Thiols as myeloperoxidase-oxidase substrates.

Area of Science:

• Biochemistry
• Enzymology

Background:

• Myeloperoxidase (MPO) is a key enzyme in neutrophil-mediated immunity.
• Understanding MPO substrates is crucial for studying oxidative stress and inflammation.

Purpose of the Study:

• To investigate the myeloperoxidase-oxidase activity of various low-molecular-weight thiols.
• To elucidate structure-activity relationships for MPO substrate potential.

Main Methods:

• Assessed oxygen consumption by MPO in the presence of nine different low-Mr thiols.
• Investigated the effects of superoxide dismutase, catalase, and mannitol on thiol oxidation.
• Determined pH optima for thiol oxidation.

Main Results:

• Methyl and ethyl esters of cysteine showed significant MPO-oxidase activity, approximately twice that of cysteamine.
• Cysteine, glutathione, N-acetylcysteine, and penicillamine were inactive or poorly active.
• Activity required free thiol and amino groups; free carboxyl groups abolished activity.
• Cysteine ester oxidation was inhibited by superoxide dismutase (<5 μg/ml) and catalase, but not mannitol.
• High concentrations of superoxide dismutase (>5 μg/ml) stimulated cysteine ester activity.
• Broad pH optima around pH 7 were observed for cysteine esters.

Conclusions:

• Cysteine esters are effective substrates for myeloperoxidase-oxidase activity.
• Structure-activity relationships suggest specific functional groups are essential for MPO substrate activity.
• Proposed a novel mechanism for cysteine ester oxidation by MPO, differing partly from cysteamine's pathway.