The biosynthesis of δ-aminolaevulinic acid in plants

Abstract

In animal and bacterial cells the first enzymic step unique to the tetrapyrrole biosynthetic pathway is the condensation of succinyl-CoA and glycine to yield δ-aminolaevulinic acid (ALA). The enzyme catalysing this reaction could not be detected in extracts from higher plants, or green or blue-green algae. Through the use of laevulinic acid, which competitively inhibits ALA dehydrase, and causes the accumulation of ALA in vivo , the ability of a number of specifically labelled ¹⁴ C radioactive compounds to label this ALA has been studied. Glycine and succinate are poor label donors, whereas α-ketoglutarate, glutamate and glutamine are able to donate ¹⁴ C to the ALA. Chemical degradation of the [ ¹⁴ C]ALA indicates that C ₅ arises from C ₁ of glutamate and the remaining four carbon atoms of the ALA arise from the remaining four carbon atoms of glutamate. This labelling pattern is incompatible with the succinyl-CoA-glycine condensation reaction and indicates a new pathway for ALA biosynthesis from the intact carbon skeleton of glutamate in greening plant tissues.