["Affinity labelling" of the cholinergic receptor of the motor end plate with diazoacetylcholine]

Preparation and application of a photoreactive thrombin analogue: binding to human platelets

alpha-Thrombin has previously been shown to bind to specific, saturable glycoproteins on the platelet surface. Modification of the thrombin active site with tosyllysyl chloromethyl ketone (TosLysCH2Cl) does not alter thrombin's binding characteristics. Interaction of alpha-thrombin with high-affinity binding sites (KD = 10(-9) M) initiates the platelet response which involves proteolytic hydrolysis of this glycoprotein. Although TosLysCH2Cl--thrombin binds to and competes for the same sites as alpha-thrombin, it cannot induce platelet stimulation because it is enzymatically inactive. In this study, we describe the preparation and application of photoreactive tritium-labeled thrombin analogues. The alpha-thrombin derivative retains its platelet-stimulating and enzymatic activities and, upon photoactivation, covalently binds to specific platelet membrane components. When freshly washed human platelets are exposed to less than saturation doses (less than or equal to 2 nM) of the thrombin derivatives in the dark and photoactivated, a single labeled complex is detected. The same experiment with greater than saturating doses (greater than or equal to 20 nM) of the thrombin derivative yields a similar complex as well as two additional ones. Molecular weight estimates of these thrombin-bound complexes were obtained by gel filtration and NaDodSO4--polyacrylamide gel electrophoresis. The low dose (high affinity) complex with TosLysCH2Cl--thrombin has an approximate molecular weight of 200 000, while that with active alpha-thrombin is smaller, approximately 120 000, due to enzymatic cleavage. The additional complexes detected with the high thrombin dose had estimated molecular weights of 400 000 and 46 000, respectively, and appeared to be the same for TosLysCH2Cl--thrombin and for the alpha-thrombin coupled platelets. These isolated complexes appear to correspond to the two previously detected populations of thrombin binding sites on the platelet.

Characterization of pharmacological receptors

The isolation of macromolecules that bind drugs and neurohumoral mediators at receptor sites will require analytical methods adequate to detect these substances in homogenates and crude mixtures. Although the binding of isotopically labeled agonists has been useful in a few instances, equilibrium binding studies of radioactive, lipophilic antagonists promises to be more useful with beta-adrenergic receptors. Alkylation by radioactive reagents that seek out specific receptors or specific functional groups on protein have been used in attempts to label other receptors. Conformationally directed labeling of receptor and receptor-related proteins by the latter group of alkylating agents has been used with receptors for cardiac glycosides and may be applicable to other systems.

Affinity-labelling of the muscarinic receptor of the guinea-pig illeum

1. The activities of some cholinergic compounds were investigated on the terminal ileum of the guinea-pig and their muscarinic potencies were found to be in the following order: acetylcholine (ACh) greater than diazoacetylcholine (DACh) greater than iodoacetylcholine (IACh) greater than azidoacetylcholine (AACh). 2. Protection experiments with atropine showed a decreased affinity of the four cholinergic compounds for the muscarinic receptor, demonstrating a direct interaction of the drugs and the receptor. 3. The maximal contractions (intrinsic activity) caused by DACh were greater than those casued by ACh, IACh and AACh. 4. Incubation of the ileum with ACh was followed by a reversible loss of its ability to contract (desensitization). The time course of recovery was similiar to that after incubation with AACh. 5. IACh caused a partial (50%), irreversible paralysis of the muscle. 6. The furaziridines seem to react partially irreversibly (30%), whereas paranitrophenyldiazonium fluoborate (p-NDP) caused a complete, irreversible blockade of the muscarinic receptor.

An examination of the utility of photogenerated reagents by using alpha-chymotrypsin

As a test of the labelling characteristics of photogenerated reagents, an aryl azide was photolysed in the aromatic-binding locus of a protein of known tertiary structure. The acyl-enzyme derived from the reaction of alpha-chymotrypsin with the p-nitrophenyl ester of p-azido[(14)C]cinnamate was isolated and photolysed. About 60% of the acyl group is covalently bound to the protein after photolysis and deacylation, and labelled enzyme is inactive. The covalently attached label is localized in the C chain of chymotrypsin, and there are firm indications that the major labelled tryptic fragment of the C chain is that which constitutes the aromatic-binding locus of the enzyme. The high degree of labelling of that portion of the protein molecule predicted on the basis of the known chemistry and structure of alpha-chymotrypsin, provides gratifying confirmation of the utility of the photo-labelling method.

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.