Alanine aminotransferase (ALT): An enzyme that is normally present in liver and heart cells. ALT is released into blood when the liver or heart is damaged.The blood ALT levels are thus elevated with liver damage (for example, from viral hepatitis) or with an insult to the heart (for example, from a heart attack).Some medications can also raise ALT levels.LT is released into blood when the liver or heart is damaged. The blood ALT levels are thus elevated with liver damage (for example, from viral hepatitis) or with an insult to the heart (for example, from a heart attack).