The effect of naloxone on enkephalin catabolism

The effect of opioids and of naloxone on Na+,K+-adenosine triphosphatase activity in frog spinal cord membrane fractions

The effects of opioids and of naloxone on ouabain-sensitive Na+,K+-adenosine triphosphatase (ATPase) activity were studied in vitro on membrane fractions from frog spinal cords. The addition of morphine and of the stable enkephalin analogue, D-Ala2,D-Leu5-enkephalin, in concentrations from 10(-7) to 10(-4) M significantly increased Na+,K+-ATPase activity. No effect was found with methionine enkephalin (Met-Enk). However, the addition of two peptidase inhibitors, captopril and phosphoramidon (10(-5) M each), significantly increased Na+,K+-ATPase activity. A further increase in enzyme activity was found when Met-Enk (10(-4) or 10(-7) M) was added simultaneously with peptidase inhibitors. On the other hand, the addition of the opiate antagonist, naloxone, at low concentration (10(-7) M) decreased the activity of Na+,K+-ATPase. These results are discussed with respect to the effect of synthetic and endogenous opioids on the activity of Na+,K+-ATPase.

Enkephalin-containing polypeptides are potent inhibitors of enkephalin degradation

Enkephalin-containing polypeptides derived from pro-enkephalin A, pro-enkephalin B, or pro-opiomelanocortin were inhibitors of enkephalin degradation by aminoenkephalinases purified from cytosol or membranes. Of the peptides, Argo-Met-enkephalin was the most potent inhibitor for the aminoenkephalinases, with an IC50 of about 0.6 microM, it was more effective than bestatin (IC50 = 0.8-1.0 microM). This inhibition was partly due to substrate competition. Argo-Met-enkephalin was hydrolyzed by aminoenkephalinases to form Arg, Tyr, and Gly-Gly-Phe-Met in a substrate-inhibited manner. The hexapeptide also inhibited the breakdown of Arg- and Tyr-beta-naphthylamide by the membrane aminoenkephalinase. Since Argo-Met-enkephalin did not inhibit leucine aminopeptidase, it was a more selective inhibitor than bestatin of Met-enkephalin breakdown by aminopeptidases. Argo-Met-enkephalin inhibited enkephalin breakdown by synaptosomal plasma membranes but not by brain slices. Our data suggest that in addition to their possible role as opioids, the enkephalin-containing polypeptides may be regulators of enkephalin levels.

Enkephalins and opiate antagonists control calmodulin distribution in neuroblastoma-glioma cells

The calcium binding protein calmodulin and the opiate receptor binding sites are unevenly distributed in various subcellular fractions of neuroblastoma-glioma NG108-15 cells. The crude mitochondrial-membrane fraction of these cells contains two membrane fractions that are separable by sucrose gradient centrifugation. These two differ in the content of both calmodulin and opiate receptors. Leucine enkephalin and D-Ala2-methionine enkephalinamide decrease the amount of membrane-bound calmodulin in the NC108-15 cells in a time- and dose-dependent manner, whereas the opiate antagonists naloxone and levallorphan have an opposite effect. Naloxone blocks the effect of leucine enkephalin and dextrallorphan has no significant effect. The opiate alkaloids entorphine and phenazocine induce changes similar to that of the enkephalins whereas morphine is inactive even at high concentrations. The alteration in the amount of membrane-bound calmodulin after a short incubation (15 min) with the enkephalins or with naloxone is reflected as an opposite change in the amount of calmodulin in the cell cytosol. Naloxone and levallorphan also increase the number of opiate receptors in NG108-15 cells but dextrallorphan has no such effect. Modulation of the intracellular distribution of calmodulin by opioid peptides and alkaloids may control the activity of various membrane-bound and cytosolic systems that are calmodulin- and/or calcium-dependent.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.