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Biomedical subjects

B D Howard

Publications and source records attributed to B D Howard.

At least 55 records · Page 3Linked to original sources
PubMed1980

Mitochondria and sarcoplasmic reticulum as model targets for neurotoxic and myotoxic phospholipases A2.

Certain neurotoxins and myotoxins from snake venoms have phospholipase A(2) activity (phosphatide 2-acylhydrolase, EC 3.1.1.4), which appears to be necessary for their toxicity. Several of these toxins inhibit the net uptake of Ca(2+) into sarcoplasmic reticulum vesicles and brain mitochondria. We have obtained evidence that the ability to inhibit this Ca(2+) uptake is a mechanistically relevant correlate of the toxicity of these proteins rather than being just a nonspecific consequence of their phospholipase A(2) activity. Two of the toxins, beta-bungarotoxin and notexin, had 5% and 50%, respectively, of the phospholipase A(2) activity of IVa phospholipase A(2)(a nontoxic enzyme), but beta-bungarotoxin was as effective as IVa in inhibiting Ca(2+) uptake into brain mitochondria and notexin was more effective. Each of the myotoxic enzymes substantially inhibited Ca(2+) uptake into sarcoplasmic reticulum, notexin being the most effective in this regard. This ability correlated better with their myotoxic potency than with their phospholipase A(2) activity. beta-Bungarotoxin lost its toxicity but not its measurable phospholipase A(2) activity after modification with ethoxyformic anhydride in the presence of dihexanoylphosphatidylcholine. The modified toxin also lost most of its ability to inhibit Ca(2+) uptake into sarcoplasmic reticulum and brain mitochondria. Sarcoplasmic reticulum vesicles reconstituted from solubilized sarcoplasmic reticulum retained their sensitivity to notexin.

Animals↗
PubMed1978

Energy utilization in the induced release of gamma-aminobutyric acid from synaptosomes.

Newly accumulated gamma-aminobutyric acid (GABA) was released from synaptosomes by treatment with 30 mM K+ or the Ca2+ ionophore A23187. Release was Ca2+-dependent and energy-dependent. The induced release of GABA was inhibited by S-13, an uncoupler of oxidative phosphorylation, by azide, a blocker of mitochondrial respiration, and by oligomycin, efrapeptin, tributyltin and dicyclohexylcarbodiimide (DCCD), which are inhibitors of Ca2+/Mg2+-ATPases, including mitochondrial ATPase. Efrapeptin blocked GABA release induced by K+ but not A23187-induced release. Azide and oligomycin appeared to inhibit GABA release as a consequence of their effects on mitochondrial ATP synthesis. However, the inhibition of GABA release by the other compounds could not be totally accounted for by their effects on synaptosomal ATP stores. It is proposed that these compounds, in addition to affecting ATP synthesis, directly affect biochemical reactions involved in GABA release. Thus, these and similar inhibitors seem to be useful probes of the transmitter release process.

Adenosine Triphosphate↗
PubMed1977

Energy utilization in the uptake of catecholamines by synaptic vesicles and adrenal chromaffin granules.

Several inhibitors of energy metabolism decreased the ATP-stimulated uptake of catecholamines by isolated synaptic vesicles from rat brain and by chromaffin granules from bovine adrenal medulla. Catecholamine uptake was inhibited by dinitrophenol, S-13 and oleic acid, which are known to block active transport by dissipating trans-membrane proton gradients. Thus a proton gradient appears to be involved in catecholamine transport. Both catecholamine uptake and vesicle-associated Ca2+/Mg2+-ATPase were inhibited by dicyclohexylcarbodiimide and tributyltin, which had previously been shown to inhibit the Ca2+/Mg2+-ATPase of mitochondria. However, mitochondrial ATPase was not involved in catecholamine uptake as oligomycin and aurovertin, more specific inhibitors of mitochondrial ATPase, did not affect catecholamine uptake. It is suggested that ATP stimulates catecholamine uptake by serving as a substrate for the ATPase. Activity of this enzyme causes translocation of protons across the vesicle membrane establishing a trans-membrane proton gradient. The proton gradient drives the transport of catecholamines.

Adenosine Triphosphate↗
PubMed1977

The buoyant density of synaptic plasma membranes from the cerebral cortex of neonatal rats.

A fraction enriched in synaptic plasma membranes was prepared from neonatal (5-6 day old) rat cerebral cortex. The procedure was based on a method used to prepare synaptic plasma membranes from adult cerebral cortex. Critical steps were monitored by electron microscopy. Synaptic plasma membranes from neonatal cerebral cortex sedimented as a broad peak between 0.9 M and 1.1 M sucrose. In contrast, the majority of adult synaptic plasma membranes have been reported to sediment to 1.2 M sucorose. The activities of various enzyme markers were determined in subfractions of neonatal preparations in order to estimate contamination. The specific activities of these markers indicated substantial contamination of the neonatal synaptic plasma membrane fractions by microsomes and glia.

2',3'-Cyclic-Nucleotide Phosphodiesterases↗
PubMed1977

Relationship between the neurotoxicity and phospholipase A activity of beta-bungarotoxin.

beta-Bungarotoxin is a protein neurotoxin that exhibits phospholipase A activity. The neurotoxin and phospholipase A activities were similarly affected by several agents that modify proteins in various ways. Both activities were very thermostable and resistant to treatment with proteases, 6 M urea, phenylmethylsulfonyl fluoride, and N-acetylimidazole. Both activities were sensitive to beta-mercaptoethanol, and to N-bromosuccinimide and ethoxyformic anhydride, which previously had been shown to inactivate rattlesnake venom phospholipase A by modifying selective amino acids. Dihexanoyllecithin, which acts as a substrate for the beta-bungarotoxin phospholipase A, and Ca2+ protect the phospholipase A activity against inactivation by ethoxyformic anhydride but not the neurotoxicity. Treatment of intact membranes with proteases reduces hydrolysis of the membranes lipids by the toxin phospholipase A.

Animals↗