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H Verweij

Publications and source records attributed to H Verweij.

9 recordsLinked to original sources
PubMed1991

Improved procedure for the second-derivative spectrophotometric analysis of carboxyhaemoglobin.

We describe a modification of the second-derivative spectrophotometric assay for carboxyhaemoglobin in blood. Using the original procedure we have often observed a time-dependent change in the signal leading to unreliable results. By using phosphate-buffered-saline (pH 7.4) instead of ammonia to dissolve the deoxygenating agent (sodium dithionite), we obtained stable and reproducible second-derivative signals from which the percentage carboxyhaemoglobin in the patients' samples can be calculated.

Blood Chemical Analysis↗
PubMed1986

Improved gas chromatographic-mass fragmentographic assay for tetrahydroaldosterone and aldosterone in urine.

A newly devised procedure for a simultaneous determination of urinary tetrahydroaldosterone and aldosterone is described. The procedure is based on deconjugation and acetalization, followed by extraction and derivatization of the urinary compounds to their trimethylsilyl ethers and subsequent gas chromatographic-mass fragmentographic detection. To evaluate the assay, aliquots of a urine sample of a healthy individual were analysed in multiplicate; a mean tetrahydroaldosterone concentration of 103 nmol/l and a within-sample, within-day- and day-to-day coefficient of variation of 1.8, 3.2 and 3.4%, respectively, were found. Determination of aldosterone in the same sample yielded a mean concentration of 25.3 nmol/l and the following coefficients of variation: 2.8% (within-sample), 3.8% (within-day) and 4.3% (day-to-day). The urinary excretion of tetrahydroaldosterone and aldosterone in 24-h urine portions was determined in twenty healthy individuals, aged 23-77 years; for tetrahydroaldosterone and aldosterone, an excretion of 94 +/- 66 nmol per 24 h and of 40 +/- 22 nmol per 24 h was found, respectively, in accord with the literature. An example of the usefulness of the described assay is given by establishing the cause of severe salt-wasting in an infant; a highly elevated tetrahydroaldosterone and aldosterone excretion was demonstrated, proving that the child suffered from unresponsiveness to aldosterone (pseudohypoaldosteronism).

Adult↗
PubMed1982

Ozone-induced formation of O,O'-dityrosine cross-linked in proteins.

Treatment of spectrin, insulin, glucagon and ribonuclease with ozone results in covalent cross-linking of these proteins. This cross-linking is not reversed by treatment with dithiothreitol and thus can not be ascribed to -S-S- bond formation. A concomitant O,O'-dityrosine formation is observed by spectrofluorometric analysis of the protein and by amino acid analysis and thin-layer chromatography of hydrolyzed protein samples. It is highly probable that the observed protein cross-linking should be attributed to interpeptide O,O'-dityrosine bonds. Several authors have shown before that oxidation of proteins with horseradish peroxidase and H2O2 also leads to O,O'-dityrosine formation. Peroxidase-induced O,O'-dityrosine formation in galactose oxidase (d-galactose:oxygen 6-oxidoreductase, EC 1.1.3.9) causes a strong increase of enzyme activity. In accordance with these observations ozone treatment of galactose oxidase also leads to O,O'-dityrosine formation with a concomitant 8-fold increase of enzyme activity.

Amino Acids↗
PubMed1981

Photodynamic protein cross-linking.

Exposure of spectrin to visible light in the presence of a photosensitizer results in photo-oxidation of sensitive amino acid residues and covalent cross-linking of the polypeptides. In a previous paper the cross-linking was ascribed to a secondary reaction between photo-oxidized histidine residues and amino groups. The following observations, described in this paper, are in accordance with this supposition. (1) During illumination of spectrin in the presence of a photosensitizer a pronounced photo-oxidation of histidine residues takes place. (2) Simultaneously a decrease of free amino groups is observed. (3) Semicarbazide protects against cross-linking and is bound to a histidine photo-oxidation product in spectrin. (4) The pH profile of histidine photo-oxidation and subsequent reaction with amino groups is similar to the pH profile of spectrin cross-linking. Amidination of NH2 groups in spectrin does not inhibit cross-linking, as visualized by gel electrophoresis. On the other hand aminidation of denatured myoglobin causes a 50% inhibition of cross-linking. These observations support the notion of NH2-involvement in cross-linking but also demonstrate, that other photodynamic cross-link mechanisms exist.

Amino Acids↗
PubMed1980

Effects of semicarbazide on oxidative processes in human red blood cell membranes.

Semicarbazide can interfere with oxidative processes in the red blood cell membrane via different modes of action. Treatment of human red blood cell membranes with O3, results, among other effects, in cross-linking of membrane proteins and inhibition of glyceraldehyde-3-phosphate dehydrogenase activity. Semicarbazide inhibits these effects by acting as an O3 scavenger. The effect of semicarbazide as an O3 scavenger is complicated by the fact that ozonolysis of semicarbazide yields a product that causes inhibition of glyceraldehyde-3-phosphate dehydrogenase. Glyceraldehyde-3-phosphate dehydrogenase inhibition can also be provoked by incubation of membrane suspensions with O3-treated phospholipids. Semicarbazide prevented this effect by interaction with an inhibitory O3-phospholipid reaction product. Protoporphyrin-induced photodynamic cross-linking of membrane proteins is chemically distinct from O3-induced cross-linking. Photodynamic cross-linking is also inhibited by semicarbazide, in this case via reaction with a histidine photooxidation product.

Erythrocyte Membrane↗