L-Arginine transport and metabolism in Giardia intestinalis support its position as a transition between the prokaryotic and eukaryotic kingdoms

Arginine is metabolized by the arginine dihydrolase pathway in Giardia intestinalis trophozoites and is an important metabolic fuel for this parasite. Radiolabelled arginine was used to characterize the transport of arginine into Giardia intestinalis trophozoites. The transporter had a high affinity for arginine (Km 15 microM) and a high activity [Vmax 76 nmol min-1 (mg protein)-1 at 25 degrees C]. Substrate specificity studies indicated an absolute requirement for the alpha-amino and carboxyl groups, but a tolerance for some substitutions in the guanidino group. The use of non-metabolized arginine analogues in combination with HPLC amino acid analysis of intra- and extracellular pools demonstrated that the arginine transporter is an arginine-ornithine antiport. Investigations of the first step of arginine metabolism, involving arginine deiminase, revealed a relatively high affinity for arginine (Km 0.16 mM) and a large maximal velocity [Vmax 550 nmol min-1 (mg protein)-1 at 37 degrees C]. Substrate specificity studies showed that the arginine deiminase had a characteristically different substrate recognition profile to that of the arginine transporter. Overall, the combination of the transporter and the deiminase result in very low intracellular arginine concentrations and their properties are consistent with the rapid transport of arginine for metabolism via the arginine dihydrolase pathway.