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Notes: Conclusions None of the synthetic analogues ofS-adenosyl-l-homocysteine was a more potent inhibitor of the enzyme thanS-adenosyl-l-homocysteine itself. Several requirements very probably had to be fulfilled for inhibition of gastric histamine methyltransferase by compounds similar in structure to S-adenosyl-l-homocysteine: (1) The side chain linked to the 5′-position of ribose must bear an amino acid residue; (2) the chain-length of this residue must be the same as that of homocysteine; (3) the heterocyclic base has to be a purine base with a nucleophilic center at position 6; (4) this nucleophilic center must not be sterically hindered by substitutes; (5) the purine base must have a nitrogen atom in position 3. These requirements indicate, that the binding sites, proposed byZappia et al. [2] for various methyltransferases were identical with those found for the fixation ofS-adenosyl-l-homocysteine or its analogues to histamine methyltransferase.

Notes: Abstract Histamine concentrations in canine whole blood and plasma were determined under several pharmacological, pathophysiological, and clinical conditions, using fluorometric methods. The specificity of the assay for whole-blood histamine was investigated by comparing 3 purification procedures for the isolation of histamine from whole blood including butanol extraction (Shore), ion-exchange chromatography on Dowex 50 W-X 8, and the combination of these 2 methods (Lorenz). Histamine in whole blood was identified in analytical and preparative samples by fluorescence spectra, thin-layer chromatography, degradation by diamine oxidase from pig kidney and inactivation by histamine methyltransferase from guinea-pig brain as well as by biological tests on the isolated guinea-pig ileum. Since butanol extraction resulted in significantly higher ‘histamine’ values than the other two purification procedures, ion-exchange chromatography on Dowex 50 was recommended as the method of choice for the specific determination of histamine in dog's whole blood. Normal values of histamine concentrations in canine plasma were tentatively estimated. They depended on the time between pretreatment of the animals (anaesthesia, operation) and the collection of blood and showed an approximately logarithmic normal distribution. The median, the lower/upper quartiles and the range of the plasma histamine levels obtained 30 minutes after the end of pretreatment were 0.2, 0–0.4 and 0–1.2 ng/ml, respectively. Nearly 50% of the values were zero (below 0.1 ng according to the sensitivity of the method), only 1% of them exceeded slightly 1 ng/ml. Thus histamine release by drugs or by other medical treatments was only stated, when plasma histamine levels exceeded 1 ng/ml and decreased in a way to give an elimination curve of approximately first-order kinetics (Bateman function). Histamine concentrations in dog's whole blood showed approximately a logarithmic normal distribution. The median, lower/upper quartiles and range were 47, 34/75 and 13–209 ng/ml respectively. The histamine levels in the whole blood of four circulatory regions did not show any significant differences. The plasma histamine concentrations in the portal vein were slightly higher than in the hepatic veins. The injection of exogenous histamine and the concomitant determination of plasma and whole-blood histamine levels in four circulatory regions showed that the plasma histamine determination was the more sensitive method for measuring histamine elimination curves than the whole-blood histamine assay. The elimination of exogenous histamine administered intravenously was influenced by several drugs including inhibitors of histamine inactivation and histamine receptor antagonists. Aminoguanidine and the H2-receptor antagonist burimamide slowed down the disappearance of histamine from the plasma, the H1-receptor antagonist dimethpyrindene enhanced it, but amodiaquine had no significant effects. Dimethpyrindene and burimamide were capable of releasing histamine in dogs, in some cases to a considerable extent. The plasma substitute Haemaccel®, a chemically modified gelatin, released histamine in dogs. Using batch 3000, from 27 animals investigated, 15 animals showed elevated plasma histamine levels and a hypotensive blood pressure response, whereas in 12 of the dogs it did not show an effect on these parameters. The plasma histamine levels at the time of maximum hypotension showed an approximately logarithmic normal distribution. This frequency distribution in combination with the varying incidence of anaphylactoid reactions depending on the batches used seemed very important for the interpretation of clinical reactions to Haemaccel in human test persons and patients. By histamine determinations in plasma and whole blood of several circulatory regions and in various tissues before and after infusion of Haemaccel it could be demonstrated that the sites of histamine release by Haemaccel in dogs were especially the skin of the upper hemisphere of the body and the liver, whereas the gastro-intestinal tract took up histamine from the circulation. These numerous results under various experimental conditions may be considered as an evidence for the high quality and reliability of the method to study histamine release in the whole animal or in human subjects by evaluating histamine elimination curves in plasma.

