Compositions and methods for the treatment of cancer are described, and, more preferably, to the treatment of cancers that do not express, or are otherwise deficient in, argininosuccinate synthetase, with enzymes that deplete L-Arginine in serum. In one embodiment, the present invention contemplates an arginase protein, such as a human Arginase I protein, comprising at least one amino acid substitution and a metal cofactor, said protein comprising an increased catalytic activity when compared with a native human Arginase I.