Arginase catabolism

The catabolic pathway of arginine is of special interest because the first step of this pathway is catalyzed by arginase, an enzyme noted for its participation in the... 

The major pathway for catabolism of arginine is arginase to form ornithine, followed by ornithine transamination and oxidation of the resulting glutamic semialdehyde to form glutamate. For the most part, this catabolism occurs... 

Arginase has been widely reported among helminths and there is evidence that arginine can be catabolized to COj (1). In some trematodes, such as F. hepatica and S. mansoni, proline is the end product of arginine catabolism. 

The catabolism of amino acids results in the release of nitrogen. Many parasites excrete the excess nitrogen in the form of ammonia. In protozoa there is no evidence for a complete urea cycle which would allow the conversion of ammonia into urea, and urea released by some species (49,50) is likely to be formed solely as a product of arginine catabolism. Urea makes up between 2 and 10% of the total nitrogenous end-product of helminths, but evidence for a functional urea cycle in parasitic species has been equivocal (1). Arginase and ornithine carbamoyltransferase do occur widely but the other urea cycle enzymes do not. Small quantities of uric acid are excreted by some cestodes and trematodes. 

Fig. 5.8. Urea cycle and its relationship with the Krebs cycle. Comparative catabolism of arginine nnder the effect of arginase and arginine deiminase...

Although the overall reaction is reversible with a measurable equilibrium constant, the reaction is probably virtually irreversible in vivo because of the ubiquitous pyrophosphatases. It is undoubtedly this step that prevents conversion of arginine to ornithine by reversal of the ornithine cycle and requires an arginase for arginine catabolism (see page 386). 

---