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

Abstract

Feeding [1-14C]-guanidinoacetic acid to shoot primordia, O2 uptake was inhibited and major products were 14C-glycine, 14CO2 and 14C-serine. The direct decarboxylation of [1-14C]-guanidinoacetic acid to 14CO2 and N-methylguanidine, the methylation of [1-14C]-guanidinoacetic acid to 14C-creatine, and the lytic cleavage to urea and 14C-glycine were all ruled out. Enzymatic transamidinations of [1-14C]-guanidinoacetic acid with amino acid acceptors occurred as arginine-rich storage proteins were being turned over and new proteins synthesized containing 14C-glycine and 14C-serine. The products of transamidination were recycled as substrates until 14C-glycine was metabolized in different directions and transported to mitochondria and peroxisomes. 14C-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 [1-14C]-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.