The pathway of arginine catabolism in Giardia intestinalis

Abstract

In Giardia intestinalis, arginine is catabolised by the arginine dihydrolase pathway. The enzymes of the pathway (arginine deiminase, ornithine transcarbamoylase and carbamate kinase) were investigated and their basic kinetic parameters determined. The specific activity of arginine deiminase was 270 ± 23 nmol min −1 (mg protein) −1; ornithine transcarbamoylase, in the direction of citrulline utilisation 170 ± 22 nmol min −1 (mg protein) −1, and in the direction of ornithine utilisation 2100 ± 100 nmol min −1 (mg protein) −1; and carbamate kinase 2100 ± 400 nmol min −1 (mg protein) −1. The activities of these enzymes are between 10 and 250 fold greater than those reported for the enzymes in Trichomonas vaginalis, the only other parasite in which the arginine dihydrolase pathway has been reported. The flux through the pathway in G. intestinalis, as determined by the liberation of 14CO 2 from 1 mM [ 14C- guanidino]arginine was 30 nmol min −1 (mg protein) −1. This flux was not affected by valinomycin (0.1 μM), nigericin (3 μM), azide (5 mM) or cyanide (1 mM). The flux was only marginally affected by glucose up to 10 mM concentration. Conversely, the flux through glucose metabolism, as determined by the release of 14CO 2 from 1 mM [1- 14C]glucose was only 2 nmol min −1 (mg protein) −1, and was unaffected by arginine concentrations up to 10 mM. These observations suggest that there is no direct metabolic interface between arginine and glucose catabolism. The potential energy yield of ATP from the arginine flux is 7–8-fold greater than that from glucose, providing evidence for the prime importance of arginine in the energy economy of G. intestinalis. It is suggested that the flux through the arginine dihydrolase pathway is the major source of energy production, particularly in anaerobic conditions.