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late 14c., "to change (something)," from Old French alterer "change, alter," from Medieval Latin alterare "to change," from Latin alter "the other (of the two)," from PIE *al- "beyond" (see alias (adv.)) + comparative suffix -ter (cf. other). Intransitive sense "to become otherwise" first recorded 1580s. Related: Altered; altering.
Alanine aminotransferase; an enzyme in serum and body tissues that catalyzes the transfer of amino acid groups from l-alanine to 2-ketoglutarate or the reverse, thus allowing nitrogen to be excreted or incorporated into other compounds.