A biosensor exploiting an electrochemically mediated enzyme-catalysed reaction was used to quantify relative contributions of cytoplasmic catalase and periplasmic cytochrome c peroxidase to the overall rate of hydrogen peroxide breakdown in cells of Paracoccus denitrificans. The effects of antimycin (an inhibitor of electron flow to cytochrome c peroxidase), the reaction rate versus substrate concentration profiles for the whole cells and subcellular fractions, and the time courses of oxygen concentration demonstrated a profound decrease in the capacity of cytochrome c peroxidase to reduce H2O2 under in vivo conditions. The reason is suggested to be a competition for available electrons between the enzyme and terminal oxidases metabolising oxygen produced by catalase.

Sinigrin, the P-D-thioglucoside of the cruciferous plant species was hydrolyzed for 15 min at pH 7 and 30°C by the enzyme myrosinase to liberate glucose and mustard oil allylisothiocyanate as aglucone. A Clark-type p02 sensor overlaid with a glucose oxidase + catalase membrane served for the glucose measurements, whereas the isothiocyanate component was measured (after conversion to allylthiourea) from the inhibition degree of a tyrosinase membrane/p02 sensor. The total amounts of glucosinolates found with the glucose probe in six assorted samples of rapeseed meal and evaluated in sinigrin equivalents (13.6 -147pmol/g) agreed with those obtained using gas chromatography as the reference. Poor agreement (results: 11 1.4-1 12.3%) was achieved when a different method of fat removal was used prior to electroanalysis. The amounts of progoitrin (not convertible to thiourea) estimated indirectly from the difference of both the glucose and the aglucone biosensor analyses were found to be in the range 6.2-103pmol/g (44.3-83.8% of the total glucosinolates).