Fluorescein is a marker-dye customary applied to the evaluation of tight-junctional permeability of epithelial cell monolayers. However, the true mechanism for the permeation has not been elucidated. Transepithelial transport of fluorescein in Caco-2 cell monolayers was therefore examined. Fluorescein transport was dependent on pH, and in a vectorical way in the apical-basolateral direction, but it was independent of the tight-junctional permeability of monolayers of these human intestinal cells. The permeation of fluorescein was concentration-dependent and saturable; the Michaelis constant was 7.7 mM and the maximum velocity was 40.3 nmol min(-1) (mg protein)(-1). Benzoic acid competitively inhibited fluorescein transport, suggesting that fluorescein is transported by a monocarboxylic acid transporter (MCT). Antioxidative polyphenolic compounds such as ferulic acid from dietary sources, competitively inhibited the permeation of fluorescein. These compounds probably share a transport carrier with fluorescein. Measurement of the effects of phenolic acids on fluorescein transport across Caco-2 monolayers would be a useful way to evaluate the intestinal absorption or bioavailability of dietary phenolic acids.
Summary:A simple and specific assay was developed for the determination of cysteine. Cysteine was reacted with /7-dimethylaminöcinnamaldehyde in methanol at 60 °C for 2 h in the presence of sulphuric acid äs a catalyst. Absorbance was ineasured at 587 nm; the chromogen was stable at 60 °C for at least 5 h. The colour reactiqn was specific for cysteine, and was negative for other amino acids but positive for cysteamine. The absorbance followed Beer's law in the ränge from 0.825 to 413« 1/1, and the apparent molar lineic absorbance was calculated to be 5000 m 2 · mol" 1 . This procedure is applicable to the determination of cysteine and cystine in amino acid mixtures and in plasma, since cystine can be quantitatively reduced to cysteine with dithiothreitol under alkaline conditions. Recovery of cysteine and cystine from human plasma was 98.5 to 103% and 94.2 to 102%, respectively.
Einfache und spezifische kolorimetrische Bestimmung von Cystein mit p-Dimethylaminocinnamaldehyd
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