A method for characterization of endogenous ligands to orphan receptors belonging to the steroid hormone receptor superfamily--isolation of progesterone from pregnancy plasma using progesterone receptor ligand-binding domain

Fifty years with bile acids and steroids in health and disease

Cholesterol and its metabolites, e.g., steroid hormones and bile acids, constitute a class of compounds of great biological importance. Their chemistry, biochemistry, and regulation in the body have been intensely studied for more than two centuries. The author has studied aspects of the biochemistry and clinical chemistry of steroids and bile acids for more than 50 years, and this paper, which is an extended version of the Schroepfer Medal Award lecture, reviews and discusses part of this work. Development and application of analytical methods based on chromatography and mass spectrometry (MS) have been a central part of many projects, aiming at detailed characterization and quantification of metabolic profiles of steroids and bile acids under different conditions. In present terminology, much of the work may be termed steroidomics and cholanoidomics. Topics discussed are bile acids in human bile and feces, bile acid production, bacterial dehydroxylation of bile acids and steroids during the enterohepatic circulation, profiles of steroid sulfates in plasma of humans and other primates, development of neutral and ion-exchanging lipophilic derivatives of Sephadex for sample preparation and group separation of steroid and bile acid conjugates, profiles of steroids and bile acids in human urine under different conditions, hydroxylation of bile acids in liver disease, effects of alcohol-induced redox changes on steroid synthesis and metabolism, alcohol-induced changes of bile acid biosynthesis, compartmentation of bile acid synthesis studied with 3H-labeled ethanol, formation and metabolism of sulfated metabolites of progesterone in human pregnancy, abnormal patterns of these in patients with intrahepatic cholestasis of pregnancy corrected by ursodeoxycholic acid, inherited and acquired defects of bile acid biosynthesis and their treatment, conjugation of bile acids and steroids with N-acetylglucosamine, sulfate-glucuronide double conjugates of hydroxycholesterols, extrahepatic 7alpha-hydroxylation and 3-dehydrogenation of hydroxycholesterols, and extrahepatic formation of C27 bile acids. The final part discusses analysis of free and sulfated steroids in brain tissue by capillary liquid chromatography-electrospray MS and suggests a need for reevaluation of the function of steroid sulfates in rat brain.

Fatty acid activation of peroxisome proliferator-activated receptor (PPAR)

Peroxisome proliferators such as clofibric acid, nafenopin, and WY-14,643 have been shown to activate peroxisome proliferator-activated receptor (PPAR), a member of the steroid nuclear receptor superfamily. We have cloned the cDNA from rat that is homologous to that from mouse, which encodes a 97% similar protein. To search for physiologically occurring activators, we established a transcriptional transactivation assay by stably expressing in CHO cells a chimera of rat PPAR and the human glucocorticoid receptor that activates expression of the placental alkaline phosphatase reporter gene under the control of the mouse mammary tumor virus promoter. 150 microM concentrations of arachidonic or linoleic acid but not of dehydroepiandrosterone, cholesterol, or 25-hydroxy-cholesterol, activated the receptor chimera. In addition, saturated fatty acids induced the reporter gene. Shortening the chain length to n = 6 or introduction of an omega-terminal carboxylic group abolished the activation potential of the fatty acid. To test whether a common PPAR binding metabolite might be formed from free fatty acids we tested the effects of differentially beta-oxidizable fatty acids and inhibitors of fatty acid metabolism. The peroxisomal proliferation-inducing, non-beta-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY-14,643, whereas the homologous beta-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, beta-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to beta-oxidation of the acyl-CoA ester.

Potential of receptor-ligand interactions for sample handling in liquid and gas chromatography

The use of receptor proteins for biospecific sample handling in liquid and gas chromatography is described. As a model system the uterine estrogen receptor was chosen for the isolation of 17 beta-estradiol and its synthetic agonist diethylstilbestrol. Biochemical characteristics relevant for the use of receptors in sample handling such as the kinetics of receptor-ligand binding, reproducibility and capacity are examined by means of an estrogen radioreceptor assay. Different techniques for the non-covalent immobilization of the estrogen receptor were investigated. Both protamine-coated glass fibre filters and silica particles which bind the receptor via electrostatic interactions have been used for this purpose. The use of the estrogen receptor in the isolation of 17 beta-estradiol and diethylstilbestrol prior to GC-MS analysis is demonstrated and discussed.

Is the A-ring lactone of brevetoxin PbTx-3 required for sodium channel orphan receptor binding and activity?

Brevetoxin PbTx-3 and non-toxic derivative 4 were investigated for their abilities to bind to the specific brevetoxin receptor site on rat brain synaptosomes and to modulate the normal function of voltage-gated sodium channels as determined by patch clamping of cultured neurons. Compounds 4 and 5 are produced from PbTx-3 by opening of the A-ring lactone to the saturated and unsaturated diols using sodium borohydride in ethanol. Natural PbTx-3 exhibited tighter binding to rat brain synaptosomes by at least 3 orders of magnitude as determined by competitive radioligand binding experiments, and was also more effective at activating voltage-gated channels. Patch clamping revealed the 3 orders of magnitude greater potency of PbTx-3 toxin over 5, although each produced delayed sodium channel opening and a pronounced delay in inactivation. Conformational modeling of the Brevetoxin B backbone indicates that the two molecules are identical except for the region of the A-Ring lactone. Thus, we conclude that the brevetoxin PbTx-3 backbone requires electrophilic functionality in the region of the lactone in PbTx-3, and that opening of the ring in 5 is sufficient to substantially reduce both binding and activity.