Myeloperoxidase Test

Myeloperoxidase (MPO) is an enzyme stored in a type of white blood cell called a neutrophil—part of your immune system’s first line of defense. When the body detects a threat, like bacteria or injury, neutrophils release MPO as part of their “attack” response. This enzyme helps generate highly reactive compounds like hypochlorous acid (the active ingredient in bleach) to kill invaders. While this is essential in short bursts, too much MPO activity can backfire, harming your own tissues and contributing to chronic inflammation.

MPO’s main job is to use hydrogen peroxide (a byproduct of cell metabolism) to create powerful oxidants—chemical substances that react with and damage microbes. However, these oxidants can also damage healthy components in the body, such as proteins, lipids (fats), and DNA. This collateral damage can trigger or worsen diseases driven by inflammation and oxidative stress.

One of the clearest links is with cardiovascular disease. MPO contributes to the oxidation of low-density lipoprotein (LDL), turning it into a form more likely to build up in artery walls. This buildup, called plaque, narrows the arteries and raises the risk of heart attacks and strokes. MPO also alters high-density lipoprotein (HDL), making it less effective at removing cholesterol from the arteries. Elevated MPO levels have been found in people with coronary artery disease, heart failure, ischemic stroke, and atrial fibrillation. In fact, some studies suggest that measuring MPO could help identify people at higher risk for a heart attack, even if traditional risk markers like LDL cholesterol or C-reactive protein appear normal.

MPO also plays a role in the brain. In conditions like Alzheimer’s and Parkinson’s disease, researchers have found increased MPO activity in areas of the brain affected by inflammation. This has led to growing interest in developing MPO inhibitors—drugs or even food-derived peptides that reduce its harmful activity.

Low MPO levels, while usually not dangerous, can be seen in a rare genetic condition called MPO deficiency. Most people with this condition don’t have serious problems, but they may be more vulnerable to fungal infections if they also have other health issues like diabetes. MPO deficiency is caused by mutations in the MPO gene and is usually inherited. Some people may also develop temporary MPO deficiency due to other illnesses or medications.

Because MPO straddles the line between immune defense and self-destruction, researchers are exploring ways to block its harmful effects without compromising its pathogen-killing abilities. Targeting MPO could become a valuable strategy in treating cardiovascular, autoimmune, and neurodegenerative diseases.