Opioid activities of human ?-casomorphins

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Abstract
Opioid activities of human β-casomorphin-4,-5,-7 and -8 and, for comparison, of the corresponding bovine β-casomorphins were studied in the guinea-pig ileum preparation. Binding parameters, i.e. K d -values and binding site concentrations, for the interaction of human and bovine β-casomorphins with opioid receptors in rat brain homogenates were determined in inhibition experiments, using [3H]-(d-Ala2, MePhe4, Gly-ol5)enkephalin, [3H]-(d-Ala2, d-Leu5)enkephalin and [3H]ethylketazocin as μ-, δ- and κ-opioid receptor ligands. Analysis of binding data was performed using a non-linear curve fitting program. All β-casomorphins examined displayed opioid activity. The affinity was highest for μ-receptors, less so for δ-receptors and lowest for κ-receptors. It is suggested that human β-casomorphins might play a role as “food hormones”.