OxyFile #244

Action of cystine in the cytotoxic response of Escherichia 
coli cells exposed to hydrogen peroxide.

Author:  Cantoni O; Brandi G; Albano A; Cattabeni F

Source:  Free Radic Res 1995 Mar; 22(3):275-83


Cystine markedly enhanced the cytotoxic response of Escherichia 
coli cells to concentrations of hydrogen peroxide resulting 
in mode one killing, but displayed little effect in mode 
two killed cells. The effect of cystine was concentration-
dependent over a range of 5-50 microM and did not further increase 
at higher levels. Cystine had similar effects in other 
bacterial systems. In order to sensitize the cells to the 
oxidative injury, the amino acid must be present during 
exposure to the oxidant since no enhancement of the cytotoxic 
response can be observed in cystine pre-loaded cells. In 
addition, no further enhancement of cytotoxicity could 
be detected when cystine was added before and left during 
challenge with the oxidant. The enhancing effect of cystine 
on oxidative injury of E. coli cells appears to be directly 
mediated by the amino acid and in fact cysteic acid, the 
most likely oxidation product, had no effect on the killing 
of bacterial cells elicited by hydrogen peroxide. Other 
disulfide compounds such as oxidized glutathione, cystamine 
and dithionitrobenzoic acid only slightly increased the 
susceptibility of bacteria to the oxidant. The effect of 
the disulfides was not concentration-dependent over a range 
of 200-800 microM and was statistically significant only 
for cystamine. Taken together, these results indicate that 
cystine markedly increases the cytotoxic response of bacteria 
to hydrogen peroxide and suggest that the amino acid might 
impair the cellular defence machinery against hydrogen 
peroxide. This effect may involve a thiol-disulfide exchange 
reaction at the cell membrane level.