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31.
Esther Senn Erwin Scharrer Siegfried Wolffram 《Biological trace element research》1992,33(1-3):103-108
The influence of glutathione (1 mmol/L) (GSH) on in vitro mucosal uptake and in vivo absorption of75Se-labeled selenite (10 μmol/L) was investigated in rat jejunum. For comparison, the effect ofl-cysteine (1 mmol/L) on in vivo absorption of75Se-labeled selenite was also studied. In the in vitro, uptake experiments, only the mucosal surface was exposed to the incubation
medium for 3 min. For the in vivo experiments, a luminal perfusion technique was employed. GSH inhibited in vitro mucosal
Se uptake, whereas absorption in vivo was stimulated by GSH.l-Cysteine also stimulated in vivo Se absorption, confirming former in vitro mucosal uptake experiments. Thus, unlikel-cysteine, GSH affected in vitro and in vivo absorption of Se from selenite differently. Enzymatic cleavage of products of
the reaction of selenite with GSH occuring more efficiently under in vivo than in vitro conditions may be a prerequisite for
the stimulatory effect of GSH on Se absorption. This apparently does not apply to the stimulatory effect of cysteine. Since,
GSH occurs in the intestinal lumen under physiological conditions, it may contribute to the high bioavailability of Se from
selenite. 相似文献
32.
Erwin Scharrer Esther Senn Siegfried Wolffram 《Biological trace element research》1992,33(1-3):109-120
The influence of several thiols (conc. 1 mmol/L) on mucosal uptake of75Se from75Se-labeled selenite (conc. 10 μmol/L) across the brush border of rat jejunum and cecum was investigated in vitro using a short-term
uptake technique.l-Cysteine (l-Cys) stimulated75Se uptake in the mid- and distal jejunum and cecum, but not in the proximal jejunum. The effect was maximal in the distal
jejunum.d-Cys was less effective in the jejunum and similarly effective in the cecum.l-Leucine (l-Leu) andl-glutamic acid significantly reduced the stimulatory effect ofl-Cys on Se uptake in the distal jejunum, whereas the respective effect ofd-Cys was not diminished byl-Leu. Cysteamine stimulated mucosal75Se uptake at all intestinal sites tested, whereas the effect of mercaptopyruvate was restricted to the distal jejunum. Thioglycolate
also enhanced75Se uptake in the distal jejunum. The stimulatory effects ofl-Cys, mercaptopyruvate, and thiologlycolate were Na+-dependent, whereas the effect of cysteamine also occurred in the absence of Na+. Mercaptosuccinate,d-penicillamine, ergothioneine, and thiosulfate did not enhance mucosa75Se uptake. It is concluded from these findings that the reaction of some thiols with selenite results in Se compounds that
are rapidly absorbed by the intestinal epithelium through various Na+-dependent and Na+-independent, mechanisms. The high bioavailability of Se from selenite found by others might thus be the result of the presence
of thiols in the gastrointestinal tract. 相似文献
33.
Vitellin and vitellogenin labelled in vitro with 125I and in vivo with 3H were incorporated into yolk by locust oöcytes incubated in an in vitro system. This incorporation was specific and linear with the duration of incubation. Uptake of vitellin by oöcytes was 3–4 times higher than 125I-bovine serum albumin in 2.1-mm oöcytes and 20 times higher than 125I-bovine serum albumin in 4.0-mm long oöcytes. The uptake of the albumin was enhanced by the presence of vitellin in the incubation medium. 3H-labelled yolk protein was incorporated at higher rates than that labelled with 125I. The addition of the juvenile hormone analogue ZR 515, caused the incorporation rates of vitellogenin to be increased. The amount of vitellin or vitellogenin taken up by the oöcytes increased with their length, and the rate of incorporation per unit surface area was highest in 3–4-mm long oöcytes. These results corroborate previously reported in vivo patterns of incorporation rates of developing oöcytes. 相似文献
34.
Summary The Ca++-mediated increase in K+-permeability of intact red blood cells (Gardos effect) was initiated by exposing cells to known concentrations of Ca++ (using EGTA buffers) in the presence of the ionophore A23187. The potency of quinine, an inhibitor of the response, was found to depend on the external K+ concentration. In K+-free solutions the concentration of quinine to achieve 50% inhibition (K
50) was 5 m, but at 5mm K+ the required concentration was increased 20-fold to 100 m. An increase in internal Na+ had the opposite effect, allowing a high potency of quinine despite the presence of external K+. Alterations in the internal K+ level, on the other hand, were without effect on theK
50, suggesting that the membrane potential is not a factor. This conclusion is supported by the lack of effect on quinine inhibition of substitution of Cl– by NO
3
–
, a considerably more permeant anion. The data are consistent with the hypothesis that quinine inhibits by competitively displacing K+ from an external binding site, the reported K+-activation site for the Ca++-mediated K+-permeability. 相似文献
35.
The monoterpenes of the fruits of Pittosporum resiniferum and P. undulatum have been identified. The essential oil of P. resiniferum contai 相似文献
36.
John Roboz Robert Suzuki Esther Rose 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1980,181(2):195-205
N-Phosphonoacetyl-l-aspartic acid (PALA), a potent inhibitor of aspartic acid transcarbamylase, is now undergoing Phase I clinical trials. Initial experiments revealed that PALA is not metabolized to phosphonoacetic acid (PAA) in humans. Thus PALA may be quantified in serum after in vitro conversion to PAA. Serum is deproteinized with perchloric acid, lipid extracted with methylene chloride, hydrolyzed with 8 N hydrochloric acid at 100° for 3 h, and evaporated to dryness with nitrogen. The residue is silylated, and PAA is quantified by monitoring the ions of the protonated molecular ions of trimethylsilyl derivatives of PAA and phosphonopropionic acid (internal standard) obtained in chemical ionization with methane. Limit of detection is 0.5 μM (150 ng/ml) PALA using 1 ml serum. PALA was given by continuous infusion to cancer patients at various doses. Maximum levels of PALA (50–500 μM range) were obtained at the end of infusion, followed in most cases by biexponential decay. Persistent residual PALA levels (5 μM for 48 h after infusion) correlated with increased toxicity. 相似文献
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