Nitrogen metabolism of Picea glauca. IV. Metabolism of uniformly labelled 14-C-L-arginine, [carbamyl-14C]-L-citrulline, and [1,2,3,4-14C]-gamma-guanidinobutyric acid during diurnal changes in the soluble and protein nitrogen associated with the onset of expansion of spruce buds.

Abstract

In spring as white spruce buds started to expand, seasonal levels of proline were high and pool sizes of most free amino acids were maximal at sunset and sunrise. Uniformly labelled 14C-L-arginine, applied to buds at sunrise, was converted to citrulline via ornithine. [Carbamyl-14C]-L-citrulline was metabolized to argininosuccinate, arginine, and urea. These reactions indicated the presence at low levels of the Krebs–Henseleit or ornithine cycle. No convincing urease activity could be extracted or detected histochemically. Urease, added to extracts of buds exposed to radioactive L-arginine or L-citrulline, released 14CO2 confirming the presence of 14C-urea. L-Arginine was converted readily to ornithine, Δ1-pyrroline-5-carboxylic acid, and proline. The appearance of 14C in monosubstituted guanidines, e.g. γ-guanidinobutyric acid and others, which remained unidentified but proven not to be homoarginine, agmatine, argininic acid, nor γ-guanidinobutyramide, accounted more readily for the metabolism of arginine than did the ornithine cycle alone. L-Citrulline was metabolized via arginine to γ-guanidinobutyric acid and to several unidentified compounds that were not amino acids or related to monosubstituted guanidines. 14C-γ-Aminobutyric acid, a product of arginine degradation, was also derived from [l,2,3,4-14C]-γ-guanidinobutyric acid suggesting the presence of a heteroarginase. γ-Guanidinobutyric acid was translocated down the shoot with little distribution of 14C into other compounds.14C from arginine and citrulline and to a much lesser extent from γ-guanidinobutyric acid was recovered in the protein fraction mainly as arginine, glutamic acid, and proline. Maximal incorporation of 14C into protein was reached at or near midnight, then rapidly fell to noon of the following day. Incorporation of 14C into bud protein was out of phase with levels of total protein N and in phase with an increase of radioactivity in compounds of the anionic and neutral fraction, reflecting dramatically the complexity of the nitrogen metabolism of buds awakening from winter dormancy.