Identification of two arginase isoenzyme activities along the nephron of Meriones shawi

Conflicting theories on the existence of several renal arginase isoenzymes remain in debate. Because the activity of arginase is high in two embryologically different nephron segments of the Meriones shawi kidney, namely the cortical (CPST) and medullary (OSPST) proximal straight tubule and the outer medullary collecting duct (OMCD), we postulate that these nephron segments may contain different isoforms. Isolated nephron segments were dissected from collagenase-treated kidneys. Tubules were permeabilized with Triton X-100 (0.25%) and incubated with increasing Arg concentrations to characterize the arginase activity. The results were as follows: (1) in OMCD, one arginase isoform (E1), characterized by a high Arg affinity (1.160 mM), was present; (2) in CPST, two arginase isoforms were discovered - one, E1, had a similar Km (1.407 mM) to that found in OMCD whereas the other (E2) had a low affinity for Arg (Km =18.8 mM); and (3) in OSPST, two isoenzymes were present - E1 which had a Km of 1.478 mM and the second isoform that we named E2 which had a Km of 9.07 mM. In addition, arginase located in CPST and OMCD was strongly inhibited by Orn and Lys. The Ki value for Lys varied between 1.635 and 2.288 mM. Therefore, this work demonstrates that two arginase isoforms are present in the kidney of Meriones shawi. Isoform E1 is present in the proximal tubule and the collecting duct whereas isoform E2 is restricted to the proximal tubule.