Summary Steady‐state rate equations are written on the basis of information obtained from the literature describing the kinetics of the four enzyme‐catalysed reactions comprising the urea cycle. These equations are formulated into a set which also accounts for fluxes of input and output compounds external to the cycle. Numerical integration of this set of equations is performed employing parameters selected to approximate those pertaining to the operation of the urea cycle in normal liver. The result is a pattern of intermediate metabolite concentrations, which forms a basis for the comparison of patterns reflecting the effects of inborn errors of metabolism. The latter are calculated by varying specified kinetic parameters in the numerical integration. Each of the observed clinical syndromes, Hyperarginemia Types I and II, Hyperarginemia, Citrullinemia and Argininosuccinicaciduria, is discussed.

The permeability of biological membranes to the rapidly penetrating compound ammonia is extremely difficult to study due to the lack of readily available radionuclides. 14N and 15N saturation transfer nuclear magnetic resonance (NMR) experiments were used to measure the erythrocyte membrane permeability of ammonia under equilibrium exchange conditions. When 14N spectra from erythrocytes suspended in NH4Cl solution were obtained in the presence of the extracellular shift reagent dysprosium tripolyphosphate, intracellular and extracellular ammonia signals were readily resolved. Comparison with 15N spectra from erythrocyte suspensions containing 15N4Cl revealed that the intracellular [14N]ammonia signals were 100% NMR visible. 14N and 15N saturation transfer NMR experiments showed similar influx rates and permeabilities, indicating no loss of saturation transfer due to quadrupolar relaxation of 14N nuclei upon membrane passage. Ammonia influx was directly proportional to concentration (0.39 +/- 0.012 fmol.cell-1.s-1.mM-1 at pH 7.0) and not saturable, which is consistent with passive diffusion. Apparent ammonia permeability increased with pH over the range of pH 6-8 as the fraction of free NH3 increased. However, diffusion through unstirred layers became increasingly rate limiting. The permeability of the unstirred layers (1.1 +/- 0.45 x 10(-3) cm/s) was considerably lower than that of NH3 (0.21 +/- 0.014 cm/s). The Arrhenius activation energy for NH3 permeability was 49.5 +/- 11.8 kJ/mol. No evidence for NH+4 influx over the time domain of these experiments was found.

A method that permits the use of measurements on the concentration of the intermediate in a coupled enzymic assay in determining the presence or absence of an interaction between the enzymes is presented. The method is shown to be closely analogous to a previously formulated procedure involving the determination of the rate of production of the final product of such a sequence and is shown to be applicable regardless of the complexity of the operative kinetic mechanisms, provided it may be assumed that all enzyme-substrate complexes are in the steady-state. Kinetic results obtained with the arginase--ornithine carbamoyltransferase couple, in which the intermediate ornithine is monitored, are examined in these terms to conclude that no heterogeneous association is operative between the enzymes.

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