Monoclonal antibody D1.2 reacts specifically with Arginase I, the final enzyme in the urea cycle, whichis responsible for the hydrolysis of arginine to urea and ornithine. The highest concentration of theenzyme is present in the liver in which the bulk of ureagenesis occurs. Two types of arginases areknown: Arginase I and II. The cytosolic enzyme found primarily in liver is Arginase I, a 35 kD proteinthat circulates as trimer. Arginase II is exclusively located in the mitochondrion. Arginase I is next to theliver in man also expressed by mature fetal and adult red blood cells and activated monocytic cells.During inflammation induction of Arginase I by inflammatory cytokines in monocytic cells is consideredto lead to a local depletion of arginine resulting in a microenvironment that prevents nitric oxideproduction and arginine dependent T cell function. Arginase II is expressed by kidney, nucleated redblood cells, brain, spinal cord, gastro-intestinal tract, mammary gland and prostate. Enhancedcirculating Arginase I levels have been reported after surgery, following haemorrhage and in asthmaticpatients. Measurement of circulating Arginase I has been used experimentally as rapid marker for liverinjury.