Oxidative Changes: Antioxidant System

Oxidative changes:

Fruit ripening has been described as an oxidative phenomenon, which requires a turnover of active oxygen species, such as H2O2 and superoxide anion (Hamilton, 1974). For this to be the case there must be a balance between the production of active oxygen species and their removal by antioxidant systems. It is likely, therefore, that the antioxidant system will play a crucial role in the ripening process (Foyer and Halliwell, 1976). The term antioxidant can be considered to describe any compound capable of quenching active oxygen species without itself undergoing conversion to a destructive radical (Nishikimi and Yagi, 1996). Antioxidant enzymes are considered as those that either catalyze such reactions and/ or are involved

In general, superoxide can arise when electrons are misdirected and donated to oxygen. Mitochondrial electron transport, for example, is a well- documented source of superoxide radicals, as is the electron transport chain of the photosynthetic apparatus within the chloroplast. An additional problem for the chloroplast is the transfer of excitation energy from chlorophyll to oxygen, which can generate singlet oxygen (Bowler et al., 1992). Hydrogen peroxide is disposed by catalases and peroxidases. In plants, catalase is found predominantly in peroxisomes (and also in glyoxysomes) where it functions chiefly to remove the H2O2 formed during photorespiration (or during β-oxidation of fatty acids in glyoxsomes) (Bowler et al., 1992). In spite of its restricted location it may play a significant role in defense against oxidative stress since H2O2 can readily diffuse across membranes. Some of these enzymes have broad substrate specificity while others can only function with one. Catalase is an enzyme related to the cellular control. Catalase catalyses the dismutation of hydrogen peroxide into water and oxygen (Redinbaugh et al.,

Phenolic compounds are oxidized to Quinones. These quinines may polymerize into coloured, usually brown, products (Amiot et al., 1997). Rate of enzymic browning depends on whether the responsible enzyme is present, active and physically available. Enzymes other than polyphenoloxidase such as laccase, peroxidase and β-galacturonase are also reported to be involved in browning in a minor way (Mayer and Harel, 1979). It has been suggested that high levels of oxidative changes, together with high levels of phenolic substrate early in fruit development may act as a defense mechanism by protecting seeds from infection or predation prior to maturity (Mayer and Harel, 1979; Knee et al.,