either of two amino acids, one of which (α-alanine) is a constituent of proteins. An especially rich source of α-alanine is silk fibroin, from which the amino acid was first isolated in 1879. Alanine is one of several so-called nonessential amino acids for birds and mammals; i.e., they can synthesize it from pyruvic acid (formed in the breakdown of carbohydrates) and do not require dietary sources. Beta-alanine is not found in proteins but occurs naturally in two peptides, carnosine and anserine, found in mammalian muscle. It is an important constituent of the vitamin pantothenic acid.