A new casomorphin pentapeptide (αS1-casomorphin) has been isolated from the sequence of human αS1-casein [αS1-casein-(158–162)], with the sequence Tyr-Val-Pro-Phe-Pro. This peptide was found to bind with high affinity to all three subtypes of the κ-opioid receptor (κ1–κ3). When amidated at the C-terminus, αS1-casomorphin amide binds to the Δ- and κ3-opioid sites. Both αS1-casomorphin and its amide inhibit in a dose-dependent and reversible manner the proliferation of T47D human breast cancer cells. This anti-proliferative activity was greater for αS1-casomorphin, which was the most potent opioid in inhibiting T47D cell proliferation. In T47D breast cancer cells, other casomorphins have been found to bind to somatostatin receptors in addition to opioid sites. In contrast, αS1-casomorphin and its amide do not interact with somatostatin receptors in our system.

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