Biochemical characterization of a leukemia-associated inhibitor (LAI) suppressing normal granulopoiesis in vitro

Low-density (less than 1.077 g/ml) marrow or blood cells from patients with acute or chronic leukemia release a high molecular weight substance called “leukemia-associated inhibitor” (LAI) that reduces the fraction of normal marrow CFU-c in S-phase as measured with the 3H-TdR suicide technique. LAI from conditioned media or 3M KCl extracts of subcellular fractions behaved homogeneously on gel chromatography, showing an apparent molecular weight greater than 500,000. However, ion- exchange chromatography and isoelectric focusing indicated considerable charge heterogeneity for LAI molecules. Results from SDS-polyacrylamide gel electrophoresis indicated that the biologic activity resides in a subunit of 150,000–170,000 daltons. The findings of marked affinity for Con-A-Sepharose, marked susceptibility to mild periodate treatment, partial susceptibility to protease digestion, and relative resistance to heating suggest that LAI is a glycoprotein. Data from radiolabeling of cell surface components and sucrose density gradient centrifugation are consistent with LAI being a peripheral cell membrane glycoprotein, which may suppress normal granulopoiesis in leukemia.