Co-identity of brain angiotensin converting enzyme with a membrane bound dipeptidyl carboxypeptidase inactivating Met - enkephalin

Enkephalinase inhibition relieves pain syndromes of central dysnociception (migraine and related headache)

Normotensive and hypertensive headache sufferers were treated with D-phenylalanine, aprotinin or captopril--all inhibitors of endogenous opioid degradation. Inconsistent results were obtained using D-phenylalanine and aprotinin (acute administration at the start of a migraine attack). Satisfactory results were obtained by prophylactic treatment with captopril (an inhibitor of the angiotensin converting enzyme and of the dipeptidyl carboxypeptidase--an enkephalin inactivating enzyme) compared with conventional drugs such as methysergide, lisuride, pizotifen, clonidine and beta-blocking agents. Excellent results were obtained with captopril on patients suffering from headache and arterial hypertension who had experienced no relief from beta-blocking agents and clonidine. Captopril could thus be a drug of choice in the therapy of headache associated with essential hypertension.

Purification of a membrane-bound metalloendopeptidase from porcine kidney that degrades peptide hormones

A porcine kidney microsomal metalloendopeptidase has been enriched 3900-fold. Gel filtration on a calibrated Toyo-Soda G-3000 SW column indicated an appropriate molecular weight for the endopeptidase of 88,000 +/- 2000. The purified enzyme is inhibited by a number of synthetic inhibitors of thermolysin. The endopeptidase hydrolyzes the succinyl (Suc)-containing fluorogenic peptide substrate Suc-Ala-Ala-Phe-(7-amino-4-methylcoumarin) at the Ala-Phe position with a Km of 2.9 X 10(-4) M. The endopeptidase also hydrolyzes a variety of peptides including corticotropin, substance P, angiotensin I and II, neurotensin, somatostatin, bradykinin, and the renin tetradecapeptide substrate. The endopeptidase hydrolyzes both [Leu]- and [Met]enkephalin at the Gly-Phe bond.

Selective protection of methionine enkephalin released from brain slices by enkephalinase inhibition

Methionine enkephalin release was evoked by depolarization of slices from rat striatum with potassium. In the presence of 0.1 microM thiorphan [(N(R,S)-3-mercapto-2-benzylpropionyl)glycine], a potent inhibitor of enkephalin dipeptidyl carboxypeptidase (enkephalinase), the recovery of the pentapeptide in the incubation medium was increased by about 100 percent. A similar effect was observed with the dipeptide phenylalanylalanine, a selective although less potent enkephalinase inhibitor. Inhibition of other known enkephalin-hydrolyzing enzymes--aminopeptidase by 0.1 mM puromycin or angiotensin-converting enzyme by 1 microM captopril--did not significantly enhance the recovery of released methionine enkephalin. These data indicate that enkephalinase is critically involved in the inactivation of the endogenous opioid peptide released from striatal neurons.

Potentiation of the morphine-induced respiratory rate depression by captopril

The effects of morphine and captopril, an angiotensin-converting enzyme inhibitor, on respiratory frequency have been investigated in non-anesthetized mice. Morphine (10.0 mg/kg) reduced the respiratory frequency which gradually returned to the control level 2 h after the injection. The same dose of morphine when given together with captopril (0.1 and 1.0 mg/kg), caused a similar reduction in respiratory rate. This respiratory depression however, persisted until the end of the observation period. Similar results were obtained with the same dose of morphine in mice pretreated with captopril. The minimal dose of morphine reducing respiratory frequency (3.0 mg/kg), when given to the mice pretreated with captopril (0.1 mg/kg) caused a significant reduction in respiratory frequency and this effect was equal to that obtained with 10.0 mg/kg morphine alone. The results are discussed from the point of the possible inhibitory effect of captopril on the enkephalin degrading enzyme(s).

Purification and substrate characterization of a human enkephalin-degrading aminopeptidase

A 5000-fold purification of the enzyme responsible for the rapid inactivation of enkephalin in human blood has been achieved: this enzyme cleaves the N-terminal tyrosine from enkephalin and from short peptides provided their first amino acid is aromatic. The enzyme, an enkephalin-degrading aminopeptidase (alpha-aminoacyl-peptide hydrolase, EC 3.4.11.11), requires a free amino group on the substrate and has a maximum activity around pH 8. Its appearance molecular weight is in the range of 80 000-90 000 and an apparent Michaelis constant of 0.4 mM was determined.

