Glutamine
Glutamine is the most abundant naturally occurring, nonessential amino acid in the human body, and one of the few amino acids that can directly cross the blood–brain barrier. Humans obtain glutamine through catabolism of proteins in foods they eat. In states where tissue is being built or repaired, like growth of babies, or healing from wounds or severe illness, glutamine becomes conditionally essential.
Boletus edulis
A comparative study of the amino acid composition of eleven Portuguese wild edible mushroom species showed "Boletus edulis" to have the highest total amino acid content, about 2.3 g per 100 g of dried mushroom. This total includes a full complement of 20 essential and nonessential amino acids. Analysis of the free amino acids (that is, those not bound up in protein) revealed glutamine and alanine to be the principal amino acids (each about 25% of total compounds); a separate analysis concluded that lysine is another predominant compound.