b-Casomorphin (1-3), (C23H27N3O5), a tri-peptide with the sequence H2N-Tyr-Pro-Phe-OH, MW= 425.48. Casomorphins are peptides derived from the digestion of milk protein casein. The distinguishing characteristic of casomorphins is that they have an opioid effect. The most important casomorphins from bovine milk are those released from the digestion of β-casein into β-casomorphins, sometimes denoted as BCM followed by a numeral indicating the number of amino acids in the sequence. Each variant fits into one of two main categories known as A1 and A2. In cattle, A1-type β-caseins have the amino acid histidine at position 67, whereas the A2-type β-caseins have the amino acid proline at position 67. Laboratory experiments show that the casomorphin known as BCM7 is released only from the A1-type β-caseins[1]. The potential release of BCM7 is about 0.4 grams per litre of milk (assuming as above that there are 12g of β-casein per litre).Human digestion may break down casomorphins into inactive dipeptides by the enzyme dipeptidyl peptidase-4[2]. This enzyme is found in the digestive tract and in some endocrine cells. Casein has been documented to break down in the stomach to produce the peptide casomorphin (above stated that "Human digestion may break down casomorphins into inactive dipeptides by the enzyme dipeptidyl peptidase-4"), an opioid that acts as a histamine releaser[3].
References:
1. Jinsmaa Y, Yoshikawa M, 1999; "Enzymatic release of neocasomorphin and beta-casomorphinfrom bovine beta-casein"; Peptides 20:957-962
2. Converse PJ, Hamosh A, McKusickVA(2005). DIPEPTIDYL PEPTIDASE IV; DPP4. Online Mendelian Inheritance inMan.
3. Kurek M, Przybilla B, Hermann K, Ring J (1992). "A naturally occurring opioid peptide from cow's milk, beta-casomorphine-7, is a direct histamine releaser in man". Int Arch Allergy Immunol 97 (2): 115–120.g.