Enkephalin degrading enzymes are present in the electric organ of Torpedo californica

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Arg-Lys-Asp-Val-Tyr (thymopentin) accelerates the cholinergic-induced inactivation (desensitization) of reconstituted nicotinic receptor

1. The thymic hormone thymopoietin blocks neuromuscular transmission and was proposed (Goldstein, 1974) as a modulator of synaptic conductivity. 2. The cholinergic-induced inactivation of nicotinic receptor reconstituted into asolectin lipid vesicles was studied in the presence and in the absence of thymopentin, a synthetic pentapeptide corresponding to positions 32-36 of thymopoietin. 3. The present data show that thymopentin accelerates desensitization of the nicotinic acetylcholine receptor, supporting the aforementioned physiological role proposed for thymopoietin. 4. They also suggest that the hormone itself and/or a yet unidentified hormine-derived peptide fragment may act as an endogenous ligand for nicotinic acetylcholine receptor desensitization.

Purification and characterization of a peptidyl dipeptidase resembling angiotensin converting enzyme from the electric organ of Torpedo marmorata

The electric organ of Torpedo marmorata contains a membrane-bound, captopril-sensitive metallopeptidase that resembles mammalian angiotensin converting enzyme (peptidyl dipeptidase A; EC 3.4.15.1). The Torpedo enzyme has now been purified to apparent homogeneity from electric organ by a procedure involving affinity chromatography using the selective inhibitor lisinopril immobilised to Sepharose via a 28-A spacer arm. The purified protein, like the mammalian enzyme, acted as a peptidyl dipeptidase in cleaving dipeptides from the C-terminus of a variety of peptide substrates, including angiotensin I, bradykinin, [Met5]enkephalin, [Leu5]enkephalin, and the model substrate hippuryl (benzoylglycyl; BzGly)-His-Leu. The hydrolysis of BzGly-His-Leu was activated by Cl-. Enzyme activity was inhibited by classical angiotensin converting enzyme inhibitors, including captopril, enalaprilat (MK422), and lisinopril (MK521). Torpedo angiotensin converting enzyme, like its mammalian counterpart, was also able to act as an endopeptidase in hydrolysing the amidated neuropeptide substance P. Hydrolysis of substance P occurred primarily at the Phe8-Gly9 bond with release of the C-terminal tripeptide, Gly-Leu-MetNH2, and this hydrolysis was blocked by selective inhibitors. The Torpedo enzyme was recognised by a polyclonal antibody to pig kidney angiotensin converting enzyme on immunoelectrophoretic (Western) blot analysis. Thus, on the basis of substrate specificity, inhibitor sensitivity, and immunological criteria, the Torpedo enzyme closely resembles mammalian angiotensin converting enzyme. However, the Torpedo enzyme appears somewhat larger (Mr = 190,000) than the pig kidney enzyme (Mr = 180,000) on sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The endogenous peptide substrate(s) for Torpedo electric organ angiotensin converting enzyme and the physiological role of the enzyme in this tissue remain to be evaluated.

Angiotensin-converting enzyme associated with Torpedo california electric organ membranes

Torpedo electric organ contains high concentrations of angiotensin converting enzyme (ACE) like activity, cleaving [Leu5]enkephalin at the Gly3-Phe4 peptide bond. Most of the activity cosediments with the cell membranes. The enzymatic preparation from membranes is inhibited by low concentrations of the ACE inhibitors, SQ 14225 and SQ 20881 (IC50 of 0.6 and 15 nM, respectively), and is weakly inhibited by the neutral endopeptidase inhibitors, phosphoramidon and thiorphan (IC50 of 30 microM and ca. 70 nM, respectively). The enzyme degrades hippuryl-His-Leu and is activated by NaCl. Hippuryl-His-Leu and [Leu5]enkephalin are degraded with Km of 93 and 41 microM, and Vmax of 21 and 10 nmol/mg protein/min, respectively. The specific activity of the ACE-like activity in homogenates of Torpedo electric organ is relatively high (6.3 nmol hippuryl-His-Leu/mg protein/min); this value is similar to that obtained for rat lung and rat striatum.

Enkephalin-degrading activity in arthropode hemolymphe

Enkephalin and related peptides are rapidly inactivated in Astacus fluviatilis and Limulus polyphemus hemolymphe. At least three different enzymes, an aminopeptidase, a carboxypeptidase and a peptidyl-dipeptidase, acting concomitantly on the peptide substrates have been identified. The properties of these enzymes were characterized and they were compared to similar enzymes of the vertebrate blood. The opioid peptides appear to be extremely short lived in invertebrate hemolymphe, which presumably is a metabolic barrier to the action of active peptides stronger than vertebrate blood.

Saturable [D-Ala2, D-Leu5]-enkephalin transport into cholinergic synaptic vesicles

Incubation of cholinergic synaptic vesicles purified from the electric organ of Torpedo ocellata with radiolabeled [D-Ala2, D-Leu5]-enkephalin results in its accumulation in the vesicles. Kinetic analysis of the initial rates of uptake reveals temperature-dependent saturation kinetics which are best fitted by a single transport system (KT = 12 +/- 2 microM and Vmax = 0.85 +/- 0.24 pmol/mg protein per min). The specific rates and extents of [3H]-[D-Ala2, D-Leu5]-enkephalin uptake, like those of [3H]acetylcholine uptake, are highest in the purified synaptic vesicles fraction. These findings suggest that Torpedo cholinergic synaptic vesicles contain an opioid peptide transporter. The physiological significance of this transporter is discussed in view of the recent observation that Torpedo nerve terminals contain an endogenous enkephalin-like peptide and presynaptic opiate receptors.

The metabolism of neuropeptides. Both phosphoramidon-sensitive and captopril-sensitive metallopeptidases are present in the electric organ of Torpedo marmorata

A membrane fraction from the electric organ of Torpedo marmorata hydrolyses the Gly3-Phe4 bond of [D-Ala2, Leu5]enkephalin as well as the Gly-His bond of benzoyl-Gly-His-Leu. The hydrolysis of benzoyl-Gly-His-Leu is completely inhibitable by Captopril (I50 = 19nM), consistent with peptidyl dipeptidase activity, but enkephalin hydrolysis is inhibited to a maximum of only 70%. The residual activity hydrolysing enkephalin is inhibited by phosphoramidon (I50 = 15nM) and therefore resembles endopeptidase-24.11, a mammalian plasma-membrane enzyme implicated in the metabolism of neuropeptides. Both enkephalin-hydrolysing activities in Torpedo electric organ are inhibited by 1,10-phenanthroline, like their mammalian counterparts. The peptidases may function in the hydrolysis of endogenous peptides or in neurotransmitter exocytosis in the electric organ.