Relationship between copper- and zinc-induced oxidative stress and proline accumulation in Scenedesmus sp.

A 4-h exposure of Scenedesmus sp. to Cu or Zn enhanced intracellular levels of both test metals and proline. The level of intracellular proline increased markedly up to 10 muM Cu, but higher concentrations were inhibitory. However, intracellular proline consistently increased with increasing concentration of Zn in the medium. Cu and Zn induced oxidative stress in the test alga by increasing lipid peroxidation and membrane permeability, and by reducing SH content. Pretreatment of the test alga with 1 mM proline for 30 min completely alleviated Cu-induced lipid peroxidation, minimized K+ efflux and also reduced depletion of the SH pool. But proline pretreatment could only slightly reduce Zn-induced oxidative stress. Interestingly, proline pretreatment increased the level of Cu (25-54%) and Zn (19-49%) inside the cells. It did not affect the activities of superoxide dismutase, ascorbate peroxidase or catalase, but improved glutathione reductase activity under Cu and Zn stress. A comparison of the effects of proline pretreatment on lipid peroxidation by Cu, Zn, methyl viologen and ultraviolet-B radiation suggests that proline protects cells from metal-induced oxidative stress by scavenging reactive oxygen species rather than by chelating metal ions. Pretreatment of cells with a known antioxidant (ascorbate) and a hydroxyl radical scavenger (sodium benzoate) considerably reduced metal-induced lipid peroxidation and proline accumulation. However, sodium benzoate had a very mild effect on Zn-induced lipid peroxidation and proline accumulation. The present study demonstrates that proline possibly acts by detoxifying reactive oxygen species, mainly hydroxyl radicals, rather than by improving the antioxidant defense system under metal stress.