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. NADP and NADPH stimulated the conversion of Mg-protoporphyrin IX to Mg-2,4-divinyl pheoporphyrin a5. The conversion required oxygen and was half saturated at 50 micromolar dissolved O2. The conversion was insensitive to inhibitors of iron-sulfur and heme-containing proteins, to Cu chelators, H2O2, and peroxide scavengers. However, 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 molar NaCl suggested a requirement for plastid integrity. |