Transamidination of [1-^sup 14^C]-guanidinoacetic acid to ^sup 14^C-glycine and decarboxylation to ^sup 14^CO2 in white spruce shoot primordia entering winter dormancy

Feeding [1-^sup 14^C]-guanidinoacetic acid to shoot primordia, O2 uptake was inhibited and major products were ^sup 14^C-glycine, ^sup 14^CO2 and ^sup 14^C-serine. The direct decarboxylation of [1-^sup 14^C]-guanidinoacetic acid to ^sup 14^CO2 and N-methylguanidine, the methylation of [1-^sup 14^C]-guanidinoacetic acid to ^sup 14^C-creatine, and the lytic cleavage to urea and ^sup 14^C-glycine were all ruled out. Enzymatic transamidinations of [1-^sup 14^C]-guanidinoacetic acid with amino acid acceptors occurred as arginine-rich storage proteins were being turned over and new proteins synthesized containing ^sup 14^C-glycine and ^sup 14^C-serine. The products of transamidination were recycled as substrates until ^sup 14^C-glycine was metabolized in different directions and transported to mitochondria and peroxisomes. ^sup 14^C-Glycine was decarboxylated by a glycine decarboxylase multienzyme complex resulting in a net carbon loss and a sharp decline in total protein rich in arginine N. Under these conditions, unlabelled arginine and ornithine contributed as substrates for reversible transamidination reactions. Peroxisomes and mitochondria are hypothesized as providing arginine-derived nitric oxide to maintain redox homeostasis in response to the stresses imposed by [1-^sup 14^C]-guanidinoacetic acid and to protect against the inhibitory activity of sulfhydryls on transamidinase activity. The destruction of a respiratory inhibitor by transamidination may comprise a mechanism associated with the awakening from of dormancy and the mobilization of storage protein reserves in conifers.[PUBLICATION ABSTRACT]