Effect of pH on phenol oxidase activity on decaying Sphagnum mosses

Inhibition of phenol oxidase (PO) activity by low pH is a potential key mechanism to limit decomposition in acidic Sphagnum mires, but few controlled experiments have been performed. The possible effects of assay pH and of auto-oxidation of the commonly used substrate L-dihydroxy phenylalanine (L-DOPA) have remained unreported. We conducted a series of experiments in order to reveal realistic response of PO activity to the pH gradient, taking into account the possible effects of auto-oxidation and assay pH. We found positive responses to pH with 60–100% increase of PO activity with one pH unit (3.5–4.5). In alkaline solutions, L-DOPA was readily auto-oxidized, which may hamper the use of this reagent with alkaline samples. Adjustment of assay pH resulted in variable responses of PO activity to temperature and sample pH. Combinations of matching sample pH and assay pH consistently showed a positive response of PO activity to pH, indicating that control of assay pH is an essential prerequisite of obtaining realistic results of PO activity. The positive relationship of PO activity and pH indicates that the inhibition of enzymic oxidation of phenolic compounds may contribute to the decay resistance of Sphagnum in acidic mire habitats.

► Phenol oxidase activity on decaying Sphagnum samples is positively related to pH.
► High pH promotes phenolic degradation.
► The phenol oxidase substrate L-DOPA is auto-oxidized in alkaline pH.
► Control of pH through the enzyme assay is important in phenol oxidase activity measurements.