Biosynthesis of 5-aminolevulinate from glutamate in Anabaena variabilis.

Anabaena variabilis filaments excrete 5-aminolevulinate into the medium when incubated in the presence of levulinic acid, a competitive inhibitor of the 5-aminolevulinate utilizing enzyme, 5-aminolevulinate dehydratase (5-aminolevulinate hydro-lyase, EC 4.2.1.24). Although 5-aminolevulinate accumulation is independent of an external supply of substrate, the accumulating 5-aminolevulinate can be readily labeled by [14C]glutamate or alpha-[14C]ketoglutarate. Glycine and succinate, substrates of the classical 5-aminolevulinate synthesizing enzyme, 5-aminolevulinate synthase (succinyl-CoA:glycine C-succinyltransferase (decarboxylating), EC 2.3.1.37), label 5-aminolevulinate only to a very small extent. Studies with glutamate and alpha-ketoglutarate labeled in specific carbon atoms show that in A. variabilis, as in higher plants and eukaryotic algae, all five carbon atoms of these substrates are incorporated into 5-aminolevulinate, with carbon 1 of glutamate or of alpha-ketoglutarate becoming carbon 5 of 5-amino-levulinate. These findings are consistent with the theory that chloroplasts evolved from cyanobacteria or from closely related organisms.