The relationship among
cancer growth, the
glutathione redox cycle and the
antioxidant system was studied in blood and in tumour cells. During
cancer growth, the
glutathione redox status (GSH/
GSSG) decreases in blood of Ehrlich
ascites tumour-bearing mice. This effect is mainly due to an increase in
GSSG levels. Two reasons may explain the increase in blood
GSSG: (a) the increase in
peroxide production by the tumour that, in addition to changes affecting the
glutathione-related and the
antioxidant enzyme activities, can lead to GSH oxidation within the red blood cells; and (b) an increase of
GSSG release from different tissues into the blood. GSH and
peroxide levels are higher in the tumour cells when they proliferate actively, however
GSSG levels remain constant during tumour growth in mice. These changes associate with low levels of lipid peroxidation in plasma, blood and the tumour cells. The GSH/
GSSG ratio in blood also decreases in patients bearing breast or
colon cancers and, as it occurs in tumour-bearing mice, this change associates with higher
GSSG levels, especially in advanced stages of
cancer progression. Our results indicate that determination of
glutathione status and oxidative stress-related parameters in blood may help to orientate
cancer therapy in humans.