Negative regulation of Scl-70/topoisomerase I by zinc and an endogenous macromolecule

Scl-70/topoisomerase I (topo I), a target of autoantibodies in the human disorder scleroderma, may be linked to collagen overproduction. We report that physiologic concentrations of zinc (greater than or equal to 80 microM) potently inactivate human and rat topo I in the presence of a 125-fold molar excess of MgCl2. We also describe a highly acidic nuclear molecule which competitively inhibits topo I. 89kD complexes of topo I and inhibitor can be separated from active 70kD monomers by glycerol density gradient centrifugation. The complexes have a 20-fold higher Km for DNA than monomers. Dissociation by dilution results in a 50-fold increase in rat liver and a 47-fold increase in human fibroblast specific activity. Zinc inhibition is non-competitive, and independent of the endogenous inhibitor. Our studies suggest that the majority of topo I is normally maintained in an inactive state by physiologic inhibitors, and loss of negative regulation may have pathogenetic consequences.