Antioxidant Enzymes

Although the relatively small-sized antioxidants, vitamin C, vitamin E, and glutathione, have received the most public attention, large antioxidant enzymes in the cell also perform a vital role in neutralizing free radi-cals.a In fact, they act more efficiently than the small antioxidant compounds because they directly catalyze the conversion of free radicals to other, less harmful free radicals or to inert or useful compounds such as water and reduced glutathione. Unlike the small antioxidant compounds, antioxidant enzymes do not become free radicals that need to be regenerated. The conversion of free radicals by antioxidant enzymes is illustrated in Figure 5.2.

The left side of the figure shows how oxygen (O2) loses an electron to produce the superoxide radical (O2* ). Superoxide is then converted to hydrogen peroxide by the enzyme SOD. Hydrogen peroxide is converted to harmless water (H2O) by the enzymes catalase, glutathione peroxidase, or both, with glutathione peroxi-dase requiring the presence of glutathione to be effective. The right side of the figure shows that the enzyme glutathione reductase converts oxidized glutathione to reduced glutathione. Reduced glutathione either can scavenge free radicals itself, or it can act with glu-tathione peroxidase as shown.

Interestingly, cancer cells are frequently deficient in the enzymes catalase and SOD, a deficiency that would have the effect of producing extra superoxide radicals and hydrogen peroxide, as long as glutathione peroxi-

a A number of other relatively large protein compounds act as antioxidants, including uric acid and albumin, but will not be treated here.

Figure 5.2

Reactive Oxygen Species and Enzyme Antioxidants

(electron loss)

Legend: ® enzyme SOD: superoxide dismutase O2: molecular oxygen

hydrogen peroxide superoxide radical

, copper/zinc SOD manganese SOD

H,O, catalase

NADH

, copper/zinc SOD manganese SOD

glutathione glutathione reductase glutathione

oxidized glutathione

H2O: water

dase is not increased. Indeed, cancer cells are known to produce large quantities of hydrogen peroxide, as discussed in Chapter 2. Superoxide concentrations are also commonly increased at cancer sites. Increased production of these ROS may give cancer cells certain survival advantages, some of which are discussed below.

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