrapyrroles, called phyllobilins (PBs), were discovered as stable degradation products accumulating in the vacuoles of the plant cell.[1]
Chl breakdown has revealed many of its mysteries, and we have a well-defined picture of the biochemical program of Chl breakdown, called pheophorbide a oxygenase (PaO)/phyllobilin pathway
Of particular interest is the esterification of the propionic acid side chain at C12; Oberhuber et al showed that introducing a methyl ester at the PluB stage by chemical synthesis changed the kinetics of the conversion to PleBs, resulting in increased stability of the esterified PluB.[12b]. An esterification of the propionic acid side chain of PBs was discovered to occur naturally, causing PluBs in banana peels to accumulate and, as a consequence, the peel of ripened bananas to luminesce blue.[30]
Summary
The biochemical degradation of the green plant pigment chlorophyll (Chl) has remained unresolved for a long time, until abundant linear tetrapyrroles, called phyllobilins (PBs), were discovered as stable degradation products accumulating in the vacuoles of the plant cell.[1]. For the plant Cercidiphyllum japonicum, in addition to 32-OH-PleB (Cj-PleB (1)),[2] a yellow colored oxidation product, a phylloxanthobilin (Cj-PxB (2)), was discovered for the first time 10 years ago[3] (Scheme 1).
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