Fungitoxicity of oxine and copper oxinate: effects of pH, metals and chelating agents on activity

Oxine and copper oxinate have a long history of use as fungicides. Ability to chelate and lipophilicity have been regarded as essential to the action of oxine. The most widely held hypothesis on the mechanism of action of oxi ne holds that oxine is only active when it can form saturated chelates with metals in the medium which enter the cell and dissociate to liberate a tox ic half-chelate. Metals, chelating molecules and pH were investigated for t heir effect on the fungitoxicity of oxine and copper oxinate. Oxine fungito xicity increased with increase in medium pH, inhibitory activity correspond ing most closely with the concentration of the neutral species. Chelation o f oxine with metals in the medium was found not to be a requirement for oxi ne fungitoxicity. Potentiation of the action of oxine by metals is explaine d by the formation of more fungitoxic and soluble metal oxinates and antago nism by the formation of less soluble or less active metal oxinates. Chelat ing amino acids, nucleic acid bases and EDTA did not antagonize oxine fungi toxicity Antagonism by riboflavin and folic acid suggests interaction of ox ine with specific cellular functions in fungi. The fungitoxicity of copper oxinate was generally not affected by metals, chelating molecules or change s in medium pH. It is proposed that the data support the inherent fungitoxi city of oxine, the role of metal oxinates as co-toxicants and independent m echanisms of action for oxine and copper oxinate.