A highly sensitive and specific solid-phase enzyme immunoassay system for terminal deoxynucleotidyl transferase (TdT, EC 2.7.7.3 1) has been developed by the use of monospecific antibody against calf thymus TdT and P-D-galactosidase from Escherichia coli as label. The immunoassay system was composed of solid phase (polystyrene beads) with immobilized F(ab')r antibody fragments and the antibody Fab' fragments labeled with 0-Bgalactosidase. The minimum detectable concentration of calf TdT was 0.1 rig/ml (0.01 rig/assay), making it more sensitive than the radioimmunoassay or enzyme immunoassay methods that use alkaline phosphatase as label, as reported previously. The assay system cross-reacted with human TdT, and TdT in neoplastic cells or sera from leukemic patients was successfully detected by the present immunoassay method.
' Abbreviations used: TdT, terminal deoxynucleotidyl transferase; ALL, acute lymphoblastic leukemia; CML-BC, chronic myelogenous leukemia in blastic crisis; AML, acute myelogenous leukemia: buffer A, IO mM sodium phosphate, pH 7.0, containing 0.1 M NaCl, 1 mM MgClr, 0.1% bovine serum albumin, and 0.1% NaNs; IgG, immunoglobulin G; F(ab')r, pepsin-treated IgG which comprises a dimer of two identical units, the Fab' fragments, each with one combining site, linked by one disulfide bond.