Lysine was isolated from casein by Drechsel
in 1889. It makes up 7–9% of meat, egg and
milk proteins. The content of this essential amino
acid is 2–4% lower in cereal proteins in which
prolamin is predominant. Crab and fish proteins
are the richest sources (10–11%). Along with
threonine and methionine, lysine is a limiting
factor in the biological value of many proteins,
mostly those of plant origin. The processing of
foods results in losses of lysine since its ε-amino
group is very reactive (cf. Maillard reaction).