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 transpo rt of arginine into Giardia intestinalis trophozoites. The transporter had a high affinity for arginine (K-m 15 mu M) and a high activity [V -max 76 nmol min(-1) (mg protein)(-1) at 25 degrees C]. Substrate spec ificity studies indicated an absolute requirement for the alpha-amino and carboxyl groups, but a tolerance for some substitutions in the gua nidino group. The use of non-metabolized arginine analogues in combina tion with HPLC amino acid analysis of intra- and extracellular pools d emonstrated that the arginine transporter is an arginine-ornithine ant iport. Investigations of the first step of arginine metabolism, involv ing arginine deiminase, revealed a relatively high affinity for argini ne (K-m 0.16 mM) and a large maximal velocity [V-max 550 nmol min(-1) (mg protein)(-1) at 37 degrees C]. Substrate specificity studies showe d that the arginine deiminase had a characteristically different subst rate recognition profile to that of the arginine transporter. Overall, the combination of the transporter and the deiminase result in very l ow intracellular arginine concentrations and their properties are cons istent with the rapid transport of arginine for metabolism via the arg inine dihydrolase pathway.