Individual differences in activity of glutathione peroxidase and catalase studied in monozygotic twins discordant for smoking.

1. Cigarette smoke contains free radicals. The enzymes glutathione peroxidase (GSH-px) and catalase are important parts of the anti-oxidative protecting system. 2. Ten pairs of monozygotic twins, who were discordant for smoking, were analysed in order to determine their erythrocyte glutathione peroxidase and catalase activities and their plasma concentrations of selenium. 3. Analysis of variance (ANOVA) revealed that the difference in activities of catalase and glutathione peroxidase was much less within pairs than between pairs, indicating a large individual variation due to genetic expression or shared environment and no major effect from smoking. 4. The plasma selenium levels of the investigated twins revealed sufficient intake of selenium to maintain maximal activity of GSH-px in erythrocytes. The mean +/- s.d. selenium concentration in plasma for smokers was 98 +/- 16 micrograms l-1 and for non-smokers 111 +/- 16 micrograms l-1. There was no correlation between plasma selenium and glutathione peroxidase in erythrocytes.