The fat directly on the heart makes more harmful chemicals than the fat around blood vessels, and this matches with it having more of the enzyme MAO-A.

The fat directly on the heart makes more harmful molecules than the fat around the big blood vessels, even without any treatment.

When fat tissue around the heart and blood vessels was soaked in a blue dye called methylene blue for a day in the lab, it made less of a harmful enzyme and produced fewer damaging chemicals, which might help explain how the dye could protect cells.

When the fat tissue was given serotonin (a chemical that MAO-A breaks down), it made more harmful molecules—but adding methylene blue stopped some of that increase.

In the fat around the heart and blood vessels of heart failure patients, a specific enzyme called MAO-A is much more common and makes more harmful chemicals than its cousin MAO-B.