ALT (alanine aminotransferase): An enzyme normally present in liver and heart cells that is released into the bloodstream when the liver or heart is damaged. The blood ALT levels are 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. ALT is also called serum glutamic pyruvic transaminase (SGPT).