THERMOCHEMISTRY OF THE HYDROLYSIS OF L-ARGININE TO (L-CITRULLINE PLUS AMMONIA) AND OF THE HYDROLYSIS OF L-ARGININE TO (L-ORNITHINE PLUS UREA)

Molar enthalpies reaction for the hydrolysis of d-arginine(aq) to {d-ornithine(aq) + urea(aq)} and for the hydrolysis of d -arginine(aq) to {d-citrulline(aq) + ammonia(aq)} have been measured by microcalorimetry. These reactions are catalyzed, respectively, by arginase and by arginine deiminase. The effects of variations in pH, temperature, and ionic strength on the molar enthalpies of reaction were studied. The results have been analyzed with a model which accounts for the complex equilibria in solution. The results obtained at T = 198.15 K for the reference reactions are: ΔrHom= -(21.4±0.5) kJ·mol-1for d-arginine+(aq) + H2O(l) = d-ornithine+(aq) + urea(aq) and ΔrHom= -(31.9±0.8) kJ·mol-1for d-arginine+(aq) + H2O(l) = d-citrulline(aq) + NH+4(aq). These results are discussed in terms of thermodynamic-cycle calculations and in terms of the metabolic urea cycle. © 1993 Academic Press Ltd.