Response to light of a specific aminotransferase for delta-aminolevulinate formation in higher plants

It is thought that the C-5 pathway is the major, possibly the sole, route for the formation of delta-aminolevulinic acid for the biosynthesis of tetrapyrroles, including chlorophylls, in higher plants; a route involving 4,5-dioxovalerate as an intermediate followed by transamination to delta-aminolevulinic acid has been supported as one of the C-5 pathways (Granick, S., and Beale, S. I. (1978) Adv. Enzymol. Relat. Areas Mol. Biol. 46, 33-203). A specific aminotransferase for L-alanine and 4,5-dioxovalerate was found in the cucumber seeds. In dark-grown cucumber seedlings, alanine:4,5-dioxovalerate aminotransferase activity in the transitional region between shoot and root was remarkably high compared with that in the cotyledons. The exposure of the dark-grown seedlings to illumination resulted in a rapid and dramatic increase in the activity only in this transitional region. In contrast, the enzyme in the cotyledons, stem, and roots did not respond to illumination. After a 27-h illumination, the enzyme activity in the transitional region was 100-fold higher than that in the cotyledons. Other aminotransferases assayed in the transitional region did not respond to illumination. Alanine:4,5-dioxovalerate aminotransferase in the transitional region was also specific for L-alanine and 4,5-dioxovalerate.