Properties of ascorbate oxidase isozymes

The ascorbate oxidase of two squash cultivars was resolved into five molecular forms by gel electrophoresis; that of cucumber was resolved into three forms. Molecular weight estimates by Sephadex gel filtration and interconversions of these forms strongly suggest the presence of a monomeric form of MW 30 000 for the cucumber enzyme and 35 000 for that of the squashes. The other two forms in the cucumber appear to be a dimer and a tetramer, whilst a tetramer, an octamer, a dodecamer, and a polymer of MW between 670 000 and 2 000 000 are likely to be the other four forms present in the squashes. The monomer was the most abundant form in the cucumber and the tetramer in the two squashes. The peel of these fruits was higher in activity than the flesh, but the isozyme pattern was the same in peel and flesh. The tetramer of the squashes and the dimer of cucumbers were the most resistant forms to heat inactivation. The enzyme is soluble and not associated with subcellular particles.