Intraplastidic localization of the enzymes that convert δ-aminolevulinic acid to protoporphyrin IX in etiolated cucumber cotyledons

Abstract

The intraplastidic localization of the enzymes that catalyze the conversion of δ-aminotevulinic acid (ALA) to protoporphyrin IX (Proto) is a controversial issue. While some researchers assign a stromal location for these enzymes, others favor a membranebound one. Etiochloroplasts were isolated from etiolated cucumber cotyledons (Cucumis sativus, L.) by differential centrifugation and were purified further by Percoll density gradient centrifugation. Purified plastids were highly intact, and contamination by other subcellular organelles was reduced five- to ninefold in comparison to crude plastid preparations. Most of the ALA to Proto conversion activity was found in the plastids. On a unit protein basis, the ALA to Proto conversion activity of isolated mitochondria was about 2% that of the purified plastids, and could be accounted for by contamination of the mitochondrial preparation by plastids. Lysis of the purified plastids by osmotic shock followed by high speed centrifugation, yielded two subplastidic fractions: a soluble clear stromal fraction and a pelleted yellowish one. The stromal fraction contained about 11% of the plastidic ALA to Proto conversion activity white the membrane fraction contained the remaining 89%. The stromal ALA to Proto conversion activity was in the range of stroma contamination by subplastidic membrane material. Complete solubilization of the ALA to Proto activity was achieved by high speed shearing and cavttation, in the absence of detergents. Solubilization of the ALA to Proto conversion activity was accompanied by release of about 30% of the membrane-bound protochlorophyllide. It is proposed that the enzymes that convert ALA to Proto are loosely associated with the plastid membranes and may be solubilized without the use of detergents. It is not clear at this stage whether the enzymes are associated with the outer or inner plastid membranes and whether they form a multienzyme complex or not.