Aminopeptidase N (CD13, EC 3.3.4.11.2) Occurs on the Surface of Resting and 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 stimulated human lymphocytes. 1) 50% of total Ala-pNA hydrolysis of viable cells was found to be due to the ectoenzyme APN as revealed by titration with the new inhibitor probestin in comparison to purified APN from human kidney (Ki value 6.5 nM). After stimulation with ConA an increase 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 expression as detected by antibody surface labeling of unstimulated (5-12% CD13 positive cells) and stimulated lymphocytes (28-36% CD13 positive lymphocytes) using polyclonal and monoclonal antibodies. 3) In vivo APN was found to be expressed on the surface of up to 50% lymphocytes isolated from the synovial fluid of rheumatoid patients.