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M Dijkstra

Publications and source records attributed to M Dijkstra.

31 records · Page 2Linked to original sources
PubMed1993

The linkage approach applied to a school-based smoking prevention program in The Netherlands.

Effective diffusion strategies are necessary to enhance use of innovative health promotion programs. One strategy uses the linkage approach to innovation-development and diffusion planning. The linkage approach enhances collaboration among three systems: resource system (university-based researchers), linkage system (district health educators), and user system (teachers). This article illustrates how the linkage approach was applied in a smoking prevention research project. Identification of the linkage system and the collaborative process between the resource system and linkage system are described. Results from a process evaluation indicated the linkage approach was feasible in a school-based smoking prevention project.

Child↗
PubMed1993

The role of glutathione in bile secretion of endogenous trace elements in rats.

To evaluate the role of glutathione in biliary secretion of endogenous trace elements, we quantitated trace element output rates by proton-induced x-ray emission under various conditions with altered biliary glutathione secretion and hepatic glutathione content in the rat. Treatment with phenobarbital (80 mg/kg body weight, 4 days), ethanol (0.9 gm/kg body weight, 4 days), or diethylmaleate (3.9 mmol/kg body weight) resulted in changes in biliary glutathione secretion of +114%, -56%, and -95%, respectively, and in hepatic glutathione content of -0%, +25%, and -86%, respectively, when compared with control values. Biliary glutathione level was below detection limits in mutant Groningen Yellow Wistar rats, whereas hepatic glutathione content was increased by 114% in these animals. Glutathione secretion showed a linear relationship with bile flow when data from all experiments were included in the analysis; the apparent choleretic activity of glutathione was 67 microliters/mumol. Six trace elements (iron, zinc, copper, manganese, molybdenum, bromine) could always be detected in bile. Potassium and calcium were measured for comparative purposes. No relation was found between biliary trace element secretion and hepatic glutathione content. Biliary output rates of iron, molybdenum, and bromine correlated, albeit poorly, with biliary glutathione efflux (r values: iron, 0.67; molybdenum, 0.40; bromine, 0.53; respectively). Copper, manganese, and zinc secretion did not show any consistent relationship with glutathione secretion. The secretion rates of iron, molybdenum, and bromine, like that of calcium, showed a highly significant correlation with bile flow (r values: iron, 0.89; molybdenum, 0.75; bromine, 0.80; and calcium, 0.90; respectively, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

The utilization of qualitative and quantitative data for health education program planning, implementation, and evaluation: a spiral approach.

The process of development of a Dutch smoking prevention project is described. An essential feature of the project is the combination and interaction of qualitative and quantitative research methods. It is advocated that each method has its own contribution and can be considered as a separate methodology contributing to social science in general and health education research in particular. Combining the two approaches in a spiral approach will result in a synergistic effect, because of the interaction of both approaches. The results of both methods suggest that qualitative methods enhanced the generation of ideas and theories. Qualitative methods were used to formulate ideas for improving quantitative data gathering, analyzing and comparing ideas with respect to program development, and for testing the internal validity of a quantitative design. The quantitative method enabled testing of results in different groups and detecting detailed differences. It also provided information that one of the assumptions of the program, the development of a teacher independent program, was not completely realized. The major advantage of using both methods is that this provides feedback between assumptions and data, thus enhancing comparison of results and critical reflection during the whole project.

Health Education↗
PubMed1991

[Meat quality in traditionally and alternatively (not exclusively milk substitute) fed veal calves].

A number of trials were done in veal calves, in which part of the milk substitutes were replaced by maize silage or a silage-concentrate mix. The milk scheme was reduced by twenty per cent in some trials or milk was only given for the first eleven weeks. When these alternative systems were adopted, growth was satisfactory and the costs of feed were considerably reduced. All calves were slaughtered when the live weigh was approximately 300 kilograms. The main effect of the alternative system was that the meat was no longer pale. The extent to which the quantity of milk substitute was reduced had no further effect on the colour of the meat. The differences in colour of the meat between traditionally and alternatively fed calves, visually as well as measured by instruments or chemically (haematin content) were significant. As regards other meat quality parameters, the differences between the two groups of calves were not significant. A taste panel only indicated significant effects of the feeding regimen in traditionally fed calves and one group of alternatively fed calves for flavour and taste but not for succulence and tenderness. As far as traditional veal and veal of alternatively fed calves are concerned, the shear-force values of meat samples taken at equal weight did not show any significant differences. In so far as it can be judged today, there is a restricted market for veal of these alternatively fed calves.

