To examine the interaction of human arginase II (EC ) with sub-strate and manganese ions, the His120Asn, His145Asn and Asn149Asp mutations were introduced separately. About 53% and 95% of wild-type arginase activity were expressed by fully manganese activated species of the His120Asn and His145Asn variants, respectively. TheKmfor arginine (– mm) was not altered and the wild-type and mutant enzymes were essen-tially inactive on agmatine.