AMINOPEPTIDASE-N (CD13, EC 3.4.11.2) OCCURS ON THE SURFACE OF RESTINGAND CONCANAVALIN A-STIMULATED LYMPHOCYTES

Using different approaches evidence is provided that aminopeptidase N (APN, EC 3.4.11.2, CD13) is expressed on the surface of resting and st imulated human lymphocytes. 1) 50% of total Ala-pNA hydrolysis of viab le cells was found to be due to the ectoenzyme APN as revealed by titr ation with the new inhibitor probestin in comparison to purified APN f rom human kidney (K(i) value 6.5nM). After stimulation with ConA an in crease of Ala-pNA hydrolysis was observed from 8 pkat up to 15 pkat/10 (6) cells. 2) This correlated with an increase of APN (CD13) surface e xpression as detected by antibody surface labeling of unstimulated (5- 12% CD 13 positive cells) and stimulated lymphocytes (28-36% CD13 posi tive lymphocytes) using polyclonal and monoclonal antibodies. 3) In vi vo APN was found to be expressed on the surface of up to 50% lymphocyt es isolated from the synovial fluid of rheumatoid patients.