Animal Feed↗
PubMed1991

Biliary secretion of trace elements and minerals in the rat. Effects of bile flow variation and diurnal rhythms.

Effects of bile flow variations on bile secretion of trace elements were systematically evaluated by Proton-Induced X-ray Emission (PIXE) in the unanesthetized rat: (i) longitudinally for a 9 day-period of bile drainage; (ii) continuously for a period of 24 h, to include circadian rhythm and (iii) during exogenous bile salt administration. Potassium and Ca were determined for comparative purposes. In rat bile, six trace elements could always be detected by PIXE (Fe, Zn, Cu, Mo, Mn, Br); occasionally some V, Cr, Sr and Pb was found. Se could not be detected in bile. Bile-plasma concentration ratios of the elements could arbitrarily be divided into three groups: Fe, Zn and Se less than 1; Ca, K, Cu, Mo and Br approx. 1 and Mn much greater than 1. After interruption of the enterohepatic circulation, bile flow and bile salt concentration dropped sharply within 4 h to 36% and 4% of their initial values, respectively. Output rates of K, Ca, Mo, Br, Zn and Fe closely followed the decrease in bile flow; bile secretion of Mn and Cu, on the other hand, was minimally affected. A circadian rhythm was observed for all detectable elements; during the night period, secretion rates of Zn, Fe, Ca and K increased maximally by 60-80% and that of Mn, Mo, Br and Cu by 30-50%. Bile salt output and bile flow increased maximally by 70% and 50%, respectively, in the same time interval.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Two pathways for biliary copper excretion in the rat. The role of glutathione.

To evaluate the role of glutathione in biliary copper excretion, we studied this process in control Wistar rats and in mutant Wistar rats (GY rats), in which the secretion of glutathione into bile is deficient. For comparison, biliary zinc excretion was determined simultaneously. In spite of the markedly reduced bile flow (-45%) in GY rats, biliary output rates of endogenous copper were virtually identical in GY and control rats. In contrast, zinc output was drastically reduced in GY rats compared to controls (-80%). Biliary excretion patterns after intravenous administration of copper, in doses ranging from 65 to 2265 nmol/100 g/body wt, showed a distinct rapid and slow phase in control rats. In GY rats, on the other hand, the rapid phase in copper excretion was absent but the slow phase appeared to be unaffected. Pretreatment of rats with diethylmaleate to deplete hepatic and biliary glutathione abolished the rapid phase of copper excretion in control rats, while the slow phase remained unaffected. No significant effect of diethylmaleate on the hepatic handling of exogenous copper was observed in GY rats. The maximal capacity of the slow copper excretion pathway was 40-45 nmol/hr/100 g body wt, both in control and GY rats; the capacity of rapid excretion pathway depended on the administered copper load. Intravenous injection of copper induced the biliary excretion of a substantial amount of zinc in control rats, but not in GY rats. These results indicate the existence of at least two distinct biliary excretory pathways for copper in the rat, i.e. a slow and a rapid pathway, with a glutathione dependency of the latter only. The basal excretion of (endogenous) copper, in contrast to that of zinc, can proceed independently of glutathione excretion. However, glutathione appears to be involved in the rapid secretion of excess copper.

Animals↗
PubMed1990

Bile secretion of trace elements in rats with a congenital defect in hepatobiliary transport of glutathione.

