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The formation of protochlorophyllide and protochlorophyllide phytyl ester was investigated during etioplast biogenesis in order to study the biosynthetic relation of these two compounds. Protochlorophyllide accumulates slowly during the first 2 days of germination, its rate of formation increases sharply during the 3rd day, and then it decreases. Protochlorophyllide phytyl ester starts accumulating a day later; its formation coincides with the initiation of xanthophyll biosynthesis. Kinetic analysis of specific radioactivities after (14)C labeling of the protochlorophyll pools does not support the currently accepted conversion of protochlorophyllide into protochlorophyllide phytyl ester, but suggests that both compounds originate simultaneously from a common precursor pool.

The porphyrin and phorbin biosynthetic activity of etiolated cucumber (Cucumis sativus, L.) cotyledons was compared to that of cotyledonary homogenates. Etiolated cotyledons incubated with delta-aminolevulinic acid accumulate protoporphyrin, coproporphyrin, small amounts of Mg protoporphyrin monoester, and trace amounts of uroporphyrin. They also incorporate 4-(14)C-delta-aminolevulinic acid into free porphyrins, protochlorophyllide, protochlorophyllide phytyl ester, and Mg protoporphyrin monoester. Homogenates incubated with delta-aminolevulinic acid likewise accumulate coproporphyrin, uroporphyrin, Mg coproporphyrin, and trace amounts of protoporphyrin. They also incorporate 4-(14)C-delta-aminolevulinic acid into Mg protoporphyrin monoester, Mg coproporphyrin, and free porphyrins. However, the capacity to synthesize protochlorophyllide and protochlorophyllide phytyl ester is lost and the endogenous protochlorophylls gradually disappear. Mg protoporphyrin monoester represents the terminal biosynthetic step in this cell-free system.