Myeloperoxidase (MPO) is an enzyme found primarily within specific white blood cells, and its measurement is increasingly recognized as a significant component of health assessments. Elevated levels of this protein in the circulation signal underlying inflammatory processes that extend beyond its initial protective function. Understanding MPO, how it works, and why its presence in the bloodstream matters provides context for evaluating long-term health risks. This article explores the dual nature of MPO, detailing its testing, mechanism of damage, and strong association with cardiovascular health.
What is Myeloperoxidase and Its Immune Function
Myeloperoxidase is a heme protein enzyme packaged in high concentrations within the azurophilic granules of neutrophils, a common type of white blood cell. Its primary, protective role occurs during the immune response when the neutrophil engulfs a pathogen. MPO catalyzes a reaction that transforms hydrogen peroxide and chloride ions into hypochlorous acid (HOCl), a potent oxidizing agent.
Hypochlorous acid is a powerful natural bleach used by the body to destroy invading bacteria and fungi inside the neutrophil’s phagosome. The enzyme’s intense green color, caused by its heme pigment, contributes to the greenish hue sometimes seen in pus or sputum. This process is a foundational component of the innate immune system.
Testing and Interpreting MPO Levels
MPO levels are typically measured via a blood test, often using an enzyme-linked immunosorbent assay (ELISA) or similar immunoassay. The test measures the amount of free MPO protein released into the plasma, which acts as a marker for systemic or localized inflammation, particularly within the vascular system. Optimal levels are often cited as less than 470 picomoles per liter (pmol/L), though ranges vary by laboratory.
Elevated MPO levels signify chronic immune system activation and the release of the enzyme from activated white blood cells. Levels exceeding 540 pmol/L are associated with an increased risk profile. While high MPO can indicate general inflammation, its presence in the bloodstream is primarily viewed as a specific marker of vascular inflammation.
The Oxidative Damage Caused by MPO
When MPO is released inappropriately outside the protective environment of the neutrophil, its powerful chemical activity turns against the host’s tissues. The hypochlorous acid it generates is highly reactive and causes widespread oxidative stress, initiating a chain of cellular damage by attacking biological molecules.
One significant consequence is lipid peroxidation, where reactive species attack the fatty components of cell membranes and lipoproteins. MPO also causes protein modification through chlorination and carbonylation, altering the structure and function of necessary proteins. This excessive consumption of protective molecules, including antioxidants, leaves host tissues vulnerable to further injury.
MPO’s Link to Cardiovascular Risk
The primary clinical concern surrounding elevated MPO levels is their strong, independent association with cardiovascular risk. MPO is actively involved in the development of atherosclerosis, the buildup of plaque within artery walls, by promoting endothelial dysfunction.
MPO’s pro-oxidant activity is particularly damaging to lipoproteins like Low-Density Lipoprotein (LDL) and High-Density Lipoprotein (HDL). It oxidizes LDL, transforming it into a modified form easily engulfed by macrophages, leading to the formation of foam cells, which are precursors of atherosclerotic plaque. MPO-catalyzed oxidation also impairs HDL, causing it to lose its protective ability to remove cholesterol from the arteries.
MPO also contributes to the destabilization of existing atherosclerotic plaques. The enzyme and its products modulate the activity of matrix metalloproteinases (MMPs) in vulnerable plaques. These MMPs degrade the structural components, like collagen, that form the fibrous cap over a plaque’s lipid core, making the plaque more likely to rupture and trigger a heart attack or stroke.
Strategies for Managing High MPO Levels
Managing elevated MPO levels focuses on reducing the underlying chronic inflammation that drives its release. Adopting a heart-healthy dietary pattern, such as the Mediterranean diet, helps by emphasizing antioxidant-rich foods like fruits, vegetables, and whole grains. These foods provide compounds that neutralize the excessive reactive species generated by MPO.
Specific nutritional components have been studied for their potential to influence MPO activity or mitigate its effects. Ascorbic acid (Vitamin C) has shown inhibitory effects on the MPO enzyme in laboratory settings. The amino acid L-cysteine and the polyphenol curcumin, derived from turmeric, have also been investigated, though clinical results for curcumin have been inconsistent.
Additionally, managing established cardiovascular risk factors—including high blood pressure, maintaining a healthy weight, and incorporating regular physical activity—reduces inflammatory load and vascular stress.