Search PubMed⌕ Search

Biomedical subjects

H L White

Publications and source records attributed to H L White.

At least 37 records · Page 2Linked to original sources
PubMed1990

An enzymatic method for detecting MAO-A and MAO-B inhibitors in plasma and its application in studies with reversible MAO-A selective inhibitors.

A general method was developed for the simultaneous enzymatic assay of MAO-A and MAO-B inhibitors in plasma of pretreated animals. After extraction of the inhibitor from plasma into an appropriate volatile organic solvent, dried aliquots of the organic layer are incubated with a buffered rat brain MAO preparation and substrates, [3H]serotonin and [14C]phenethylamine. Under conditions employed, the presence in plasma of MAO-A and/or B inhibitors can be detected.

Animals↗
PubMed1990

Acetyl-L-carnitine as a precursor of acetylcholine.

Synthesis of [3H]acetylcholine from [3H]acetyl-L-carnitine was demonstrated in vitro by coupling the enzyme systems choline acetyltransferase and carnitine acetyltransferase. Likewise, both [3H] and [14C] labeled acetylcholine were produced when [3H]acetyl-L-carnitine and D-[U-14C] glucose were incubated with synaptosomal membrane preparations from rat brain. Transfer of the acetyl moiety from acetyl-L-carnitine to acetylcholine was dependent on concentration of acetyl-L-carnitine and required the presence of coenzyme A, which is normally produced as an inhibitory product of choline acetyltransferase. These results provide further evidence for a role of mitochondrial carnitine acetyltransferase in facilitating transfer of acetyl groups across mitochondrial membranes, thus regulating the availability in the cytoplasm of acetyl-CoA, a substrate of choline acetyltransferase. They are also consistent with a possible utility of acetyl-L-carnitine in the treatment of age-related cholinergic deficits.

Acetylcarnitine↗
PubMed1988

Effects of acetylcholine and other agents on 32P-prelabeled phosphoinositides and phosphatidate in crude synaptosomal preparations.

Experimental conditions are described which permit effects of various agents on polyphosphoinositides and phosphatidic acid (PA) to be evaluated simultaneously in crude nerve-ending preparations from rat brain. Acetylcholine (3-100 microM) or carbachol (30-1,000 microM) induced the hydrolysis of prelabeled polyphosphoinositides and, at the same time, stimulated the net label incorporated in phosphatidic acid. All muscarinic effects were blocked by atropine or pirenzepine. Non-muscarinic agonists (glutamate, adenosine, norepinephrine) stimulated polyphosphoinositide hydrolysis in this preparation, but of these only norepinephrine affected phosphatidic acid turnover. A potentiation of acetylcholine-induced phosphoinositide turnover by KCl was observed, as well as an apparent selective inhibition of PIP2 hydrolysis by LiCl. Acetylcholine-stimulated turnover of PA was not necessarily coupled to phosphoinositide hydrolysis.

Acetylcholine↗
PubMed1988

Increased airways responsiveness to histamine induced by platelet activating factor in the guinea-pig: possible role of lipoxygenase metabolites.

In anaesthetized guinea-pigs pretreated with propranolol (1 mg/kg, i.v.), platelet activating factor (Paf, 0.02 micrograms/kg, i.v.) caused an acute increase in airways response to histamine (0.5-3.0 micrograms/kg, i.v.) measured as intratracheal pressure. Treatment with the cyclooxygenase inhibitors, aspirin (10 mg/kg, i.v.) or indomethacin (5 mg/kg, i.v.), enhanced the magnitude and duration of this effect but a combined lipoxygenase/cyclooxygenase inhibitor, BW 755C (20 mg/kg, i.v.), prevented the increase in responsiveness. In aspirin treated animals, a putative lipoxygenase inhibitor NDGA (10 mg/kg, i.v.). or atropine methyl nitrate (1 mg/kg, i.v.) or bilateral vagotomy reduced the magnitude of Paf-induced increased histamine responses but did not prevent the effect. Bronchoconstriction induced by Paf was variably influenced by the drug treatments. These data suggest that Paf causes an acute increase in airways responsiveness to histamine in the guinea-pig through a mechanism that may, in part, be dependant on the release of lipoxygenase metabolites.