Bile secretion of trace elements, analyzed by proton-induced x-ray emission, was studied in rats with a congenital defect in hepatobiliary transport of organic anions [Groningen Yellow (GY) rats], in which the process of bile secretion resembles that of the neonatal period. Bile flow (-41%) and biliary glutathione secretion (-99%) were drastically impaired in GY rats compared with controls. Plasma concentrations of all detectable trace elements (Fe, Cu, Zn, Mo, Br, and Se), as well as that of simultaneously determined Ca, were similar in GY and age-matched control Wistar rats. Bile concentrations of Fe, Mo, Br, and Ca were also similar in both groups, resulting in a approximately 40% reduction of their secretion rates in GY rats. The concentrations of Zn (-62%) and Mn (-64%) were significantly lower in GY rats in contrast to that of Cu, which was 50% higher. Se could not be detected in bile of either group. Recovery in bile (% dose/3 h) after i.v. injection of MnCl2, CuSO4, or SeO2 (1 mg metal/kg) was lower in GY rats than in controls: Mn, 26 and 35%; Cu, 2.6 and 5%; and Se, 1.5 and 5%, respectively. Injection of ZnSO4 did not lead to increased Zn secretion in GY rats, and only 1.1% of the dose was recovered in controls. Thus, the hepatic handling of different endogenous and exogenously administered trace metals is affected to a variable extent in the GY rat. For a number of metals (e.g. Fe, Mo), this may be related to the reduced bile flow; for others (e.g. Zn, Mn, Cu), other regulatory factors appear to be responsible.

Animals↗
PubMed1989

Studies on electron transfer from methanol dehydrogenase to cytochrome cL, both purified from Hyphomicrobium X.

Ferricytochrome cL isolated from Hyphomicrobium X is an electron acceptor in assays for homologous methanol dehydrogenase (MDH), albeit a poor one compared with artificial dyes. The intermediates of MDH seen during the reaction are identical with those observed with Wurster's Blue as electron acceptor, indicating that the reaction cycles are similar. The assay showed a pH optimum of approx. 7.0 and scarcely any stimulation by NH4Cl, this being in contrast with assays with artificial dyes, where strong activation by NH4Cl and much higher pH optima have been reported. From the results obtained with stopped-flow as well as steady-state kinetics, combined with the isotope effects found for C2H3OH, it appeared that the dissimilarities between the electron acceptors can be explained from different rate-limiting steps in the reaction cycles. Ferricytochrome cL is an excellent oxidant of the reduced MDH forms at pH 7.0, but the substrate oxidation step is very slow and the activation by NH4Cl is very poor at this pH. At pH 9.0 the reverse situation exists: ferricytochrome cL is a poor oxidant of the reduced forms of MDH at this pH. No C2H3OH isotope effect was observed under these conditions, indicating that substrate oxidation is not rate-limiting, so that activation by NH4Cl cannot be found. Since just the opposite holds for assays with artificial dyes, the poor electron-acceptor capability and the different pH optimum of ferricytochrome cL as well as the insignificant activating effect of NH4Cl (all compared with artificial assays) can be explained. Although different views have been reported on the rate-limiting steps in the systems from Methylophilus methylotrophus and Methylobacterium sp. strain AM1, these are most probably incorrect, as rate-limiting electron transfer between ferrocytochrome cL and horse heart ferricytochrome c can occur. Therefore the conclusions derived for the Hyphomicrobium X system might also apply to the systems from other methylotrophic bacteria. Comparison of the assays performed in vitro (at pH 7.0) having ferricytochrome cL and Wurster's Blue as electron acceptor with methanol oxidation by whole cells shows that the former has similarity whereas the latter has not, this being although ferricytochrome cL is a poor electron acceptor in the assay performed in vitro. The reason for this is the absence of a (natural) activator able to activate the (rate-limiting) substrate oxidation step at physiological pH values.

Alcohol Oxidoreductases↗
PubMed1988

Kinetic and spectral studies on the redox forms of methanol dehydrogenase from Hyphomicrobium X.

