Medical Definition of ALANINE AMINOTRANSFERASE. : an enzyme which promotes transfer of an amino group from glutamic acid to pyruvic acid and which when present in abnormally high levels in the blood is a diagnostic indication of liver disease—abbreviation ALT; called also glutamic pyruvic transaminase.weet. : an enzyme which promotes transfer of an amino group from glutamic acid to pyruvic acid and which when present in abnormally high levels in the blood is a diagnostic indication of liver disease—abbreviation ALT; called also glutamic pyruvic transaminase.