4,5-Dihydro-1-(3-(trifluoromethyl)phenyl)-1H-pyraz↗
PubMed1988

Inhibition of lyso-PAF: acetyl-CoA acetyltransferase by salicylates and other compounds.

Diflunisal and benoxaprofen (20-100 microM) produced dose-dependent inhibitions of lyso-platelet activating factor: acetyl-CoA acetyltransferase in a lysate of rat pleural neutrophils. Salicylate and aspirin were inhibitory at concentrations of 1 mM and above. Nordihydroguaiaretic acid was a relatively potent inhibitor (I50 = 6 microM). Other compounds, including anti-inflammatory steroids, cyclooxygenase and 5-lipoxygenase inhibitors, appeared ineffective at relevant concentrations. Inhibitions by diflunisal and salicylate occurred at concentrations similar to expected plasma levels in humans at therapeutic doses. An inhibition of platelet-activating factor synthesis may contribute to the antiinflammatory, analgesic, or antipyretic actions of these compounds.

Acetyl-CoA C-Acetyltransferase↗
PubMed1985

Arachidonate metabolic pathways in cells harvested from rat pleural cavity at various times after carrageenan administration.

Cells were harvested from rat pleural cavity before and during the inflammatory response stimulated by carrageenan injection. The conversion of [14C]arachidonate by intact cells into products of the cyclooxygenase and 5-lipoxygenase pathways was studied in the absence and presence of ionophore. Incorporation of arachidonate into phosphatidic acid was also followed. In the absence of ionophore, the principal arachidonate metabolites of resident macrophages were the cyclooxygenase products, prostacyclin and thromboxane, while mobilized monocytes produced thromboxane as a major product, but very little prostacyclin. Arachidonate-metabolizing enzymes in mobilized monocytes were, in general, less active than those of resident macophages. Cells harvested at times when mobilized neutrophils were the predominant cell type were capable of converting arachidonate into thromboxane and prostaglandins, but not prostacyclin. These cells exhibited the most active turnover of arachidonate into phosphatidic acid, and the extent of this turnover appeared to be temporally related to the presence of edema in the pleural cavity at the time of cell harvest. Enzymatic formation of 5-lipoxygenase products was dependent on calcium and was markedly stimulated by ionophore A23187 in both resident and mobilized pleural cells. Among several non-steroidal drugs tested, cyclooxygenase inhibitors were the most effective in preventing the inflammatory response in the carrageenan model of inflammation.

Animals↗
PubMed1984

Species differences in monoamine oxidase-A and -B as revealed by sensitivity to trypsin.

Monoamine oxidase-A and -B of brain and peripheral tissues of human, dog, rat, and rabbit have been characterized with respect to kinetic parameters and response to limited trypsin treatment. MAO-A in all extracts was inactivated by trypsin to an extent that was species dependent, but not related to the nature of the tissue, to MAO-A/B ratios, or to kinetic parameters. MAO-B activity of human, dog, and rat was unaffected by trypsin under the conditions employed, but rabbit MAO-B was sensitive. Inactivation patterns obtained with the trypsin treatment described in this study provide additional evidence that structures of both MAO-A and MAO-B show species-specific variations.

Animals↗
PubMed1984

Synthesis of bridged catechol-homocysteine derivatives as potential inhibitors of catechol O-methyltransferase.

Catechol derivatives, covalently joined to homocysteine by sulfide or sulfonium linkages, were synthesized as potential catechol O-methyltransferase multisubstrate inhibitors which might bridge the enzymatic binding sites for the catechol substrate and the amino acid portion of the methyl donor S-adenosylmethionine. These compounds were found to be less effective inhibitors than the product inhibitor S-adenosylhomocysteine.

Animals↗
PubMed1984

Selective effects of proteases and phospholipase A2 on monoamine oxidases A and B of human brain and liver.