Conversion of Met-enkephalin-Arg6-Phe7 by a purified brain carboxypeptidase (cathepsin A)

A carboxypeptidase A-like enzyme known as cathepsin A was purified from rat brain by extraction with Triton X-100, followed by chromatography on DEAE-Sephadex A-50 and gel-filtration. Purified enzyme was devoid of contamination of tryptic-like enzymes, by dipeptidyl carboxypeptidase (angiotensin converting enzyme) and of enkephalinnases cleaving the Tyr-Gly and Gly-Phe bonds of Met-enkephalin. Incubation of purified enzyme with Met-enkephalin-Arg6-Phe7, a naturally occurring enkephalin surrogate, was accompanied by the release of three products as detected by reverse phase HPLC. Subsequent amino acid analysis identified these as Phe, Met-enkephalin-Arg6, and Met-enkephalin, indicating cleavage at the Arg6-Phe7 and Met5-Phe6 bonds. Breakdown followed a precursor-product-relationship with the hexapeptide appearing as an intermediate and the pentapeptide as the final product. The Km for cleavage of the Arg-Phe site was 0.09 mM. Rates of cleavage of hexa- and heptapeptide accord with those found for synthetic N-protected dipeptide substrates. Cathepsin A does not act as an enkephalinase in the accepted sense, since no breakdown of Met-enkephalin was observed.

Purification and characterization of human converting enzyme (kininase II)

We purified peptidyl-dipeptidase (converting enzyme, EC 3.4.15.1) to homogeneity from the membrane fraction of human lung and for comparison, from human and hog kidney. The membrane-bound lung enzyme was purified 1800-fold with 19% yield, and the kidney enzyme 640-fold with 10% yield. The specific activities with Bz-Gly-His-Leu were 81 mumol/min/mg for the lung and 65 for the kidney enzyme. The lung enzyme was homogeneous in gel electrophoresis with Mr = 155,000 and Sw,20 = 8.0 in ultracentrifugation. Antibodies elicited against lung or kidney enzyme cross-reacted with enzyme from other organ, but not with the hog enzyme. In isoelectric focusing both human enzymes had a major form with a pI of 5.2. The lung preparation also contained more acidic forms (pI = 4--5), which were eliminated by treatment with neuraminidase. Lung and kidney converting enzyme hydrolyzed bradykinin, angiotensin I, and enkephalins and had similar kinetic constants. Bradykinin was the best substrate, as indicated by its kcat/Km, but Met5-enkephalin had the highest turnover number. The hydrolysis of Bz-Gly-His-Leu was inhibited by captopril (SQ 14225) competitively, and by Keto-ACE, a non-peptide derivative of Bz-Phe-Gly-Pro, non-competitively.

Angiotensin converting enzyme in the brain of spontaneously hypertensive rats

1. Angiotensin converting enzyme (ACE) was measured in homogenates of regions of rat brain using the substrate Hip-His-Leu. 2. The enzyme resembles classical ACE in its marked Cl- dependence and inhibition by both SQ 20,881 (24 micro mol/1) and EDTA (1 mmol/1). 3. Spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls (NT-WK) were killed at 20-22 weeks of age their brains dissected into eight regions. 4. There were marked region variations of ACE with highest levels in striatum, hippocampus, cerebellum and pituitary and lower levels in hypothalamus and cerebral cortex. 5. In three brain regions ACE was significantly lower in SHR compared to NT-WK: medulla oblongata (P < 0.05), hypothalamus (P < 0.02) and cerebral cortex (P < 0.05). In the other sites the levels were not different. 6. These region-specific differences of ACE in the SHR could lead to altered production or metabolism of central neuropeptides postulated to be involved in the control of blood pressure.

