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Metabolism and Enzymology

Arginine Metabolism in Developing Soybean Cotyledons

III. Utilization ¹

Department of Horticultural Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada

Tracerkinetic experiments were performed using L-[guanidino-¹⁴C]arginine, L-[U-¹⁴C]arginine, L-[ureido-¹⁴C]citrulline, and L-[1-¹⁴C]ornithine to investigate arginine utilization in developing cotyledons of Glycine max (L.) Merrill. Excised cotyledons were injected with carrier-free ¹⁴C compounds and incubated in sealed vials containing a CO₂ trap. The free and protein amino acids were analyzed using high performance liquid chromatography and arginine-specific enzyme-linked assays. After 4 hours, 75% and 90% of the ¹⁴C metabolized from [guanidino-¹⁴C]arginine and [U-¹⁴C]arginine, respectively, was in protein arginine. The net protein arginine accumulation rate, calculated from the depletion of nitrogenous solutes in the cotyledon during incubation, was 17 nanomoles per cotyledon per hour. The data indicated that arginine was also catabolized by the arginase-urease reactions at a rate of 5.5 nanomoles per cotyledon per hour. Between 2 and 4 hours ¹⁴CO₂ was also evolved from carbons other than C-6 of arginine at a rate of 11.0 nanomoles per cotyledon per hour. It is suggested that this extra ¹⁴CO₂ was evolved during the catabolism of ornithine-derived glutamate; ¹⁴C-ornithine was a product of the arginase reaction. A model for the estimated fluxes associated with arginine utilization in developing soybean cotyledons is presented.

The maximum specific radioactivity ratios between arginine in newly synthesized protein and total free arginine in the ¹⁴C-citrulline and ¹⁴C-ornithine experiments indicated that only 3% of the free arginine was in the protein precursor pool, and that argininosuccinate and citrulline were present in multiple pools.

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