Does fish represent an intermediate stage in the evolution of ureotelic cytosolic arginase I?

Arginase 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 arginase 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.