Recombinant giardial arginine deiminase (rADI) was characterized. The enzyme was found to have a specific activity of 12 U (mg protein)-1under at pH 7.4 and 1 mM arginine. The maximum velocity was 14.75 U (mg protein-1) and the KM was 0.167 mM. The specific activity and maximum velocity values are significantly lower than the values reported previously for giardial rADI, while the KM value is quite similar. The optimum pH for the giardial rADI was 6-9, a broad range compared to other arginine deiminases. Recombinant ADI is very specific in its binding specificity, with canavanine (KI 2.4 mM) and ornithine (KI 2.1 mM) being the only substrate analogues giving significant inhibition from the wide variety of analogues tested. None of the analogues could be shown to act as alternative substrates. The contribution of conserved, catalytic and C-terminal residues proposed by previous research towards ADI activity was investigated by site-directed mutagenesis. Mutations of catalytic site residues Asp175, Glu226, His280 and Cys424 decreased the rADI activity to below 2%. Conservative mutations showed significant activity for Asp175 to Glu175 (DE175) and Glu226 to Asp226 (ED226). Site directed mutagenesis on the conserved non-catalytic site Leu46 showed activities below 15%, with the highest activity observed for the mutation to Val46 (LV46), which differs in one CH2 to Leu46 on its side chain. The KM of the mutant LV46 was 3.64 mM while for LA46 (Leu to Ala mutation) was 1.33 mM. Excising 5, 13, 16 amino acids from the C-terminal residues resulted in activity decreasing to 0.8% of the wild type, while excising 54 amino acids caused the rADI to be insoluble. Sequence alignment of Giardia and Dictyostelium suggests a homologous area within the C-terminal extension. Site directed mutagenesis on the Tyr567 residue in this region resulted in a decrease in activity, with the highest activity observed for a Tyr to Phe mutation. Studies using specific cysteine protease inhibitors demonstrated partial protection against proteolysis of ADI against giardial proteases. Degradation of ADI by giardial proteases was more rapid at pH 6 than at pH 7.4.