Modification of the tetrodotoxin receptor in Electrophorus electricus by phospholipase A2

Effect of oxidative stress on receptors and signal transmission

Reactive oxygen metabolites affect binding of ligands to membrane receptors and also coupling of receptors to G-proteins and effector enzymes. Peroxidation of membrane lipids may lead to a lowered receptor density and also will alter the viscosity of the plasma membrane, which affects receptor coupling. Reactive oxygen species may also interact with thiol/disulfide moieties on receptor proteins or on other factors in the receptor system, which is responsible for alterations in receptor binding or coupling. Moreover, lipid peroxidation is associated with the phospholipase A2 pathway, which might indirectly affect receptor function. Moreover, oxidative stress may lead to a disturbance in cellular Ca(2+)-homeostasis. This might be related to an effect on Ca(2+)-mobilizing receptors, but there is also evidence for a decreased Ca(2+)-sequestration by ATPases. In addition, peroxidation of membrane lipids increases membrane permeability to Ca2+. Finally, reactive oxygen species interfere with actions of nitric oxide, thus affecting another pharmacological messenger system.

Effects of cold shock and phospholipase A2 on intact boar spermatozoa and sperm head plasma membranes

Head plasma membranes (HPM) isolated from cryopreserved boar spermatozoa show an excessive fluidization, which might be involved in the loss of fertility. The current study assessed the ability of cold shock (5 degrees C) and phospholipase A2 (PA2) to duplicate these effects on membrane structure and to affect 45Ca2+ uptake and gross morphological characteristics of whole, fresh boar-sperm. The HPM from cold-shocked sperm showed a significantly greater rate of fluidization over time than did HPM from control sperm. Addition of PA2 (bee or snake venom, 0.1 or 10.0 ng/ml) to HPM from control sperm caused fluidization similar to cold shocking, but to a lesser degree (P less than 0.05). Cold-shocked intact sperm exhibited severe acrosomal disruption, loss of motility, and increased 45Ca2+ uptake relative to control sperm. Addition of PA2 (bee or snake venom, 0.1, 1.0., 10.0, and 1,000 ng/ml) to control sperm had no effect on gross morphology or motility while maintaining or increasing sperm extrusion of 45Ca2+. Therefore, although PA2 can, to some extent, duplicate the effects of cold shock on HPM molecular organization, its lipid hydrolytic action is insufficient to cause all the gross disruptions of severe thermal shock. Both PA2 and cold shock disrupted HPM structure, but only cold shock increased 45Ca2+ uptake, suggesting that cold shock may be increasing 45Ca2+ uptake in areas other than the head. Cold shock disrupts sperm on three levels; membrane molecular organization, intracellular Ca2+ regulation, and gross morphology/motility.

Modulation of synaptic GABA receptor binding by membrane phospholipids: possible role of active oxygen radicals

Pretreatment of cerebral synaptic membrane preparations with phospholipase (PLase) A2 invariably induced a significant enhancement of [3H]muscimol binding in a dose-dependent manner with a concomitant elevation of the content of total free fatty acids in the membrane. In vitro addition of various free fatty acids exhibited no profound alteration in [3H]muscimol binding, whereas a significant enhancement of the binding was induced by the pretreatment of the membrane with unsaturated free fatty acids such as arachidonic acid and linoleic acid, but not by that with saturated free fatty acids. None of the inhibitors of arachidonic acid metabolism including indomethacin (an inhibitor of cyclo-oxygenase) and nordihydroguaiaretic acid (an inhibitor of lipoxygenase), however, had a significant preventive action on the augmentation of [3H]muscimol binding. On the other hand, various scavengers for superoxide anion radical such as superoxide dismutase, tiron and nitroblue tetrazolium (NBT) not only suppressed the PLase A2-induced enhancement of [3H]muscimol binding, but also diminished the augmentation of the binding due to PLase C and arachidonic acid. It was also found that a remarkable facilitation of the formation of superoxide anion radical was induced by the treatment of synaptic membrane with PLase A2, PLase C and arachidonic acid, all of which exhibited a prominent stimulation of the binding. In addition, treatment of the membrane with xanthine and xanthine oxidase, a superoxide anion radical generating system, resulted in a profound stimulation of the binding. The PLase A2-induced enhancement of the binding was also attenuated by the scavengers for hydrogen peroxide like catalase as well as by those for hydroxyl radical such as dimethylnitrosoaniline, mannitol, methanol and ethanol, but not by those for singlet oxygen radical including alpha-tocopherol and beta-carotene. The present results suggest that membrane phospholipids may play an important role in the modulation of the association of GABA with its relevant receptor through the generation of active oxygen radicals from unsaturated free fatty acids which are yielded by the catalytic action of PLase A2 and/or PLase C.

Effect of phospholipase digestion and lysophosphatidylcholine on dopamine receptor binding

[3H]Spiperone specific binding by microsomal membranes isolated from sheep caudate nucleus is decreased by trypsin and phospholipase A2 (Vipera russeli), but is insensitive to neuraminidase. The inhibitory effect of phospholipase A2 is correlated with phospholipid hydrolysis. After 15 min of phospholipase (5 micrograms/mg protein) treatment, a maximal effect is observed; the maximal lipid hydrolysis is about 56% and produces 82% reduction in [3H]spiperone binding. Equilibrium binding studies in nontreated and treated membranes showed a reduction in Bmax from a value of 388 +/- 9.2 fmol/mg protein before phospholipase treatment to a value of 52 +/- 7.8 fmol/mg protein after treatment, but no change in affinity (KD = 0.24 +/- 0.042 nM) was observed. Albumin washing of treated membranes removes 47% of lysophosphatidylcholine produced by phospholipid hydrolysis without recovering [3H]spiperone binding activity. However, the presence of 2.5% albumin during phospholipase A2 action (1.5 micrograms/mg protein) prevents the inhibitory effect of phospholipase on [3H]spiperone binding to the membranes, although 28% of the total membrane phospholipid is hydrolysed. Lysophosphatidylcholine, a product of phospholipid hydrolysis, mimics the phospholipase A2 effect on receptor activity, but the [3H]spiperone binding inhibition can be reversed by washing with 2.5% defatted serum albumin. Addition of microsomal lipids to microsomal membranes pretreated with phospholipase does not restore [3H]spiperone stereospecific binding. It is concluded that the phospholipase-mediated inhibition of [3H]spiperone binding activity results not only from hydrolysis of membrane phospholipids, but also from an alteration of the lipid environment by the end products of phospholipid hydrolysis.