Biosynthesis of δ -aminolevulinic acid from the Intact Carbon Skeleton of Glutamic Acid in Greening Barley

The customary route in animals and bacteria for δ -aminolevulinic acid biosynthesis is from glycine and succinyl CoA, catalyzed by the enzyme δ -aminolevulinic acid synthetase [succinyl-CoA:glycine C-succinyltransferase (decarboxylating), EC 2.3.1.37]. Attempts to demonstrate this route in plants have been unsuccessful. Evidence is given for a new enzymic route of synthesis of δ -aminolevulinic acid in plants. This route involves the incorporation of the intact five-carbon skeleton of glutamic acid into δ -aminolevulinic acid. Demonstration of the new pathway in plants has been made by feeding specifically labeled [14C]glutamic acid to etiolated barley shoots greening in the light. In the presence of levulinate, a competitive inhibitor of δ -aminolevulinic acid dehydratase [porphobilinogen synthase; δ -aminolevulinate hydro-lyase (adding δ -aminolevulinate and cyclizing); EC 4.2.1.24], δ -aminolevulinate accumulates. The δ -aminolevulinate formed was chemically degraded by periodate to formaldehyde and succinic acid. The C5(formaldehyde) fragment was separated, as the 5,5-dimethyl-1,3-cyclohexanedione (dimedone) derivative, from the C1-C4(succinic acid) fragment. The C5atom contained radioactivity predominantly derived from C1of glutamic acid. Conversely, the labeled C3and C4atoms of glutamic acid were found primarily in the succinic acid (C1-C4) fragment of δ -aminolevulinate. This labeling pattern for δ -aminolevulinic acid is consistent with a biosynthetic route utilizing the intact five-carbon skeleton of α -ketoglutarate, glutamate, or glutamine, and is inconsistent with the δ -aminolevulinic acid synthetase pathway utilizing glycine and succinyl CoA as precursors.