Platinum catalysts are reported for the direct, low-temperature, oxidative conversion of methane to a methanol derivative at greater than 70 percent one-pass yield based on methane. The catalysts are platinum complexes derived from the bidiazine ligand family that are stable, active, and selective for the oxidation of a carbon-hydrogen bond of methane to produce methyl esters. Mechanistic studies show that platinum(II) is the most active oxidation state of platinum for reaction with methane, and are consistent with reaction proceeding through carbon-hydrogen bond activation of methane to generate a platinum-methyl intermediate that is oxidized to generate the methyl ester product.
Urinary excretion of human liver-type fatty acid-binding protein (hL-FABP
The peroxisome proliferator-activated receptor ␣ (PPAR␣) is a member of the nuclear receptor superfamily and mediates the biological effects of peroxisome proliferators. To determine the physiological role of PPAR␣ in cardiac fatty acid metabolism, we examined the regulation of expression of cardiac fatty acid-metabolizing proteins using PPAR␣-null mice. The capacity for constitutive myocardial -oxidation of the medium and long chain fatty acids, octanoic acid and palmitic acid, was markedly reduced in the PPAR␣-null mice as compared with the wild-type mice, indicating that mitochondrial fatty acid catabolism is impaired in the absence of PPAR␣. In contrast, constitutive -oxidation of the very long chain fatty acid, lignoceric acid, did not differ between the mice, suggesting that the constitutive expression of enzymes involved in peroxisomal -oxidation is independent of PPAR␣ . Indeed, PPAR␣-null mice had normal levels of the peroxisomal -oxidation enzymes except the D-type bifunctional protein. At least seven mitochondrial fatty acid-metabolizing enzymes were expressed at much lower levels in the PPAR␣-null mice, whereas other fatty acid-metabolizing enzymes were present at similar or slightly lower levels in the PPAR␣-null, as compared with wild-type mice. Additionally, lower constitutive mRNA expression levels of fatty acid transporters were found in the PPAR␣-null mice, suggesting a role for PPAR␣ in fatty acid transport and catabolism. Indeed, in fatty acid metabolism experiments in vivo, myocardial uptake of iodophenyl 9-methylpentadecanoic acid and its conversion to 3-methylnonanoic acid were reduced in the PPAR␣-null mice. Interestingly, a decreased ATP concentration after exposure to stress, abnormal cristae of the mitochondria, abnormal caveolae, and fibrosis were observed only in the myocardium of the PPAR␣-null mice. These cardiac abnormalities appeared to proceed in an age-dependent manner. Taken together, the results presented here indicate that PPAR␣ controls constitutive fatty acid oxidation, thus establishing a role for the receptor in cardiac fatty acid homeostasis. Furthermore, altered expression of fatty acid-metabolizing proteins seems to lead to myocardial damage and fibrosis, as inflammation and abnormal cell growth control can cause these conditions.
Intestinal bile acid-binding protein (I-BABP) is a cytosolic protein that binds bile acids (BAs) with a high affinity. In the small intestine, its expression is restricted to the ileum where it is involved in the enterohepatic circulation of BAs. Using the human enterocytelike Caco-2 cell line, we have recently shown that BAs increased I-BABP gene expression. To determine whether this regulation occurs in vivo, the effect of BA depletion or supplementation was studied in mice. A dramatic drop in I-BABP mRNA levels was observed in mice treated with the BA-binding resin cholestyramine, whereas an increase was found in animals fed with taurocholic acid. BAs are physiological ligands for the nuclear farnesoid X receptor (FXR). Both FXR and I-BABP are co-expressed along the small intestine and in Caco-2 cells. To determine the role of FXR in the regulation of I-BABP expression, the promoter of the human I-BABP gene was cloned. In Caco-2 cells, cotransfection of FXR and RXR␣ is required to obtain the full transactivation of the I-BABP promoter by BAs. Deletion and mutation analyses demonstrate that the FXR/RXR␣ heterodimer activates transcription through an inverted repeat bile acid responsive element located in position ؊160/؊148 of the human I-BABP promoter. In conclusion, we show that FXR is a physiological BA sensor that is likely to play an essential role in BA homeostasis through the regulation of genes involved in their enterohepatic circulation. Primary BAs1 are synthetized from cholesterol in the liver where they are conjugated with glycine or taurine prior to secretion into bile (1). In most mammals, bile is stored in the gall bladder. During a meal, BAs are released into the duodenum where they are required for the efficient absorption of dietary fat and lipid-soluble vitamins. In the distal gut, conjugated BAs may undergo bacterial modifications leading to the formation of secondary BAs. In humans, more than 90% of BAs are reabsorbed throughout the intestine and return, via the portal blood, to the liver where they are secreted again into bile. This enterohepatic circulation is essential for the maintenance of BA and cholesterol homeostasis (1).Intestinal absorption of BAs takes place as a function of their chemical form (hydrophobicity index) through three complementary mechanisms: passive nonionic diffusion and facilitated and active protein-mediated transports. After bacterial deconjugation, passive diffusion of protonated BAs occurs in the ileum and colon. Passive absorption of glycine-conjugated BAs has also been recently reported in the jejunum of guinea pig (2). Conjugated dihydroxy-BAs are primarily absorbed at the jejunal level via facilitated transport, whereas taurine and glycine trihydroxy-BA are actively transported in the ileum (3). The relative contribution of jejunal carrier-mediated transport under physiological conditions remains to be determined. By contrast, the active ileal absorption of conjugated BAs has been the subject of extensive research. BA uptake in the ileum is mediated by an...
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