Use of N-Bromoacetyl-l-ornithine to Study l-Ornithine and l-Arginine Biosynthesis in Soybean (Glycine max L.) Cell Cultures

The effect of the inhibitor N²-bromoacetyl-l-ornithine (NBAO) on the biosynthesis of ornithine in higher plants, was investigated using soybean cells (Glycine max L. var Mandarin), grown in suspension culture. The NBAO was found to reduce the specific activity of the enzyme N²-acetyl-l-ornithine: l-glutamate N-acetyltransferase (EC 2.3.1.35). In contrast, the specific activity of the enzyme acetyl coenzyme A:L-glutamate N-acetyltransferase (EC 2.3.1.1), which is also involved in N-acetylglutamate biosynthesis, was not significantly changed. Estimation of the concentrations of free amino acids in the soluble fraction of the cells showed that while ornithine levels were decreased, glutamic acid levels were increased in the presence of NBAO. While arginine levels initially increased in the presence of NBAO, they finally decreased near the end of the growth period. Evidence was obtained that the initial increase in arginine levels was due to the inhibition of arginase (EC 3.5.3.1) by N²-bromoacetyl l-ornithine. We conclude that the reaction catalyzed by N²-acetyl-l-ornithine:l-glutamate N-acetyl transferase is a rate limiting reaction in vivo.