Several reaction rate constants in the catalytic cycle of methanol dehydrogenase (EC 1.1.99.8) in vitro were determined with stopped-flow spectrophotometry. The studies revealed that the high pH required for adequate activity of the enzyme is related to the strong pH dependency of the oxidation rates of the reduced and semiquinone enzyme forms, MDHred and MDHsem, with the artificial electron acceptor Wurster's blue. The rate-limiting step in the catalytic cycle is associated with the conversion of oxidized enzyme-substrate complex (MDHox.S) into reduced enzyme (MDHred) and product. The effect of activator (ammonium salts) was also confined to this step, but even saturating concentrations were unable to remove the limitation completely. Making use of the large deuterium isotope effect associated with substrate oxidation in the absence of activator, the transient MDHox.C2H3OH complex could be isolated and its slow decomposition into MDHred and formaldehyde could be demonstrated. Further evidence is presented to support the view that the different absorption spectra observed originate from genuine redox forms of methanol dehydrogenase with different redox states of the cofactor PQQ and not from enzyme-electron acceptor complexes. The results confirm and extend our original view on the mechanism of action and contradict the mechanism proposed by others [Parkes, C. & Abeles, R. H. (1984) Biochemistry 23, 6355-6363].

Alcohol Oxidoreductases↗
PubMed1988

The soluble cytochromes c of methanol-grown Hyphomicrobium X. Evidence against the involvement of autoreduction in electron-acceptor functioning of cytochrome cL.

Hyphomicrobium X, grown on methanol with O2 or nitrate as electron acceptor, contains two major soluble cytochromes c. These were isolated in electrophoretically homogeneous form. They are related to cytochromes c already described for other methylotrophic bacteria and designated cytochromes cH and cL (properties indicated in that order) in view of the following characteristics: absorption maxima of the reduced forms (414, 520 and 551 nm and 414, 520 and 550 nm); molar absorption coefficients of the alpha-bands (23,700 M-1.cm-1 and 21,600 M-1.cm-1); maxima of the alpha-bands (no splitting) at 77 K (547.6 nm and 548.5 nm); Mr values of the native proteins (15,000 and 19,500); pI values (7.4 and 7.5, and 4.3); midpoint potentials at pH 7.0 (+292 mV and +270 mV). Both were monomers containing 1 haem c group per protein molecule, the oxidized forms binding cyanide at high pH. Autoreduction also occurred at high pH but at a rate significantly lower than that reported for other ferricytochromes c. On the other hand, the reverse situation applies to the reduction of ferricytochrome cL by reduced methanol dehydrogenase, the reduction occurring instantaneously at pH 7 but much more slowly at pH 9 (ferricytochrome cH was reduced at a 7-fold lower rate, but the rates at pH 7 and 9 were similar). Insignificant reduction was observed with cyclopropanol-inactivated enzyme or with enzyme in the presence of EDTA. In view of the dissimilarities, it is concluded that different mechanisms operate in the autoreduction of ferricytochrome cL and in its reduction by reduced methanol dehydrogenase.

Alcohol Oxidoreductases↗
PubMed1984

Inactivation of quinoprotein alcohol dehydrogenases with cyclopropane-derived suicide substrates. .

Quinoprotein alcohol dehydrogenases can be inactivated by cyclopropanol, cyclopropanone hydrate, and, depending on whether they can oxidize secondary alcohols, also by cyclopropanone ethyl hemiketal. Only enzyme molecules containing the oxidized coenzyme (PQQ), but not those with the coenzyme in the semiquinone form (PQQH), become inactivated with these compounds. The inactivation process proceeds without proton production or electron acceptor consumption and free radical is not observed in the inactivated enzyme. It could be demonstrated that a stoichiometric relationship exists between enzyme inactivation, PQQ converted, PQQ adduct formed, and cyclopropanol added. Thus the dimeric and monomeric enzyme become fully inactivated with two and one molecule of cyclopropanol, respectively, indicating that the dimeric enzyme contains two independently acting catalytic sites. Inactivation of the enzyme by cyclopropanol and cyclopropanone hydrate produces chromatographically different PQQ adducts. Since cyclopropanemethanol, cyclobutanol and cyclohexanol are not suicide substrates, the inactivation presumably proceeds via a ring opening such as proposed for the metal-ion-catalysed degradation of cyclopropane derivatives. The results are in accordance with our view on the reaction mechanism of these enzymes but not with that of others [Mincey et al. (1981) Biochemistry 20, 7502-7509]. The reasons why their model has to be refuted are discussed.

Alcohol Oxidoreductases↗