Zinc protoporphyrin levels. ZPP levels are higher in Sod2^–/– peripheral red cells compared with Sod2^+/+. Treatment of SOD2-deficient (transplant recipient) mice with the antioxidant EUK-189 (duration 4 weeks) leads to a further increase in ZPP levels. The increase occurs despite a concomitant increase in hematocrit (from 0.237 ± 0.02 [23.7% ± 2%] without treatment to 0.305 ± 0.03 [30.5% ± 3%] after therapy) and a decrease in reticulocyte count (from 0.175 ± 0.06 [17.5% ± 6%] prior to therapy to 0.072 ± 0.007 [7.2% ± 0.7%] after therapy). Data presented are mean ± standard deviation; ANOVA was used for statistical analysis with Tukey-Kramer multiple comparison test between the 3 groups (n = 5 animals per group). In a second experiment (right), the effect of Euk-189 on Sod2^+/+ red cells was assessed following therapy for 21 days (n = 4 animals per group). Although these data are suggestive of an effect of Euk-189 on ZPP in Sod2^+/+ cells, results from this trial were not statistically significant (2-tailed t test, P = .07).