In Vitro Synthesis of the Chlorophyll Isocyclic Ring

Abstract

Developing chloroplasts of Cucumis sativus L., cv. Beit Alpha which were incubated with either Mg-protoporphyrin IX or Mg-protoporphyrin IX monomethyl ester in darkness produced a partially phototransformable protochlorophyllide species that was tentatively identified as Mg-2,4-divinyl pheoporphyrin a5. S-adenosylmethionine stimulated Mg-2,4-divinyl pheoporphyrin a5 formation irrespective of the starting material used. In the case of Mg-protoporphyrin IX monomethyl ester, this stimulation was attributed to the need to remethylate substrate that had been hydrolyzed by an endogenous methylesterase which converts part of the added Mg-protoporphyrin IX monomethyl ester to Mg-protoporphyrin IX. NADPand NADPH stimulated the conversion of Mg-protoporphyrin IX to Mg-2,4-divinyl pheoporphyrin a5. The conversion required O2 and was half saturated at 50 .mu.M dissolved O2. The conversion was insensitive to inhibitors of Fe-S and heme-containing proteins, to Cu chelator, H2O2 and peroxide scavengers. The conversion was extremely sensitive to phenazine methosulfate, methylene blue and methyl viologen. A decrease of the plastids'' ability to convert Mg-protoporphyrin IX to Mg-2,4-divinyl pheoporphyrin a5 after lysis in 0.1 M NaCl suggested a requirement for plastid integrity.