Biosynthesis of delta-Aminolevulinic Acid in Chlamydomonas reinhardtii: Study of the Transamination Mechanism Using Specifically Labeled Glutamate

The first committed intermediate of chlorophyll biosynthesis, δ-aminolevulinic acid (ALA), is synthesized from glutamate in the plant cell. The last step of ALA synthesis is a transamination reaction which converts glutamate-1-semialdehyde (GSA) to ALA. The mechanism of the transamination was examined by using glutamate, specifically labeled with either 1-¹³C or ¹⁵N, as substrate for ALA synthesis. After incubating with crude enzymes extracted from Chlamydomonas reinhardtii, the distribution of labels in purified ALA molecules was examined by nuclear magnetic resonance spectroscopy and gas chromatography-mass spectrometry. We found that both isotopes were present in the same ALA molecule. We interpret the results to mean that intermolecular transamination occurs during the conversion of GSA to ALA.