Transport and metabolic degradation of hydrogen peroxide in Chara corallina: model calculations and measurements with the pressure probe suggest transport of H2O2 across water channels

Abstract

A mathematical model is presented that describes permeation of hydrogen peroxide across a cell membrane and the implications of solute decomposition by catalase inside the cell. The model was checked and analysed by means of a numerical calculation that raised predictions for measured osmotic pressure relaxation curves. Predictions were tested with isolated internodal cells of Chara corallina, a model system for investigating interactions between water and solute transport in plant cells. Series of biphasic osmotic pressure relaxation curves with different concentrations of H₂O₂ of up to 350 mol m⁻³ are presented. A detailed description of determination of permeability (P_s) and reflection coefficients (σ_s) for H₂O₂ is given in the presence of the chemical reaction in the cell. Mean values were P_s=(3.6±1.0) 10⁻⁶ m s⁻¹ and σ_s=(0.33±0.12) (±SD, N=6 cells). Besides transport properties, coefficients for the catalase reaction following a Michaelis‐Menten type of kinetics were determined. Mean values of the Michaelis constant (k_M) and the maximum rate of decompositon (v_max) were k_M=(85±55) mol m⁻³ and v_max=(49±40) nmol (s cell)⁻¹, respectively. The absolute values of P_s and σ_s of H₂O₂ indicated that hydrogen peroxide, a molecule with chemical properties close to that of water, uses water channels (aquaporins) to cross the cell membrane rapidly. When water channels were inhibited with the blocker mercuric chloride (HgCl₂), the permeabilities of both water and H₂O₂ were substantially reduced. In fact, for the latter, it was not measurable. It is suggested that some of the water channels in Chara (and, perhaps, in other species) serve as ‘peroxoporins’ rather than as ‘aquaporins’.