Terminal deoxynucleotidyl transferase activity in human leukemic cells and in normal human thymocytes

Peripheral leukocytes from patients with and without leukemia were assayed for presence of terminal deoxynucleotidyl transferase. Activity of this enzyme was detected in circulating leukemic cells from 11 to 13 patients with acute lymphoblastic leukemia, and in one of four with chronic myelogenous leukemia in blast crisis, but not in leukocytes from patients with other kinds of leukemia or in normal leukocytes. Its presence in a patient with chronic myelogenous leukemia in blast crisis lends biochemical support to the suggestion that some patients with chronic myelogenous leukemia undergo a lymphoblastic rather than a myeloblastic crisis. The thymocyte and leukemic-cell enzyme have the same substrate and primer preference. Normal thymocytes and leukemic cells contain two forms of terminal deoxynucleotidyl transferase that can be separated by phosphocellulose chromatography. The enzyme may provide a means for classifying leukemic cells on a biochemical basis independently of classic morphologic and clinical criteria.