THE HEPATOBILIARY DISEASE MARKER SERUM ALANINE AMINOPEPTIDASE PREDOMINANTLY COMPRISES AN ISOFORM OF THE HEMATOLOGICAL MYELOID DIFFERENTIATION ANTIGEN AND LEUKEMIA MARKER CD-13 GP150

In clinical chemistry, determination of serum alanine aminopeptidase ( AAP) levels has been found to be useful for detecting or confirming bi liary obstructions from either intra- or extrahepatic disorders. In ha ematology, 'gp150' is a surface-expressed protein molecule, recognised by monoclonal antibodies belonging to the so-called 'Cluster of Diffe rentiation' (CD-) 13, which has independently been found to be a usefu l marker of myeloid leukaemia in addition to providing potential progn ostic value. The current report links these two independent research s treams and provides evidence that the hepatobiliary disease marker, se rum AAP, predominantly comprises a circulating isoform of 'gp 1 50'. T hus, a (CD- 1 3) monoclonal antibody raised to, and specifically react ive with, cell surface myeloid 'gp 1 50' is able to specifically and a lmost completely block serum (or plasma) AAP activity otherwise observ ed in its absence. This holds true for serum (or plasma) derived both from normal individuals or from patients suffering hepatic dysfunction , with or without associated biliary obstruction. In the case of patie nts with obstructive jaundice, raised levels of AAP are observed, whic h fall to near normal levels following preincubation with this monoclo nal antibody. In addition, data are presented to support the view of v arying isoforms of AAP within flowing blood. Finally, preliminary data is provided on AAP activity in cases of leukaemia. These studies shou ld thus prove of use to clinical laboratories investigating the involv ement of AAP activity in various pathological processes.