Abstract
Enzymatic removal of the methoxycarbonyl group of pheophorbide (Pheid) a in chlorophyll degradation was investigated in cotyledons of radish (Raphanus sativus). The enzyme pheophorbidase (PPD) catalyzes the conversion of Pheid a to a precursor of pyropheophorbide (PyroPheid), C-13(2)-carboxylPyroPheid a, by demethylation, and then the precursor is decarboxylated nonenzymatically to yield PyroPheid a. PPD activity sharply increased with the progression of senescence in radish, suggesting de novo synthesis of PPD. The enzyme activity was separated into two peaks in anion-exchange and hydrophobic chromatography; the terms type 1 and type 2 were applied according to the order of elution of these enzymes in anion-exchange chromatography. PPD types 1 and 2 were purified 9,999- and 6,476-fold, with a yield of 0.703% and 2.73%, respectively. Among 12 substrates tested, both enzymes were extremely specific for Pheids of the dihydroporphyrin and tetrahydroporphyrin types, indicating that they are responsible for the formation of these PyroPheids. Both PPDs had molecular masses of 113,000 kD on gel filtration and showed three bands of 16.8, 15.9, and 11.8 kD by SDS-PAGE. The partial N-terminal amino acid sequences for these bands of PPD (type 2) were determined. Based on their N-terminal amino acid sequences, a full-length cDNA of PPD was cloned. The molecular structure of PPD, particularly the molecular mass and subunit structure, is discussed in relation to the results of SDS-PAGE.
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