We have investigated the compartmentalization of
aminopeptidase-N-like activity in various blood fractions obtained from patients with acute lymphoid (ALL) or myeloid (AML)
leukemia. The primary difference appears not to be the absolute level of overall activity, but rather the relative proportions of the different forms of activity detected. Thus, despite similar levels of total
aminopeptidase-N-like activity detected in cells from different leukemic groups, true
aminopeptidase-N/CD13 activity was only detected in cells derived from AML patients. Even in these patients, however, most of the detected
aminopeptidase-N-like activity ( > 80%) could not be attributed to
aminopeptidase-N/CD13. In marked contrast, plasma from leukemic patients also contained substantial total
aminopeptidase-N-like activity, of which (irrespective of leukemic group) most could be attributed to
aminopeptidase-N/CD13. Whilst slightly higher levels of total activity were obtained in plasma from AML patients compared to ALL patients, there was no difference in the relative proportion attributable to
aminopeptidase-N/CD13 (approximately 80% of total
aminopeptidase-N-like activity). Evaluation of total
aminopeptidase-N-like activity present in whole blood gave differential patterns, and whilst only a proportion (20-40% of total
aminopeptidase-N-like activity) could be attributed to true
aminopeptidase-N/CD13, blood from patients with CD13+ AML showed the greatest activity so attributable. In total, our results outline the complexities of
peptidase activities present within blood of leukemic individuals, and may, in part, explain the variability of previous studies attempting to associate prognostic features with phenotypic expression of CD13.