How can troponin relate to myocardial infarctions?

Injured heart muscle. Injured heart muscle leak troponin into the blood stream and cause elevated blood levels.
Enzyme from heart. Troponin is contained within heart muscle cells and in a heart attack the damaged cells 'leak" the troponin into the blood and the elevated levels can tell doctors the heart has been injured. If other clinical evidence is consistent this can help diagnose a heart attack.