Peptide chirality and opioid receptor modulation: Hepatoprotective effect of d-Met-enkephalin in acetaminophen-induced liver injury

l-Met-enkephalin is a neuropeptide known to exert protective effects in various experimental models of autoimmune and inflammatory diseases. These effects are mediated through opioid receptors and can be abolished by the opioid receptor antagonist naltrexone. Investigation of peptide enantiomerism and the incorporation of d-amino acids are crucial for designing novel peptides with altered structural and biological properties compared with their native l-forms. Since no data are currently available on the properties or biological activity of the d-Met-enkephalin enantiomer, we evaluated its hepatoprotective potential in a mouse model of acetaminophen-induced hepatotoxicity. Male CBA mice were treated with d-Met-enkephalin, and hepatoprotection was assessed by measuring plasma alanine aminotransferase (ALT) and aspartate amino-transferase (AST) activities, along with histological liver necrosis scores. The peptide’s secondary structure and antisense peptide binding were analysed using circular dichroism and fluorescence spectroscopy, respectively.

d-Met-enkephalin demonstrated dose-dependent hepatoprotective effects within the range of 0.5–20 mg kg–1, with maximal protection observed at 5 mg kg^–1, a dose comparable to that of the l-enantiomer (7.5 mg kg^–1). This preservation of biological activity may be attributed to the presence of the achiral amino acid glycine at positions 2 and 3, which maintains the functional conformation of the d-enantiomer. The role of opioid receptor involvement was further examined through direct receptor blockade using naltrexone and indirect inhibition with the antisense peptide IPPKY.