An enzymatic route to L-ornithine from arginine--activation, selectivity and stabilization of L-arginase

The non-proteinogenic amino acid L-ornithine (L-Orn) can be conveniently obtained by enzymatic hydrolysis of arginine (Arg) with arginase (EC 3.5.3.1). Arginase from calf liver (Vmax = 459 mumol/(min.mg), Km = 25.5 mM) is inhibited competitively by L-Orn (Ki = 480 mM). The enzyme was found to be completely enantioselective (E-value > 100) so that D,L-Arg can be split into D-Arg and L-Orn. Operational stability at 25 degrees C (deactivation rate constant kdeact = 3.8 x 10(-3) h-1; tau 1/2 = 182 h) is sufficient for use in a continuous process but is significantly smaller than temperature stability (kdeact = 4.1 x 10(-4) h-1; tau 1/2 = 1682 h); mechanical stress through stirring and unsteady Mn2+ supply owing to oxidation in the continuous process are believed to cause the difference. Addition of ascorbic acid stabilizes calf liver arginase at temperatures higher than 25 degrees C (at 60 degrees C, delta delta G not equal to = 2.9 kJ/mol).