The metabolism of 14C-urea by white spruce seedlings in light and darkness

Abstract

Weak urease activity was detected in 2-week-old white spruce seedlings. The addition of urea to these seedlings increased urease activity in light compared to darkness. Urease was localized in the cytoplasm mainly in the epidermal cells, and to a lesser extent in vascular tissues and at the shoot apex.In light, the early products of 14C-urea indicated the release of 14C-carbon dioxide and ammonium ions. Radioactivity appeared in the anionic and neutral fractions and then in alanine, serine, and glycine, just as in the fixation of 14C-bicarbonate. By 4 h most radioactivity resided in glutamic acid and alanine. This pattern was associated with increased protein synthesis, and with high levels of free glutamine. Serine, glycine, alanine, and carbamyl aspartic acid had high specific activities.In darkness, radioactivity in alanine persisted, but levels in serine, glycine, and protein were low. Most radioactivity eventually resided in asparagine. Radioactive urea and citrulline were kept at higher levels than in light. Glutamic acid, citrulline, and carbamyl aspartic acid had the greatest specific activities. In light and darkness, carbamyl derivatives were formed more readily from 14C-urea than from 14C-bicarbonate.Carbamyl phosphate was implicated as a precursor of citrulline and arginine as well as for carbamyl aspartic acid. In light, radioactivity was recovered from ribosomal (25, 23, 18, 16, and 5 S) and transfer RNA (4 S). This resided mainly in uracil and cytosine. By contrast, in DNA, radioactivity was recovered mainly from thymine, cytosine, and 5-methylcytosine. In rapidly growing seedlings, urea contributed to de novo nucleic acid synthesis mainly through pyrimidine pathways via carbamyl aspartic acid.