FIGURE 2.

a, dimer; and b, peroxide formation from Tyr-containing peptides. Samples were incubated with HRP, acetaldehyde, and XO and the respective peptide as described under “Experimental Procedures.” These peptides were shown to be good substrates for HRP, with initial rates of dimer formation measured under similar conditions of 2–16 times that of Tyr. Dimers were detected by fluorescence and hydroperoxides by the FOX assay. Zero time blanks were subtracted and error bars represent the range of means from two experiments. Peroxide yields are expressed as H₂O₂ equivalents using a calibration curve that was developed with known amounts of H₂O₂. However, it should be noted that isotope tracer studies have demonstrated that tyrosine hydroperoxide concentrations are underestimated by the FOX assay and are approximately 6 times higher than when expressed as H₂O₂ equivalents (5).