Biosynthesis of Δ-aminolevulinate in greening barley leaves: Glutamate 1-semialdehyde aminotransferase

Abstract

L-Glutamate-1-semialdehyde was synthesized by catalytic hydrogenation of N-carbobenzoxy-L-glutamyl-1-chloride-5-benzyl ester. Soluble protein extracts of chloroplasts isolated from greening barley leaves enzymically converted L-glutamate-1-semialdehyde to δ-aminolevulinate. The enzyme was partially purified by gel filtration on a Biogel column excluding proteins larger than 500,000 daltons. The enzyme had a broad pH optimum around 8.0 and required no specific cofactors for activity. Aminooxyacetate (20 mM), cycloserine (20 mM), ρ-chloromercuribenzoate (0.1 mM), glyoxylate (20 mM) and pyridoxal phosphate (5 mM) inhibited δ-aminolevulinate formation from L-glutamate-1-semialdehyde. However, β- hydroxyglutamate (1 mM) a potent inhibitor of L-glutamate-U-14C conversion to δ-aminolevulinate, had no effect on L-glutamate-1-semialdehyde aminotransferase. The aminotransferase activity was eluted from the Biogel column together with the enzyme activity that converted L-glutamate-U-14C into δ-aminolevulinate. Soluble proteins prepared from etiolated plastids and mature chloroplasts of barley had a low specific activity of L-glutamate-1-semialdehyde aminotransferase compared to soluble proteins from greening plastids. It is proposed that L-glutamate-1-semialdehyde aminotransferase catalyses a part reaction in the conversion of L-glutamate to δ-aminolevulinate in greening barley plastids. © 1978 Carlsberg Laboratory.