Catalyzes the hydrolysis of L-arginine to urea and L-ornithine. The latter can be utilized in the urea cycle or as a precursor for the synthesis of both polyamines and proline. H2O + L-arginine = L-ornithine + urea Binds 2 manganese ions per subunit. kcat is 490 sec(-1) with L-arginine as substrate. Nitrogen metabolism; urea cycle; L-ornithine and urea from L-arginine: step 1/1. Belongs to the arginase family.