A 45-year-old man experiences crushing substernal chest pain after arriving at work one morning. Over the next 4 hours, the pain persists and begins to radiate to his left arm. He becomes diaphoretic and short of breath but waits until the end of his 8-hour shift to go to the hospital. An elevated serum value of which of the following laboratory tests would be most useful for the diagnosis of this patient on admission to the hospital?