Hypobromous acid (
HOBr) generated by activated eosinophils has been implicated in tissue injury observed in
asthma,
allergic reactions, and some
infections.
Proteins are major targets for this
oxidant, but the mechanisms by which
HOBr induces loss of function are not well-established. In this study, we have examined the effect of
HOBr on
protein structure (as assessed by
amino acid loss, side chain oxidation, fragmentation, aggregation, and unfolding) and activity of a model
protease inhibitor, soybean
trypsin inhibitor (
STI), and the protective
enzyme lysozyme. Exposure of both
proteins to low
oxidant concentrations (< or = 5-fold molar excess) results in loss of function. In each case, loss of activity is associated with the selective oxidation of His, Trp, and Tyr residues, which results in protein unfolding (with
lysozyme) and
protein aggregation (with
STI). Reaction with these residues accounts for 25 and 50% of the
HOBr with
STI (25-fold excess) and
lysozyme (4-fold excess), respectively. These processes are believed to lead to changes in the structure of the
proteins, which disrupts substrate binding. With both
proteins, the oxidation of other residues, including Met, does not appear to play a major role. Bromamines, formed by reaction with
amine groups, are major products, which account for 45 and 35% of the
HOBr with
STI (25-fold excess) and
lysozyme (4-fold excess), respectively. Decomposition of these species correlates with secondary oxidation reactions, and with
lysozyme, a time-dependent loss in activity. Overall, 70% of the
HOBr can be accounted for with
STI and 95% with
lysozyme.