The hepatobiliary disease marker serum alanine aminopeptidase predominantly comprises an isoform of the haematological myeloid differentiation antigen and leukaemia marker CD-13/gp150.

In clinical chemistry, determination of serum alanine aminopeptidase (AAP) levels has been found to be useful for detecting or confirming biliary obstructions from either intra- or extra-hepatic disorders. In haematology, 'gp150' is a surface-expressed protein molecule, recognised by monoclonal antibodies belonging to the so-called 'Cluster of Differentiation' (CD-) 13, which has independently been found to be a useful marker of myeloid leukaemia in addition to providing potential prognostic value. The current report links these two independent research streams and provides evidence that the hepatobiliary disease marker, serum AAP, predominantly comprises a circulating isoform of 'gp150'. Thus, a (CD-13) monoclonal antibody raised to, and specifically reactive with, cell surface myeloid 'gp150' is able to specifically and almost completely block serum (or plasma) AAP activity otherwise observed 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 patients with obstructive jaundice, raised levels of AAP are observed, which fall to near normal levels following preincubation with this monoclonal antibody. In addition, data are presented to support the view of varying isoforms of AAP within flowing blood. Finally, preliminary data is provided on AAP activity in cases of leukaemia. These studies should thus prove of use to clinical laboratories investigating the involvement of AAP activity in various pathological processes.