A peptide hormone which consists of two polypeptide chains, A- and B- chains which are linked together by disulfide bonds. The amino acid sequence of insulin varies across species and certain segments of the molecule are highly conserved. In most species, the A chain consists of 21 amino acids and the B chain consists of 30 amino acids. In mammals, insulin is synthesised in the pancreas within the β cells whereas in certain species of fish, the insulin-producing tissue is uniquely located in separate structures called Brockmann bodies.