Activities of apocynin in cytotoxicity assays of potential pathological relevance

Low levels of some reactive oxygen and nitrogen species (ROS, RNS) are of physiological importance; but high levels result in oxidative stress and can perturb many cell functions including signal transduction and transport, and contribute to aging and chronic diseases. Apocynin (1-(4-hydroxy-3-methoxyphenyl)ethanone) is a phytochemical with reported antioxidant activities in some experimental models of human disease. The major objectives of the current study were to test the antioxidant capacity of apocynin in a hemin-peroxide assay, and test its capacity to moderate pro-oxidant-dependent inhibition of a cell function—endocytic transport. Apocynin, tested at concentrations up to 20 μM, did not exhibit statistically significant antioxidant activity (94.3 ± 7.8% relative to controls, p > 0.05) in the oxidation assay. When tested against the inhibition of endocytic transport by hydrogen peroxide, apocynin treatment did not significantly rescue such inhibition in the cell types tested (p > 0.05, relative to peroxide alone). When cells were treated with a cytotoxic protein aggregate that increased both ROS and RNS, apocynin treatment only inhibited production of the latter (30.0 ± 3.6% inhibition relative to controls without apocynin, p < 0.05). The results provide evidence that apocynin, unlike other phytochemicals such as curcumin, does not exhibit antioxidant activity in the heme-peroxide assay. The results also provide the first evidence that apocynin does not rescue hydrogen peroxide-mediated inhibition of endocytic transport, nor prevent hydrogen peroxide production by a cytotoxic protein aggregate. In the latter toxicity assay, however, apocynin could moderate oxidative stress by decreasing cellular levels of RNS.