myoglobin

Myoglobin is a small oxygen-holding protein packed inside muscle cells, including heart muscle. Because it is small, it escapes into the blood very quickly when muscle is injured — making it one of the earliest markers to rise after a heart attack, sometimes within an hour or two.

Its job inside the cell is to store and ferry oxygen for the muscle's energy needs; the same protein gives meat its red color. After heart muscle damage, myoglobin appears in the blood before troponin does, which historically made it attractive as an early alarm.

The catch is that myoglobin is in all muscle, so any bruise, fall, strenuous exercise, or kidney problem can raise it too. Its lack of specificity means a high myoglobin alone cannot confirm a heart attack. In modern practice, sensitive troponin assays have largely made myoglobin obsolete for diagnosing cardiac injury.