Aiginase catalyses the last step of the urea cycle At least two isoenzymes of arginase are known, cytosolic ARG I and mitochondrial ARG II ARG I is predominantly expressed in liver cytosol, as a part of urea cycle in ureotelic animals The second isoform ARG II is primarily responsible for non-ureogenic functions, expressed in mitochondria of both hepatic and non-hepatic tissues in most vertebrates Most micro-organisms and invertebrates are known to have only one type of arginase. whose function is unrelated to ornithine-urea cycle (OUC) However. in ureo-osmotic marine elasmobranchs aiginase is localized in liver mitochondria as a part of OUC to synthesize Urea for osmoregulation An evolutionary transition occurred in arginase enzyme in terrestrial ureotelic vertebrates, with the evolution of ARG I from a pre-existing ancestral mitochondrial ARG II. This cytosolic ARG I activity is supposed to have first appeared in lung fishes, but the 40% and 60% distribution of arginase I and II activity in liver and kidney tissue of Heteropneustes fossilis indicates reconsideration of the above fact (C) 2009 Elsevier Inc All rights reserved.