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Myeloperoxidase-catalyzed oxidation of tyrosine
1Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania, 16802, USA. mxt3@psu.edu
Abstract:
The oxidation of tyr by myleoperoxidase (MPO) is postulated to play a role in atherosclerotic plaque formation. MPO has been localized in plaques and a product of MPO-catalyzed oxidation of tyr, dityrosine, also found in plaques, is proposed to be a protein cross-linking agent. We have performed kinetic studies on the oxidation of tyr by MPO and investigated the role of substrate size on its oxidation. The kinetics of MPO-catalyzed oxidation of tyr where the tyr is free tyr, the dipeptides, tripeptides, and polypeptides were studied by stopped-flow methods. The rate of reaction with enzyme intermediates compound I and compound II are decreased with increasing substrate size. The amount of dityrosine formed was also decreased with increasing substrate size. The ability of sulfhydryl compounds to inhibit MPO-dependent dityrosine formation was investigated with reduced glutathione, cys, and met. Glutathione and cys both served as substrates for MPO compound I but not compound II, whereas met was not a substrate for either compound I or II. Met, an amino acid postulated to act as a "last chance" antioxidant for proteins, was not able to inhibit dityrosine formation from MPO-catalyzed oxidation of tyr. Glutathione and cys caused partial inhibition; however, it is possible that this inhibition was due to their ability to react directly with MPO rather than trapping the tyr radicals.
Insights
Myeloperoxidase (MPO) oxidation of tyrosine (tyr) contributes to atherosclerosis. Larger substrates reduce dityrosine formation, a proposed protein cross-linker, suggesting size influences MPO
Area of Science:
- Biochemistry
- Enzymology
- Cardiovascular Research
Background:
- Myeloperoxidase (MPO) is implicated in atherosclerosis.
- MPO-catalyzed tyrosine (tyr) oxidation produces dityrosine, a potential protein cross-linker found in plaques.
- The role of substrate size in MPO-catalyzed tyr oxidation is not fully understood.
Purpose of the Study:
- To investigate the kinetics of MPO-catalyzed tyr oxidation with varying substrate sizes.
- To determine the effect of substrate size on dityrosine formation.
- To evaluate the inhibitory potential of sulfhydryl compounds on MPO-dependent dityrosine formation.
Main Methods:
- Stopped-flow kinetic studies were employed to analyze MPO-catalyzed oxidation of free tyr, dipeptides, tripeptides, and polypeptides.
- Quantification of dityrosine formation was performed.
- The inhibitory effects of reduced glutathione, cysteine (cys), and methionine (met) were assessed.
Main Results:
- The rate of MPO reaction with enzyme intermediates (compound I and II) decreased as substrate size increased.
- Dityrosine formation significantly decreased with larger tyr-containing substrates.
- Methionine did not inhibit dityrosine formation, while glutathione and cysteine showed partial inhibition, possibly through direct MPO reaction.
Conclusions:
- Substrate size is a critical factor modulating MPO-catalyzed tyr oxidation and dityrosine formation.
- The findings suggest that larger substrates are less prone to forming dityrosine, potentially impacting plaque cross-linking.
- Sulfhydryl compounds like glutathione and cysteine exhibit complex interactions with MPO, not solely acting as radical scavengers.