In vitro cyclic AMP-mediated lipolytic activity of endorphins, enkephalins and naloxone

The maximum lipolytic activity (L_max) of β-endorphin is two and one half times that of Leu⁵-enkephalin and twice that of Met⁵-enkephalin, D-Ala², Met⁵-enkephalinamide, α-endorphin and γ-endorphin in the rabbit adipocyte. D-Met², Pro⁵-enkephalin-amide, however, has an L_max 1.6 times greater than that of Met⁵-enkephalin. The potencies (A₅₀) of Met⁵-enkephalin and its analogs and that of Leu⁵-enkephalin lie between 1.4 and 3 μM. The A₅₀ values for α-endorphin, β-endorphin and γ-endorphin are significantly less (1.2 × 10^?1 μM). Naloxone acts as an agonist in this system (A₅₀ = 2.5 μM; L_max 1.4 × Met⁵-enkephalin). All of the peptides and naloxone stimulated adenylate cyclase activity.