Proposed mechanisms for the involvement of lactoferrin in the hydrolysis of nucleic acids

Abstract

Lactoferrin has recently been proposed to have ribonuclease activity in the absence of bound iron. We and others have demonstrated previously that lactoferrin interacts with DNA and will bind a number of transition metal ions via surface-exposed histidyl residues. In the present study, we investigated the possibility that surface-bound copper ions on lactoferrin may catalyze the production of active oxygen species responsible for the hydrolysis of nucleic acids. Purified lactoferrin (apo- and holo-forms) was incubated with CuCl2 in solution to obtain lactoferrin with surface binding sites saturated by Cu(II) ions. The lactoferrin-Cu(II) complex was purified by Bio-Gel P-6 chromatography columns and tested for hydrolytic activity against DNA and RNA as analyzed by agarose gel electrophoresis. Incubation of lactoferrin-Cu(II) complexes with supercoiled plasmid Bluescript II SK DNA led to the rapid formation of relaxed open circular or linear forms of DNA characterized by changed electrophoretic mobility. Lactoferrin with bound Cu(II) also caused extensive degradation of yeast tRNA molecules in the presence of hydrogen peroxide. Covalent modification of surface-exposed histidyl residues by carboxyethylation with diethylpyrocarbonate abolished the lactoferrin-associated hydrolytic activity. These results indicate that lactoferrin-bound Cu(II) can indeed facilitate the hydrolysis of DNA and RNA molecules. Copper-binding sites on lactoferrin appear to serve as centers for repeated production of hydroxyl radicals via a Fenton-type Haber-Weiss reaction. Enhanced nuclease activity associated with elevated local concentrations of lactoferrin would promote microbial degradation.