Conversion of ornithine into proline by enzymes from germinating peanut cotyledons

Abstract

During germination a marked synthesis of proline occurs in the cotyledons of peanuts. A soluble enzyme system which converted ornithine into proline was extracted from acetone powders of 3–5-day-old peanut cotyledons. The reaction required ornithine, α-ketoglutarate, and a reduced pyridine nucleotide. NADPH was much more effect than NADH. An l-ornithine:2-oxoacid aminotransferase (EC 2.6.1.13) has been purified about six-fold from the extracts. The reaction produces a pyrroline carboxylate as indicated by a positive reaction with o-aminobenzaldehyde. An optimum pH of 8·0 was found with tris-HCl. The K m for α-ketoglutarate was 2·5 mM and for l-ornithine 5·0 mM. Exogenous pyridoxal phosphate was not required, but 0·1 mM hydroxylamine inhibited the reaction by 90 per cent. α-Ketoglutarate was quite specific as the acceptor, pyruvate, oxaloacetate, and glyoxylate showing very little activity. Fractionation of germinating cotyledon homogenates showed that the mitochondrial fraction had a specific activity more than eight-fold that in the remaining soluble cytoplasm, but the total soluble activity was six times that of the mitochondrial fraction.