THE MAIN NEUTRAL AMINOPEPTIDASE ACTIVITY OF HUMAN LYMPHOID TUMOR-CELLLINES DOES NOT ORIGINATE FROM THE AMINOPEPTIDASE N-(APN, CD13) GENE

Lymphocytes and related cell lines are predominantly CD13-negative, ho wever, there are reports describing neutral aminopeptidase activity in or on these cells. The aim of this study was to answer the question, whether this activity originates from APN-gene expression. The total c ellular activities (Ala-pNA hydrolysis) of lymphoid cell lines are up to 15 times higher than that of normal lymphocytes. Despite weak or la cking CD13 surface expression all lymphoid cell lines tested contain A PNmRNA as quantified by competitive RT-PCR as well as low enzymatic ac tivity in their particulate fractions. By isoelectric focusing two enz yme species with isoelectric points of 5.4 or between 3.5 to 4.8, resp ectively, were detected. To investigate whether these activities resul t from APN-gene we established transfectants lacking cellular APN expr ession of the CD13-positive histiocytic cell line U937 and the CD13-ne gative T cell line H9. Studies on these transfectants proved (I) that the main neutral aminopeptidase activity expressed in lymphoid cells i s definitively not related to APN and (II) that APN is also expressed in lymphoid cells, although on a low level only.