Saccharides as efficacious solubilisers for highly lipophilic compounds in aqueous media

The bioavailability of lipophilic substrates is critical for biotransformations with isolated enzymes as well as with whole cells. With the example of a series of lipophilic ketones the suitability of saccharides as potent solubilisers for highly lipophilic substrates was demonstrated. Best results were obtained for d-glucose, which increased substrate solubility up to 50 times. In whole-cell biocatalysis the sugar acts both as solubiliser and as carbon source for which reason this procedure does not impair cell physiology and is unique in being environmentally benign. The capability of saccharides to solubilise lipophilic compounds in aqueous media sources from their ability to form hydrophilic and lipophilic domains at hydrophobic interfaces, thus forming cyclodextrin-like structures around the lipophilic substrate.

Biliary excretion of phenolphthalein sulfate in rats

Glucuronide and glutathione conjugates have been reported to be substrates of multidrug resistance protein 2 (Mrp2), whereas sulfates of nonbile acid organic anions have never been reported as substrates of Mrp2. To further examine the substrate specificity of Mrp2, we examined the effects of bile acid sulfates on the biliary excretion of phenolphthalein sulfate in rats. The biliary excretion of phenolphthalein sulfate was markedly delayed in Eisai hyperbilirubinemic rats, an Mrp2-deficient strain, and was markedly inhibited by taurolithocholate-3-sulfate. The biliary excretion of leukotriene C(4) metabolites and sulfobromophthalein was inhibited by phenolphthalein sulfate infusion to some extent. These findings suggest that phenolphthalein sulfate is a unique sulfated nonbile acid organic anion which is a substrate of Mrp2.

Phenolphthalein metabolite inhibits catechol-O-methyltransferase-mediated metabolism of catechol estrogens: a possible mechanism for carcinogenicity

Phenolphthalein (PT), used in over-the-counter laxatives, has recently been identified as a multisite carcinogen in rodents, but the molecular species responsible for the carcinogenicity is not known. A catechol metabolite of PT, hydroxyphenolphthalein (PT-CAT), was recently identified and may be the molecular species responsible for at least part of the toxicity/carcinogenicity of PT. We hypothesize that PT-CAT inhibits the enzyme catechol-O-methyltransferase (COMT) and therefore potentiates genotoxicity by either PT-CAT itself or the endogenous catechol estrogens (CEs) in susceptible tissues. The present studies were conducted to determine the effects of PT treatment and PT-CAT itself on the COMT-mediated metabolism of 4- and 2-hydroxyestradiol both in vitro and in vivo. Female mice were treated with PT (50 mg/kg/d) for 21 days and then euthanized. PT-CAT concentration in urine reached plateau levels by 7 days of exposure. An O-methylated metabolite of PT-CAT was detected in feces. In vitro experiments demonstrated that PT treatment resulted in an increase in free CEs, which are normally cleared by COMT and a concurrent decrease in the capacity of hepatic catechol clearance by COMT. In vitro, PT-CAT was a substrate of COMT, with kinetic properties within the range measured with endogenous substrates. PT-CAT was an extremely potent mixed-type inhibitor of the O-methylation of the catechol estrogens, with 90-300 nM IC50s. The above data, when taken together, suggest that chronic administration of PT may enhance metabolic redox cycling of both PT-CAT and the catechol estrogens and this, in turn, may contribute to PT-induced tumorigenesis.