 A small fraction of metabolism – less than 5% in all tissues except the testes where it accounts for about 33% – is initially catalyzed by leucine aminomutase, producing β-leucine, which is subsequently metabolized into (β-KIC), β-ketoisocaproyl-CoA, and then acetyl-CoA by a series of uncharacterized enzymes.Leucine is an essential amino acid in the diet of animals because they lack the complete enzyme pathway to synthesize it "de novo" from potential precursor compounds.Consequently, they must ingest it, usually as a component of proteins.