Human brain and liver mitochondria contain membrane-bound monoamine oxidase of both A and B types. Monamine oxidase-A (MAO-A), either membrane-bound or in detergent-solubilized extracts from these tissues, was selectively inhibited during incubations with trypsin, chymotrypsin, thermolysin, or papain. MAO-A in solubilized, but not in membrane-bound, preparations was also very sensitive to the action of phospholipase A2, while MAO-B was unaffected. Membrane-bound MAO-A of rat brain mitochondria was more sensitive to phospholipases and less sensitive to proteases than was human brain enzyme, indicating that these agents may reveal species differences in MAO properties. Human brain and liver MAO-A, either solubilized or bound in mitochondrial membranes, apparently contains basic and aromatic peptide moieties that are available to proteases. Hydrolysis of these peptide bonds leads to rapid denaturation unless substrate molecules stabilize the active site. Phospholipase A2 may disrupt the phospholipid microenvironment of MAO-A, the integrity of which is essential for MAO-A activity, but not for MAO-B. No interconversion of the two activities was observed. After phospholipase A2 treatment, remaining MAO-A activity was recovered in low-molecular-weight regions of a gel filtration gradient, suggesting that MAO-A subunits were released. Although these experiments argue against the proposal that phospholipids may regulate the ratio of A/B activities of a single enzyme molecule, it is conceivable that endogenous phospholipases or proteases in mitochondrial membranes may influence MAO-A activity independently of MAO-B activity.

Animals↗
PubMed1982

A novel inhibitor of gamma-aminobutyrate aminotransferase with anorectic activity.

1-(n-decyl)-3-Pyrazolidinone (BW357U) is a potent, selective inhibitor of gamma-aminobutyrate aminotransferase (GABA-T) in vitro and in vivo. After acute or chronic, oral or intraperitoneal administration of BW357U to rats, brain GABA levels were elevated in a dose-dependent manner. When inhibition of brain GABA-T exceeded 50%, whole brain GABA levels were elevated approximately threefold, and an anorectic effect was observed in the absence of other symptoms. This compound, because of its potency and selectivity, may be useful in studies relating to the function of GABA-containing neurons in appetite regulation.

4-Aminobutyrate Transaminase↗
PubMed1982

Monoamine oxidases A and B as components of a membrane complex.

The activities of monoamine oxidase A (MAO A) and monoamine oxidase B (MAO B) represent two independent types of substrate binding site, as indicated by experiments with selective inhibitors and also by substrate competition. We have tried to determine whether A and B active sites of human brain and liver MAO are located on physically separable enzyme forms or as subunits in large membrane-bound complexes. MAO was extracted from several sources by a procedure that was designed to give solubilized enzyme in high-speed supernatants, with ratios of MAO A/MAO B activities similar to those in initial crude homogenates. This solubilized enzyme gave gel filtration profiles that suggested the presence of large molecular complexes. Affinity binding experiments indicated that both MAO A and B activities may occur in the same complexes in tissues that initially contain both activities. These complexes were broken down to enzymatically active subunits by treatment with either low concentrations of sodium dodecyl sulfate, with phospholipase A2, or with a combination of both agents. Results of this study support a concept of MAO as part of a membrane unit in which A and B are two distinct enzymes embedded in a phospholipid structure. The enzymatic activity of MAO A is critically dependent on associated phospholipids, whereas that of MAO B is not.

Animals↗
PubMed1981

Glutamate as a precursor of GABA in rat brain and peripheral tissues.

