The aim of this paper was to assess the total antioxidant capacity of some commercial fruit juices (namely citrus), spectrophotometrically and by the biamperometric method, using the redox couple DPPH· (2,2-diphenyl-1-picrylhydrazyl)/DPPH (2,2-diphenyl-1-picrylhydrazine). Trolox^® was chosen as a standard antioxidant. In the case of the spectrophometric method, the absorbance decrease of the DPPH· solution was followed. For the biamperometric method, the influence of some parameters like the potential diference, ΔE, DPPH· concentration, and Trolox^® concentration was investigated. The calibration graph obtained for Trolox^® presents linearity between 5 and 30 µM, (y = 0.059 x + 0.0564, where y represents the value of current intensity, expressed as μA and x the value of Trolox^® concentration, expressed as μM; r² = 0.9944). The R.S.D. value for the biamperometric method was 1.29% (n = 10, c = 15 μM Trolox^®). In the case of the spectrophotometric method, the calibration graph obtained for Trolox^® presents linearity between 0.01 and 0.125 mM (y = -9.5789 x+1.4533, where y represents the value of absorbance and x, the value of Trolox^® concentration, expressed as mM; r² = 0.9963). The R.S.D. value for the spectrophotometric method was 2.05%. Both methods were applied to total antioxidant activity determination in real samples (natural juices and soft drinks) and the results were in good agreement.

A method was developed for assessing ascorbic acid concentration in commercial fruit juice by cyclic voltammetry. The anodic oxidation peak for ascorbic acid occurs at about 490 mV on a Pt disc working electrode (versus SCE). The influence of the potential sweep speed on the peak height was studied. The obtained calibration graph shows a linear dependence between peak height and ascorbic acid concentration in the domain (0.1–10 mmol⋅L−1). The equation of the calibration graph wasy=6.391x+0.1903(whereyrepresents the value of intensity measured for the anodic peak height, expressed asμA andxthe analyte concentration, as mmol⋅L−1,r2=0.9995,r.s.d.=1.14%,n=10,Cascorbicacid=2 mmol⋅L−1). The developed method was applied to ascorbic acid assessment in fruit juice. The ascorbic acid content determined ranged from 0.83 to 1.67 mmol⋅L−1for orange juice, from 0.58 to 1.93 mmol⋅L−1for lemon juice, and from 0.46 to 1.84 mmol⋅L−1for grapefruit juice. Different ascorbic acid concentrations (from standard solutions) were added to the analysed samples, the degree of recovery being comprised between 94.35% and 104%. Ascorbic acid determination results obtained by cyclic voltammetry were compared with those obtained by the volumetric method with dichlorophenol indophenol. The results obtained by the two methods were in good agreement.

The Ca²⁺‐dependent response to oxidative stress caused by H₂O₂ or tert‐butylhydroperoxide (tBOOH) was investigated in Saccharomyces cerevisiae cells expressing transgenic cytosolic aequorin, a Ca²⁺‐dependent photoprotein. Both H₂O₂ and tBOOH induced an immediate and short‐duration cytosolic Ca²⁺ increase that depended on the concentration of the stressors. Sublethal doses of H₂O₂ induced Ca²⁺ entry into the cytosol from both extracellular and vacuolar sources, whereas lethal H₂O₂ shock mobilized predominantly the vacuolar Ca²⁺. Sublethal and lethal tBOOH shocks induced mainly the influx of external Ca²⁺, accompanied by a more modest vacuolar contribution. Ca²⁺ transport across the plasma membrane did not necessarily involve the activity of the Cch1p/Mid1p channel, whereas the release of vacuolar Ca²⁺ into the cytosol required the vacuolar channel Yvc1p. In mutants lacking the Ca²⁺ transporters, H₂O₂ or tBOOH sensitivity correlated with cytosolic Ca²⁺ overload. Thus, it appears that under H₂O₂‐induced or tBOOH‐induced oxidative stress, Ca²⁺ mediates the cytotoxic effect of the stressors and not the adaptation process.