DNA Ligase II of Mammalian Cells

A number of early reports on the purification of DNA ligase activities from mammalian sources suggested the possibility that more than one enzyme existed in these cells. The main indication arose from molecular weight estimates which revealed wide variations in different tissues. The DNA ligase from rabbit tissue had a mol. wt. of 95,000 [1] compared to 240,000 for the mouse enzyme [2]. Pedrali Noy et al. [3] purified the enzyme from a human heteroploid cell line, EUE. The enzyme was fractionated into two forms, one with a mol. wt. of 190,000, the other 95,000. In freshly prepared cell extracts the DNA ligase was present as the high molecular weight form, prolonged incubation, or enzyme purification lead to the appearance of the low molecular weight form, without variations in the total activity. A similar observation was made during purification of the calf thymus enzyme [4]. The smaller form of DNA ligase, in these instances, is therefore present as a result of dissociation of a high molecular weight form of the enzyme into a low molecular weight form (possibly a dimer into a monomer), or to proteolytic cleavage of the ligase.