an amino acid released in the hydrolysis of many common proteins but present in small amounts or lacking in certain plant proteins; e.g., gliadin from wheat, zein from corn (maize). First isolated from casein (1889), lysine is one of several so-called essential amino acids for warm-blooded animals; i.e., they cannot synthesize it and require dietary sources. (It is formed in plants, algae, and fungi by two distinct biosynthetic pathways.) Human populations dependent on grains as a sole source of dietary protein suffer from lysine deficiency.