Reduced γ-Cystathionase Protein Content in Human Malignant Leukemia Cell Lines as Measured by Immunoassay with Monoclonal Antibody¹

A murine hybrid cell line has been produced which secretes immunoglobulin G₂b with specificity for human γ-cystathionase (EC 4.2.1.15). The antibody has been iodinated and used in combination with quantitative immunoelectrophoresis in an assay which is capable of detecting as little as 1.5 ng enzyme protein. Human lymphoblastic leukemia cell lines CEM and Laz-221 contain undetectable enzyme protein, corresponding to their behavior as cysteine auxotrophs. In contrast, nonmalignant lymphoblastoid lines contain easily detectable enzyme protein which correlates with their behavior as cysteine prototrophs. Other malignant leukocyte cell lines contained detectable but variable amounts of enzyme protein, suggesting that the enzyme may be a useful marker of cellular differentiation.