Arginine is metabolized by the arginine dihydrolase pathway in trophozoites and is an important metabolic fuel for this parasite. Radiolabelled arginine was used to characterize the transport of arginine into trophozoites. The transporter had a high affinity for arginine ( 15 üM) and a high activity [ 76 nmol min (mg protein) at 25 °C]. Substrate specificity studies indicated an absolute requirement for the α-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 ( 0.16 mM) and a large maximal velocity [ 550 nmol min (mg protein) at 37 °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.


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