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Biomedical subjects

I Silman

Publications and source records attributed to I Silman.

At least 145 records · Page 8Linked to original sources
PubMed1973

Molecular structures of acetylcholinesterase from electric organ tissue of the electric eel.

Three purified molecular forms of acetylcholinesterase (EC 3.1.1.7) with sedimentation coefficients of 18 S, 14 S, and 11 S were studied by analytical ultracentrifugation and electron microscopy. The three species have molecular weights of (1.1 +/- 0.1) x 10(6), (7.5 +/- 1.5) x 10(5), and (3.35 +/- 0.25) x 10(5), respectively. Electron micrographs reveal that the 18S and 14S forms are asymmetric, composed of a head, containing a large number of subunits, and an elongated tail. The 11S form of acetylcholinesterase is apparently a tetrameric structure devoid of the tail. Maleylation of 18S and 14S acetylcholinesterases abolishes their tendency to aggregate at low ionic strength.

Acetylcholinesterase↗
PubMed1973

The role of a reactive disulphide bond in the function of the acetylcholine receptor at the frog neuromuscular junction.

1. The effects of disulphide bond reduction and reoxidation on synaptic transmission in the frog neuromuscular preparation have been studied.2. The amplitudes of end-plate potentials (e.p.p.s) and miniature e.p.p.s (m.e.p.p.s) were decreased irreversibly by the reducing agent dithiothreitol (DTT). Recovery of e.p.p.s and m.e.p.p.s could be achieved, however, by subsequent reoxidation employing 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB).3. No change in e.p.p. quantal content was produced by treatment with DTT and DTNB. The reduction of m.e.p.p. frequency found in DTT was attributed mainly to our inability to distinguish small m.e.p.p.s from the background noise. However, a genuine reduction in m.e.p.p.s frequency could not be ruled out.4. The ;reactive' disulphide bond which is acted upon by DTT and DTNB could be located within a few Angstrom from the anionic site of the receptor for acetylcholine (ACh), by employing the active-site directed reagent bromoacetylcholamine (BACA).5. Reduction of the ;reactive' disulphide bond did not cause changes in the post-synaptic membrane input impedance nor was the e.p.p. reversal potential affected. Treatment with DTT and DTNB was found to modify only the conductance of the synaptic membrane.6. No synaptic effects were produced by DTT in a non-cholinergic synapse, the crab neuromuscular preparation.7. It is concluded that the receptor for ACh, besides including the well-known anionic site for binding quaternary ammonium groups, also contains a unique ;reactive' disulphide bond. This bond controls synaptic conductance but not the relative permeability of the synaptic membrane to Na(+) and K(+). It is not yet clear whether the ;reactive' bond controls the interaction of ACh with the receptor or the translation of this interaction into ionic permeability changes.

Acetylcholine↗
PubMed1972

Purification by affinity chromatography of the molecular forms of acetylcholinesterase present in fresh electric-organ tissue of electric eel.

An acetylcholinesterase inhibitor-Sepharose conjugate was prepared by coupling a derivative of the powerful acetylcholinesterase inhibitor, N-methylacridinium, to CNBr-activated Sepharose. Use of this conjugate permitted direct purification, by affinity chromatography, of the two molecular forms of acetylcholinesterase, 14 and 18 S, present in fresh electric organ tissue. The purified 14S and 18S acetylcholinesterases retained the capacity to aggregate at low ionic strength displayed by crude extracts of the enzyme. The major polypeptide components of the 14S and 18S enzymes, as revealed by acrylamide gel electrophoresis, closely resemble those observed in the 11S form of acetylcholinesterase, previously purified after tryptic digestion of electric-organ tissue.

Acetylcholinesterase↗
PubMed1969

Acetylcholine receptor: covalent attachment of depolarizing groups at the active site.

Following reduction of the acetylcholine receptor in the electroplax with dithiothreitol, the quaternary ammonium compounds bromoacetylcholine bromide and the p-nitrophenyl ester of (p-carboxyphenyl) trimethylammonium iodide react near the active site probably with a sulfhydryl group. The covalently attached quaternary ammonium moieties additionally interact with the active site noncovalently to activate the receptor and cause depolarization of the cell.

Acetylcholine↗