Ascorbate oxidation reduction in Helicoverpa zea as a scavenging system against dietary oxidants

Abstract

We provide evidence of an important role for ascorbate free radical (AFR) reductase, dehydroascorbate (DHA) reductase, glutathione, and glutathione reductase as components of an oxidant-scavenging system in the midgut of larval Helicoverpa zea. Also, midgut ortho-quinone reductase is a potentially important constituent of the protective system against quinones. The midgut activities of AFR reductase, DHA reductase, glutathione reductase, and orthoquinone reductase were, respectively, 168,22.1,6, and 39.5 nmol/min/mg protein. The relatively high activity of these enzymes in the midgut provides circumstantial evidence for a protective mechanism utilizing ascorbate as an antioxidant and glutathione and/or NADPH as reductants. To our knowledge, the enzymes AFR reductase and DHA reductase have not been reported in insects. The particular relevance of this system to antioxidant protection, and in particular to the detoxication of quinones formed in damaged leaf tissues, is discussed.

Ascorbate peroxidase: A novel antioxidant enzyme in insects

“…1) (Arrigoni et al, 1981;Hossain et al, 1984;Coassin Felton and Summers, 1995). These enzymes, DHAR and AFRR, are present in insects (Felton and Duffey, 1992;Summers and Felton, 1993). As APOX loses its activity in an ascorbate-depleted medium, the importance of DHAR and AFRR should not be overlooked in recycling ascorbate and keeping tissue levels of ascorbate sufficient for enzyme activity.…”

Salivary glucose oxidase: Multifunctional roles forHelicoverpa zea?

“…Coupled with lumenal catalase and/or ascorbate peroxidase to remove H 2 O 2 , GOX may serve as part of an antioxidant enzyme system in the midgut (Felton and Duffey, 1991;Mathews et al, 1997) and prevent H 2 O 2 from degrading into other more reactive oxygen species, e.g., hydroxyl radical (Ahmad and Pardini, 1990 (Tsai et al, 1995) leading to synthesis of NADPH, nucleosides, fatty acids, or sorbitol (Friedman, 1985). The production of NADPH could enhance reactions requiring NADPH as a reductant including ascorbate free radical reductase, quinone reductase, glutathione reductase, and polysubstrate monoxygenases (Ahmad et al, 1986;Felton and Duffey, 1992;Mathews et al, 1997).…”

Antioxidant Mechanisms of Enzymes and Proteins

“…However, at this juncture it is not clear whether this enzyme is same as in plants; the NADH dependence has not been established and the mammalian enzyme may depend on other sources of reducing equivalents. The occurrence of an AFR-like enzyme was recently claimed for an insect species Helicoverpa zea (Felton and Duffy, 1992). Clearly, more work is needed to characterize the animal vs. plant enzyme.…”

“…Nonetheless, the two enzymes may be complementing each other in recovering ascorbate from its two oxidized states (for more details see chapters 9 and 10). The occurrence of DHA reductase in animal species has been speculated, and the first reliable claim for the presence of this enzyme in the insect species, H. zea, was made very recently (Felton and Duffy, 1992). The enzyme has since been isolated and its characteristics are that of the plant enzyme (Summers and Felton, 1993).…”