Notes: Abstract In 9 dogs, whose maximum gastric acid response to pentagastrin was evoked by 6 μg/kg, the total gastric secretion as well as the peak gastric secretion was enhanced by amodiaquine. The optimum dose of this antimalarial drug was 2 mg/kg, whereas 0.25 mg/kg were without effect and 3 mg/kg reduced already the augmentation of gastric secretion by this substance. The increase in acid output by amodiaquine was greater than that in volume. The total secretion was more enhanced than the peak secretion, which means a longer duration of the amodiaquine potentiated gastric secretion elicited by pentagastrin, than that without application of amodiaquine contrary to that stimulated by exogenous histamine. Amodiaquine itself did not stimulate gastric acid secretion, in contrast to prostigmine and carbachol. Thus amodiaquine seemed not to enhance gastric secretion by a direct or indirect parasympathomimetic action. The question whether amodiaquine acted on gastric secretion in a specific way and not by parasympathomimetic effects, led to investigations in several exocrine glands. In salivary glands, amodiaquine did neither stimulate the secretion in all doses investigated nor did it enhance the pilocarpine and acetylcholine induced salivation with any significance and regularity. Also the pancreatic and biliary secretion was neither stimulated by amodiaquine nor was the secretin induced secretion of the pancreas and liver augmented by amodiaquine. Thus the enhancing effect of this drug on the histamine and pentagastrin stimulated gastric secretion was very likely specific for the gastric mucosa and not due to a parasympathomimetic action of the drug. In contrast to the findings in various exocrine glands of the gastrointestinal tract, the arterial hypotension following the i.v. injection of acetylcholine was increased specifically by a preceeding i.v. injection of amodiaquine, whereas the equi-effective actions of histamine, serotonin and bradykinin as well as the hypertension by epinephrine and norepinephrine were not influenced by amodiaquine. This specific effect of the antimalarial drug very probably was not caused by an inhibition of the unspecific choline esterase in the blood. Since in exocrine glands no evidence could be found for a parasympathomimetic action or other modes of action of amodiaquine, it seemed probable that amodiaquine potentiated the histamine and pentagastrin stimulated gastric secretion by an inhibition of histamine methyltransferase in vivo.

Notes: Conclusion These results indicate that even small alterations of the incubation procedure witho-phthaldialdehyde may considerably influence the specificity of the fluorometric histamine assay. Tests for identification of this amine are always necessary in studies on histamine in body fluids or following the administration of drugs.

Notes: Abhandlungen (mit summaries)

Notes: Abstract Histamine methyltransferase from pig antrum mucosa was inhibited by 33 H1-receptor antagonists, by the H2-receptor antagonists burimamide and metiamide and by the burimamide analogue 5-methylburimamide, which did neither act on H1-nor on H2-receptors. Whereas all H1-receptor blocking agents as well as metiamide and 5-methylburimamide inhibited the enzyme in a competitive manner, the type of inhibition found for burimamide was a mixture of non-competitive and uncompetitive with respect to histamine, which is similar to that one observed with 1-methylhistamine, the product of the histamine methylation reaction. Furthermore, all compounds tested — with the only exception of burimamide — activated the gastric histamine methyltransferase in lower concentrations, the most potent activator being piprinhydrinate (180% increase of the enzyme activity). This enhancement of 1-methylhistamine formation by antihistaminic drugs was not due to a true activation of the enzyme by increasingV max, but was caused by partially abolishing the inhibition of histamine methyltransferase by so-called ‘optimum’ concentrations of histamine. Two explanations were given for the different mode of action of burimamide compared with that of metiamide and the other antihistaminic drugs: (1) The change in the type of inhibition from burimamide to metiamide seemed to be due to the introduction of a methylgroup into position 5 of the imidazole ring. (2) Burimamide and 1-, 2- and 3-methylhistamine were the only compounds tested in which the imidazole nucleus was substituted at position 4, but not at position 5, and which thus probably produced substrate or product inhibition.

Notes: Summary High histamine concentrations and histamine methyl transferase activity were demonstrated in the gastric mucosa of man, dog, pig and cow. Modified methods for the determination of histamine and histamine methyltransferase were developed. Histamine was identified by its fluorescence spectrum, by thin-layer chromatography in 8 different solvent systems and by bioassay. Histamine methyltransferase from pig antral mucosa was purified 6-fold by ultra-centrifugation and fractional precipitation by ammonium sulfate. ItsK m for histamine as substrate was 2.3×10−5 M, for S-adenosylmethionine 4.3×10−5 M, the pH-optimum was found to be pH 7.4. Nicotinamide in concentrations up to 1×10−2 M had no effect on the activity of the enzyme.

Notes: Zusammenfassung Verschiedene Isolierungsverfahren für Histamin zur fluorimetrischen Bestimmung wurden geprüft. Für Histamingehalte von mehr als 1 μg/g Gewebe oder Milliliter Vollblut eignet sich sowohl die Butanolextraktion als auch die Ionenaustausch-Chromatographie an Dowex 50W-X8, für Vollblut des Hundes nur das letztere Verfahren, für Plasma und Magensaft des Menschen nur die Kombination der beiden Isolierungsmethoden. Histamin wurde identifiziert durch Dünnschicht-Chromatographie auf Cellulose, Umsatz durch angereicherte Diaminoxydase und Histaminmethyltransferase, durch die Fluorescenzspektren nach Kondensation mit o-Phthaldialdehyd und durch seine biologische Aktivität. Die Histamingehalte in Geweben und Körperflüssigkeiten des Menschen und einiger Versuchstiere werden mitgeteilt.