Enkephalin degradation stimulated by captopril

Major rat brain membrane-associated and cytosolic enkephalin-degrading aminopeptidases: comparison studies

The major cytosolic and membrane-associated enkephalin-degrading aminopeptidases were purified in parallel by column chromatography successively on DEAE-cellulose, AH-Sepharose, hydroxylapatite, Sephadex G-200, Affigel Blue, AH-Sepharose, and hydroxylapatite. With the final hydroxylapatite column, the cytosol (S) and the membrane (M) enzymes could each be resolved into two peaks, one eluted with 0.05 M phosphate (SI, MI) and the other with 0.25 M phosphate (SII, MII). The overall purification, with Arg BNA as substrate, for the SI and MI was about 450-fold; for SII and MII, 1,200-fold. The yield for each enzyme was about 2%. the major protein integral units of the four enzymes are similar; they are single polypeptide chains with a molecular weight of 100,000 daltons. Their pH optimum, substrate specificity, and sensitivity to puromycin show that they are similar to lysosomal aminopeptidase (EC 3.4.11.-). The two forms of the cytosol and the membrane enzymes have slightly different kinetic constants. With the inhibitors, SII is more sensitive to proctolin, whereas MII is more sensitive to bestatin and Arg-Phe-Ala. Mn2+ activates SI on Met-enkephalin degradation, but inhibits SII, MI, and MII.

Enalapril, a new nonsulfhydryl angiotensin converting enzyme inhibitor, does not potentiate morphine analgesia

Sprague-Dawley rats received oral doses of enalapril maleate (5 mg/kg), a potent, nonsulfhydryl, angiotensin converting enzyme inhibitor, or saline. Sixty min later, morphine sulfate, 5 mg/kg, or saline was injected subcutaneously. Response to a thermal stimulus was monitored before and up to 5 h ter morphine injection using the rat tail flick test. Serum ACE activity was greater than 90% inhibited by enalapril throughout the experiment. Enalapril did not exhibit analgesic activity nor did it potentiate morphine analgesia.

The effect of naloxone on enkephalin catabolism

Naloxone strongly inhibited the breakdown of Met- and Leu-enkephalin when the substrate was incubated with brain homogenate, supernatant, or partially purified soluble aminopeptidase, but it had no effect on the mitochondrial fraction. Arylamidase was also inhibited by naloxone. The inhibitory effect of naloxone on the soluble aminopeptidase was in a linear relationship with concentration in the range of 8-500 microM. The KI for Met-enkephalin is 0.6 mM and for Leu-enkephalin is 2.0 mM. When the naloxone was administered intraperitoneally to the mice, its effects were somewhat different from its effects in vitro. It inhibited the brain catabolism of Met-enkephalin but not of Leu-enkephalin or Tyr-beta NA in vitro. In vivo, acute morphine in mice decreased the Met-enkephalin hydrolysis but increased the Leu-enkephalin hydrolysis. Chronic morphine (by morphine pellet implantation) increased the breakdown rate of enkephalin and of Tyr-beta NA. Naloxone (1 mg/kg) could not reverse the effects of morphine, although abstinence syndrome and stereotypical jumping were precipitated. Naloxone decreased the Leu-enkephalin level in the control mice but did not affect it in the addicted mice.