Methionine oxidation enhances opioid activity of an enkephalin analog.

D-ala2-met-sulfoxide5-enkephalinamide, DALA(0), was synthesized by oxidizing the 5-methionine residue of D-ala2-met5-enkephalinamide (DALA). Antinociception was assessed on the hot-plate and catalepsy estimated using an immobility test in rats administered DALA, DALA(0) and morphine intraventricularly. By comparing areas under time-effect curves, DALA(0) was 30 times more antinociceptive and up to 40 times more cataleptogenic than DALA. For comparison, morphine induced one-tenth the antinociception and one-fortieth the immobility caused by DALA(0). These results demonstrate that the opiate activity of DALA is clearly enhanced by oxidation of its terminal methionine.