In Vitro Synthesis of the Chlorophyll Isocyclic Ring : Transformation of Magnesium-Protoporphyrin IX and Magnesium-Protoporphyrin IX Monomethyl Ester into Magnesium-2,4-Divinyl Pheoporphyrin A(5).

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 a(5). S-Adenosylmethionine stimulated Mg-2,4-divinyl pheoporphyrin a(5) 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 a(5). The conversion required oxygen and was half saturated at 50 micromolar dissolved O(2). The conversion was insensitive to inhibitors of iron-sulfur and heme-containing proteins, to Cu chelators, H(2)O(2), 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 a(5) after lysis in 0.1 molar NaCl suggested a requirement for plastid integrity.