A method for determination of the capacity of urea synthesis in the rat

The relationship between total blood alpha-amino nitrogen concentration and urea synthesis rate was investigated with alanine as nitrogen source in 24 rats. Alanine was given as prime-continuous doses for 70 min so that constant amino acid concentration was attained between 5.5 and 34 mmol/l. Urea synthesis rate was assessed as accumulation in body water, corrected for intestinal hydrolysis. There was a positive correlation between nitrogen balance and alpha-amino nitrogen concentration. Urea synthesis rate in relation to amino acid concentration suggested barrier-limited substrate inhibition kinetics and data were examined accordingly by non-linear regression analysis. The estimated kinetic constants (mean +/- standard deviation) were: Vmax: 19.2 +/- 3.3 mumol (min X 100 g BW)-1, Km: 1.74 +/- 0.5 mmol/l, Ki: 6.84 +/- 1.9 mmol/l, and the barrier: 5.4 +/- 0.13 mmol/l. Because of the substrate inhibition, saturation cannot be attained, but the maximum synthesis rate, i.e. the capacity of urea nitrogen synthesis (CUNS), can be measured within 95% of the theoretical maximum in the concentration interval 7.3-11.6 mmol/l. CUNS was 9.16 +/- 0.81 mumol (min X 100 g BW)-1 (mean +/- standard deviation). Substrate-independent regulation of urea synthesis, e.g. by changes in liver mass or hormonal concentration, can be studied by this measure.