DNA ligases during rat liver regeneration

Abstract

DNA ligase catalyses the formation of phosphodiester bonds at single-strand breaks in double-stranded DNA¹. Since DNA is replicated discontinuously, and radiation-induced DNA single-strand breaks are effectively repaired in both bacteria and eukaryotes, bacteria and mammalian cells will require DNA ligase function both for replication and repair of DNA^2–5. A DNA ligase activity has also been found in and purified from mammalian cell extracts⁶. It required ATP as a cofactor and was found in highest amounts in tissues containing rapidly dividing cells. In agreement with these data it has been reported that the DNA ligase activity increased five- to tenfold in rat liver extracts during liver regeneration^7,8. We have shown that mammalian cells contain two different DNA ligases (I and II) both of which are found in nuclei and cytoplasm, but neither of which is predominantly a mitochondrial enzyme^9,10. Both enzymes require ATP but have different molecular weights, fractionation properties, and heat stability. Furthermore, rabbit antibodies against purified calf thymus DNA ligase I effectively inhibit DNA ligase I from calf, mouse, rabbit, and human tissues, whereas DNA ligase II from the same sources is not inhibited. It was therefore of interest to determine whether the increase in total DNA ligase activity reported to occur during liver regeneration was due to an increase in both DNA ligase I and DNA ligase II activities or due to an increase in only one of the two enzymes. Such determinations are now possible when chromatographic properties of the enzymes are known, and antibodies against DNA ligase I are available. This report shows that the level of DNA ligase I activity increases about 15-fold during rat liver regeneration, whereas the level of DNA ligase II activity remains essentially unchanged.