Blood pressure responses of conscious normotensive and spontaneously hypertensive rats to intracerebroventricular and peripheral administration of captopril

In conscious spontaneously hypertensive rats, intraxerebroventricular injection of captopril (2 mg/kg) resulted in a rapid hypotensive response that lasted several hours. The same dose given by intracerebroventricular injection had no significant effect on blood pressure (BP) of normotensive Wistar-Kyoto (WK) rats over 7 hours. There was no significant change in BP of conscious spontaneously hpertensive rats (SHR) in response to intracerebroventricular injection of vehicle and only a transitory fall in BP in response to intravenous injection of captopril (2 mg/kg). There was no significant differences between plasma renin activity (PRA) of conscious normotensive WKY rats and the PRA of SHR. These results suggest biochemical differences between the brains of SHR and normotensive WKY control rats. These differences could involve the brain renin-angiotensin system or other neuropeptides.

Alterations by captopril of pain reactions due to thermal stimulation of the mouse foot: interactions with morphine, naloxone and aprotinin

The effects of Captopril (SQ 14,225) were studied in mice both by measuring the reaction time and observing the number of jumping mice in the 'hot-plate' test. Subcutaneous injections of captopril (0.1--0.4 mg/kg) produced a significant increase in the reaction time at the 4th hour after the injection while no difference in the reaction time was observed at other times. Captopril caused a clear-cut increase in the number of jumping mice which was found to be greater with lower doses of the compound. Morphine alone did not cause an increase in the number of jumping mice, but increased the reaction time which reached a maximum 1 h after administration and gradually returned to the control level after 6 h. Injection of morphine to the captopril-pretreated mice caused a highly significant increase in reaction time remaining above the control values for 3 h. This effect of morphine was antagonized by naloxone (2 mg/kg, s.c.). Captopril, when given together with morphine, prevented the increase in reaction time over 2 h, but caused a progressive and significant increase within 6 h. Morphine also prevented the jumping behaviour induced by captopril. Aprotinin (50,000 KIU/kg i.p.) when given together with captopril, produced a significant increase in reaction time but almost completely blocked the jumping reaction. Aprotinin also prevented the captopril-induced increase in the number of jumping mice. The possible mechanisms of analgesia and jumping behaviour due to thermal stimulation are discussed in the light of these findings.

Separation of enkephalin degradation products by ligand-exchange chromatography

Enkephalin degradation products can be accurately analyzed by cascade chromatography through XAD polystyrene and copper--Chelex columns. One of the main degradation products generated during the incubation of enkephalin with rat striatal membranes, the N-terminal amino acid Tyr, is absorbed quantitatively on the copper--Chelex columns whereas the minor, but probably specific, product Tyr--Gly--Gly is not.

Resistance of FK 33-824 and other enkephalin analogues to peptidase degradation

The stability of analogues of Met-enkephalin containing different chemical substitutions in the amino acid sequence to enzymatic degradation by a mouse brain extract and by rat striatal membranes have been compared. On incubation with the mouse brain extract, the naturally occurring Met-enkephalin was completely destroyed within 5 min and 50% of (D-Ala2)Met-enkephalinamide was degraded within 2 1/2 hr, with the release of Tyr, Phe, Met, D-Ala-Gly and partially (D-)Ala and Gly. (D-Ala2, MePhe4, Met(O)ol5) enkephalin (FK 33-824) was degraded to 50% within 5 hr, giving merely Tyr and the tetrapeptide D-Ala-Gly-MePhe-Met(O)ol as degradation products. No metabolism was observed after incubation for 24 hr of (MeTyr1, D-Ala2, MePhe4, Met(O)ol5) enkephalin (FW 34-569) with the mouse brain extract. On incubation with rat striatal membranes in the presence of puromycin, Met-enkephalin was rapidly inactivated, principally by cleavage of the Gly3-Phe4 bond, whereas FK 33-824 was not split.

Enkephalinase: selective inhibitors and partial characterization

There are at least two types of enzymes in brain, endopeptidases and aminopeptidases, which metabolize enkephalins. Evidence is presented to suggest that enkephalinase, an endopeptidase cleaving at the Gly-Phe bond, is specific for the endogenous enkephalinergic system. Selective inhibitors are described for each enzyme. These are parachloromercuriphenylsulfonic acid and puromycin in the case of aminopeptidases and various enkephalin fragments in the case of enkephalinase. Some characteristics of the two types of enzymes are described. Enkephalinase has many properties in common with the well-characterized brain angiotensin-converting enzyme. These two enzymes, however, behaved differently when tested for chloride dependence, for activity in several buffers and for susceptibility to specific inhibitors.