Phosphorylation of purified Novikoff hepatoma topoisomerase I

The purified Novikoff hepatoma nuclear phosphoprotein with a molecular weight of 110 kdalton and pI 8.4, was found to be a type I topoisomerase. When isolated from 32P-labeled Novikoff ascites cells or incubated in vitro with protein kinase, phosphoserine was found to be its major phosphorylated amino acid. The enzymatic activity of topoisomerase I was altered by changes in phosphorylation. Its activity was increased by protein kinase and it was decreased by alkaline phosphatase.