Alternative Routes for the Synthesis of 5-Aminolevulinic Acid in Maize Leaves

Abstract

Intact plastids from greening maize (Zea mays L.) leaves converted [(14)C]glutamate and [(14)C]2-ketoglutarate (KG) to [(14)C]5-aminolevulinic acid (ALA). Glutamate appeared to be the immediate precursor of ALA, while KG was first converted to glutamate, as shown by the effect of various inhibitors of amino acid metabolism. Plastids from greening leaves contained markedly higher activity as compared with etioplasts or chloroplasts. The synthesis of ALA by intact plastids was light dependent. The enzyme system resides in the stroma of plastids or may be lightly bound to membranes. The solubilized system showed maximal activity around pH 7.9 and required Mg(2+), ATP, and NADPH although dependence on the latter was not clear-cut. A relatively high level of activity could be extracted from etioplasts. Maximal activity was obtained from plastids of leaves which had been illuminated for 90 minutes, after which activity declined sharply. The enzyme system solubilized from plastids also catalyzed the conversion of putative glutamate 1-semialdehyde to ALA in a reaction which was not dependent on the addition of an amino donor.The system in maize greatly resembled the one which had been reported from barley. It is suggested that this system is the one responsible for the biosynthesis of ALA destined for chlorophyll formation.