THE PREDOMINANT FORM OF MAMMALIAN DNA TOPOISOMERASE-I IN-VIVO HAS A MOLECULAR-MASS OF 100 KDA

DNA topoisomerase I was purified to apparent homogeneity from human He La cells as a single polypeptide with a molecular mass of 100 kDa, as assayed by both gel filtration column chromatography and SDS-polyacryl amide gel electrophoresis. No smaller forms of the enzyme were detecte d in the purified fraction. Therefore, smaller forms, which have been observed by other investigators, are likely to be the result of proteo lysis during isolation and are not relevant to the in vivo activity of DNA topoisomerase I.