The role of the urea cycle and polyamines in albumin synthesis

Albumin synthesis is stimulated by those amino acids which increase urea synthesis and membrane bound polysome aggregation. Ornithine, an amino acid not incorporated into protein and produced from arginine in the urea cycle, is an albumin-stimulating amino acid and is the precursor of the polyamines, and we have shown that the polyamine spermine promotes bound polysome aggregation. To test the concept that ureogenesis with its generation of ornithine might play a key role in albumin synthesis regulation via the polyamine pathway, isolated livers from fasted donors were perfused with ornithine, alpha-difluoromethyl ornithine (DFMO), and spermine. In control experiments, albumin synthesis was 13.4 +/- 0.8 mg per 100 gm liver per hr and polysome aggregation was 47%. These were increased in the presence of ornithine (26.0 +/- 2.6 mg and 59%); if the livers were preperfused with DFMO before the addition of ornithine, then the expected increase in albumin synthesis and polysome reaggregation did not occur (16.3 +/- 1.4 mg and 47%). However, if spermine was present with DFMO during the preperfusion, then the addition of ornithine had the expected effect (albumin synthesis = 26.1 +/- 1.2 mg and polysome aggregation = 62%). This suggests that if the ornithine to putrescine pathway is blocked, ornithine does not stimulate albumin synthesis and offers support to the concepts that (a) ornithine stimulation of albumin production is via polyamine synthesis and (b) that the urea cycle plays a more important role in protein metabolism than simply the pathway for nitrogen disposal.