Degradation of chlorophylls: Two reaction pathways in the formation of pyropheophorbide a

Abstract

Demethoxycarbonyl reaction of chlorophylls in plants and algae was investigated. We found that two reaction pathways were present in the formation of pyropheophorbide a. One pathway proceeds by two reaction steps; first, the enzyme designated "pheophorbidase (Phedase)" catalyzes the conversion of pheophorbide a to a precursor of pyropheophorbide a by demethylation, and then, the precursor is decarboxylated non-enzymatically to yield pyropheophorbide a. The other pathway is direct conversion of pheophorbide a to pyropheophorbide a by demethoxycarbonylation. This enzyme was termed "pheophorbide demethoxycarbonylase (PDC)" as an operational name. Phedase was purified from cotyledons of radish (Raphanus sativus), and PDC from the mutant cells of Chlamydomonas reinhardtii. Both enzymes consisted of two types, senescence-induced and constitutive enzymes. Molecular weight and Km value against pheoporbide a were 113,000 and 15 mM for Phedase, and 170,000 and 283 mM for PDC, respectively. We examined the effect of various inhibitors on both enzymes. The activity of Phedases was inhibited by the reaction product, methanol, whereas PDC had no effect. The presence of these enzymes was tested for 21 species from 14 different families. The Phedase activity was found in 9 species from three families and PDC activity from only one species of Chlamydomonas, suggesting that these enzymes are specific for a certain family of plants and algae. From these results, we conclude that Phdase and PDC are different enzymes, and at least two pathways exist in the formation of pheophorbide a.