The formation of GABA from L-glutamate was investigated in homogenates of rat brain, liver, and kidney, using highly purified [14C]-L-glutamic acid as substrate and a thin-layer chromatographic separation of products. In agreement with other workers, liberation of [14C]-CO2 was found to be stoichiometric with GABA formation in brain homogenates, but not in liver or kidney extracts. Subcellular fractionation and dialysis experiments suggested that most of the GABA synthesis in these peripheral tissues, unlike brain, does not occur via a direct decarboxylation of glutamate and requires one or more cofactors other than pyridoxal phosphate. NAD stimulated GABA formation in dialyzed extracts, and inhibition of GABA-transaminase, both in vitro and in vivo, caused marked inhibition of GABA formation from glutamate in peripheral extracts. Although a very low GAD activity in liver and kidney cannot be excluded, these experiments suggest a major pathway from glutamate to GABA in these homogenates which includes (1) conversion of glutamate to alpha-ketoglutarate by glutamate dehydrogenase or transaminases, (2) conversion of alpha-ketoglutarate to succinic semialdehyde, and (3) formation of GABA from succinic semialdehyde and glutamate by GABA-transaminase.

4-Aminobutyrate Transaminase↗
PubMed1980

Platelet monoamine oxidase activity and the classification of depression.

An attempt was made to compare platelet monoamine oxidase (MAO) activity in descriptively based types of nonbipolar depression. Platelet MAO activity was significantly higher in depression secondary to chronic anxiety, compared with primary unipolar depression and depression secondary to borderline personality, and in women compared with men. No significant differences were observed between endogenous-nonendogenous, delusional-nondelusional, psychomotor states, or different age groups.

Adult↗
PubMed1980

Effects of delta 9-tetrahydrocannabinol and cannabidiol on phospholipase and other enzymes regulating arachidonate metabolism.

delta 9-Tetrahydrocannabinol (THC) and cannabidiol (CBD) caused a marked stimulation of phospholipase A2 when incubated with intact human platelets that were prelabeled with [14C] arachidonate. CBD was about 1.5 x as potent as THC in the same concentration range (10 leads to 80 microM) Most of the released arachidonate was converted to lipoxygenae products. When [14C] arachidonate was incubated with lysed platelet extracts, THC inhibited both thromboxane synthetase and prostaglandin cyclooxygenase, so that the net effect was a redistribution of products toward the lipoxygenase pathway at the same time that a decrease in total cyclooxygenase product formation occurred. THC did not directly affect arachidonate lipoxygenase. Both TCH and CBD also stimulated release from prelabeled neuroblastoma cells (NBA2), which do not contain an active lipoxygenase pathway. In this case, accumulation of free arachidonate was detected by autoradiography. The multiple effects of THC and CBD on phospholipase A2 and arachidonate metabolism may mediate some of the pharmacological actions of these compounds, such as their anticonvulsant, anti-inflammatory, and hypotensive properties.

Animals↗
PubMed1979

4-aminobutyrate:2-oxoglutarate aminotransferase in blood platelets.

Platelet lysates obtained from blood of humans, dogs, and rats catalyzed the transamination of 4-aminobutyrate with 2-oxoglutarate as cosubstrate. Human platelet 4-aminobutyrate:2-oxoglutarate aminotransferase (36.5 +/- 3.2 picomoles per minute per milligram of platelet protein) resembled the brain enzyme in kinetic properties and in response to cofactors and inhibitors.

4-Aminobutyrate Transaminase↗
PubMed1979

Prostaglandin synthesis in homogenates of cultured neuroblastoma cells.

Homogenates of 5 neuroblastoma cell lines found to produce prostaglandin products from exogenous [14C]arachidonate, with specific enzyme activities ranging from 60 to 365 pmol per min per mg protein. Under identical conditions a glial cell line was much less active. PGF2 alpha and PGE2 were the major products from neuroblastoma cells, with PGF2 alpha predominating in all cases. The prostaglandin synthesizing activity of neuroblastoma extracts was at least an order of magnitude higher than activities reported for endogenous prostaglandin synthesis in brain tissues. The pattern of products was similar to that achieved after incubation of a rat brain microsomal extract with [14C]arachidonate, although the enzyme activity of neuroblastoma was about 200-fold higher. The presence of a relatively high prostaglandin cyclooxygenate activity in cultured neuroblastoma cells is of particular interest in that these cells may be useful model systems for studies of some aspects of neuronal prostaglandin synthesis.

Animals↗