Membrane inactivation

Vauquelin G, Bottari S, KanarekL, Strosberg AD. Evidence for essential disulfide bonds in Pj-adrenergic receptors of turkey erythrocyte membranes. Inactivation by dithiothreitol. J Biol Chem 1979 254 4462-4469. 

In contrast to these salts, sodium succinate increases thylakoid damage during freezing if present as the only solute. As will be discussed later, the situation becomes more complicated if significant concentrations of other solutes are also present with sodium succinate during freezing. Membrane inactivation by high concentrations of sodium succinate occurs even at 0°C (21). 

Salts of weak organic acids that are soluble in lipids are also injurious to thylakoids. Examples are the salts of phenylpyruvic add (Figure 3) and caprylic acid (58). These salts, even if present at very low concentrations, cause extensive membrane inactivation during freezing, if cryo-protectants are absent. At 0°C, moderate concentrations of these salts will slowly inactivate thylakoids. 

Protein Release in Relation to Loss of Membrane Function. When thylakoids are frozen in the presence of sucrose, membrane function is preserved. If a cryotoxic salt such as NaCl is also present, retention of membrane functionality during freezing depends on the ratio of sucrose to salt (5). Loss of cyclic photophosphorylation is the most sensitive parameter of membrane inactivation. Photophosphorylation is largely lost before significant protein release from the membranes can be detected (Figure 8). Since photophosphorylation requires membranes with un-... 

Membrane inactivation depends on how closely anions can approach cationic binding sites. Poorly solvated ions show the strongest binding. They are also known to be the most effective protein denaturants (65). The Stokes law hydrated radius of the toxic bromide anion is about 1.2 A, that of the relatively nontoxic fluoride about 1.6 A and that of the cryoprotective acetate anion 2.2 A. Biological membranes usually appear in thin sections as three-layered structures 60 to 100 A thick. In view of this relatively large cross section, accessibility of binding sites becomes of obvious importance. 

Colligative Protection. The principles of colligative protection were first outlined by Lovelock (35) for the red blood cell. These principles are also valid for the thylakoid system (14,21,68). If only one solute is present in a membrane suspension, its concentration, regardless of its initial concentration, will rise dining freezing to a level determined solely by the final freezing temperature. If the solute is a cryotoxic compound, this final level may be sufficient to cause membrane inactivation. When several solutes are present and only one is a cryotoxic solute, the same... 

Native enzyme is inactivated initially without loss of antibody reactivity. The inactivation reaction is catalyzed by a membrane protein present in all liver membranes but at highest specific activity in plasma membranes and at lowest activity in lysosomal membranes. Inactivation is greatly accelerated in the presence of disulfides such as oxidized glutathione or cystine and retarded by thiols. Disulfides on the membrane protein are implicated because treatment of membranes with dithiothreitol in the presence of iodoacetamide destroys the capacity to inactivate phosphoenolpyruvate carboxykinase. This treatment would reduce and fix protein disulfides. Inactivation requires a membrane protein that shows some tissue specificity, since plasma membranes from reticulocytes or erythrocytes are not active, nor are liposomes prepared from the lipids of liver microsomes. 

Use of ultrafiltration (UF) membranes is becoming increasingly popular for clarification of apple juice. AH particulate matter and cloud is removed, but enzymes pass through the membrane as part of the clarified juice. Thus pasteurization before UF treatment to inactivate enzymes prevents haze formation from enzymatic activity. Retention of flavor volatiles is lower than that using a rack-and-frame press, but higher than that using rotary vacuum precoat-filtration (21). 

Resistance. Resistance to the cephalosporins may result from the alteration of target pencillin-binding sites (PBPs), decreased permeabdity of the bacterial ced wad and outer membrane, or by inactivation via enzyme mediated hydrolysis of the lactam ring (80,81,138—140). This resistance can be either natural or acquired. Although resistance is often attributed speciftcady to one of these factors, in reaUty it reflects the interplay of several factors. In most instances, however, resistance results from the production of a P-lactamase enzyme, which opens the P-lactam ring as depicted in Figure 2. 

The Class I antiarrhythmic agents inactivate the fast sodium channel, thereby slowing the movement of Na" across the cell membrane (1,2). This is reflected as a decrease in the rate of development of phase 0 (upstroke) depolarization of the action potential (1,2). The Class I agents have potent local anesthetic effects. These compounds have been further subdivided into Classes lA, IB, and IC based on recovery time from blockade of sodium channels (11). Class IB agents have the shortest recovery times (t1 ) Class lA compounds have moderate recovery times (t 2 usually <9 s) and Class IC have the longest recovery times (t 2 usually >9 s). 

In the following, the cardiac action potential is explained (Fig. 1) An action potential is initiated by depolarization of the plasma membrane due to the pacemaker current (If) (carried by K+ and Na+, which can be modulated by acetylcholine and by adenosine) modulated by effects of sympathetic innervation and (3-adrenergic activation of Ca2+-influx as well as by acetylcholine- or adenosine-dependent K+-channels [in sinus nodal and atrioventricular nodal cells] or to dqjolarization of the neighbouring cell. Depolarization opens the fast Na+ channel resulting in a fast depolarization (phase 0 ofthe action potential). These channels then inactivate and can only be activated if the membrane is hyperpolarized... 

Although freeze-fracture experiments have demonstrated that monomers are assembled into stable tetramers in the membranes, radiation inactivation studies and, later, expression studies revealed that each monomer is a functional water channel (Fig. lc). 

The protein-C pathway is one of the most important anticoagulant mechanisms. It is activated by thrombin. Thrombin binds to a cofactor in the membrane of endothelial cells, thrombomodulin (TM). TM bound thrombin no longer activates clotting factors or platelets but becomes an effective protein C (PC) activator. Activated PC (APC) forms a complex with Protein S, which inactivates FVIIIa and FVa. Hereby generation of Flla by the prothrombinase complex is inhibited (Fig. 9). Thus, the PC-pathway controls thrombin generation in a negative feedback manner. 

Table 1). Further determinants of blocking potency are the membrane potential and the state in which the sodium channel is in (resting, activated, inactivated). The tertiary amine group can be protonated giving most local... 

Na+-dependent inactivation. There is a sequence of 20-aminoacids (219-23 8), located in the intracellular f loop near the membrane lipid interface, that seems to be involved in the Na+-dependent inactivation. This autoinhibitory 20-aminoacid sequence might interact with another portion of the f loop (562-679) producing NCX inhibition. In accordance with this view, the synthetic peptide provided with the same sequence of XIP region blocks NCX activity. 

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