Mammalian DNA ligases. Serological evidence for two separate enzymes

Mammalian cells contain two DNA ligase activities with different chromatographic properties, referred to as DNA ligase I and II. The major ligase activity present in calf thymus cell extracts, DNA ligase I, has been purified 1000-fold. After repeated injections of this enzyme with complete Freund's adjuvant into a rabbit, antibodies were induced that inhibit DNA ligase I from calf, human, mouse, and rabbit tissues. This antiserum did not affect DNA ligase II from the same sources to a detectable extent, even at a concentration 10-fold higher than that required for 98% inhibition of DNA ligase I. These data strongly indicate that the two mammalian DNA ligase activities are due to two separate enzymes, and not to two forms of the same enzyme. Both enzymes are present in the nuclear fraction, but are also found in the cytoplasmic fraction. Rapidly dividing cells (mouse ascites tumor cells and calf thymus) contain higher amounts of DNA ligase I than other cells (calf liver and spleen, human placenta, and rabbit spleen), while no such correlation was observed for DNA ligase II.