Taurine conjugation in metabolism of 3-phenoxybenzoic acid and the pyrethroid insecticide cypermethrin in mouse

Pyrethroid Insecticide Cypermethrin Accelerates Pubertal Onset in Male Mice via Disrupting Hypothalamic-Pituitary-Gonadal Axis

Pyrethroids, a class of insecticides that are widely used worldwide, have been identified as endocrine-disrupting chemicals (EDCs). Our recent epidemiological study reported on an association of increased pyrethroids exposure with elevated gonadotropins levels and earlier pubertal development in Chinese boys. In this study, we further investigated the effects of cypermethrin (CP), one of the most ubiquitous pyrethroid insecticides, on hypothalamic-pituitary-gonadal (HPG) axis and pubertal onset in male animal models. Early postnatal exposure to CP at environmentally relevant doses (0.5, 5, and 50 μg/kg CP) significantly accelerated the age of puberty onset in male mice. Administration of CP induced a dose-dependent increase in serum levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone in male mice. CP did not affect gonadotropin-releasing hormone (GnRH) gene expression in the hypothalamus, but CP at higher concentrations stimulated GnRH pulse frequency. CP could induce the secretion of LH and FSH, as well as the expression of gonadotropin subunit genes [chorionic gonadotropin α (CGα), LHβ, and FSHβ] in pituitary gonadotropes. CP stimulated testosterone production and the expression of steroidogenesis-related genes [steroidogenic acute regulatory (StAR) and Cytochrome p 450, family 11, subfamily A, polypeptide 1 (CYP11A1)] in testicular Leydig cells. The interference with hypothalamic sodium channels as well as calcium channels in pituitary gonadotropes and testicular Leydig cells was responsible for CP-induced HPG axis maturation. Our findings established in animal models provide further evidence for the biological plausibility of pyrethroid exposure as a potentially environmental contributor to earlier puberty in males.

The nonenzymatic reactivity of the acyl-linked metabolites of mefenamic acid toward amino and thiol functional group bionucleophiles

Mefenamic acid (MFA), a carboxylic acid-containing nonsteroidal anti-inflammatory drug, is metabolized into the chemically-reactive MFA-1-O-acyl-glucuronide (MFA-1-O-G), MFA-acyl-adenylate (MFA-AMP), and the MFA-S-acyl-coenzyme A (MFA-CoA), all of which are electrophilic and capable of acylating nucleophilic sites on biomolecules. In this study, we investigate the nonenzymatic ability of each MFA acyl-linked metabolite to transacylate amino and thiol functional groups on the acceptor biomolecules Gly, Tau, l-glutathione (GSH), and N-acetylcysteine (NAC). In vitro incubations with each of the MFA acyl-linked metabolites (1 μM) in buffer under physiologic conditions with Gly, Tau, GSH, or NAC (10 mM) revealed that MFA-CoA was 11.5- and 19.5-fold more reactive than MFA-AMP toward the acylation of cysteine-sulfhydryl groups of GSH and NAC, respectively. However, MFA-AMP was more reactive toward both Gly and Tau, 17.5-fold more reactive toward the N-acyl-amidation of taurine than its corresponding CoA thioester, while MFA-CoA displayed little reactivity toward glycine. Additionally, mefenamic acid-S-acyl-glutathione (MFA-GSH) was 5.6- and 108-fold more reactive toward NAC than MFA-CoA and MFA-AMP, respectively. In comparison with MFA-AMP and MFA-CoA, MFA-1-O-G was not significantly reactive toward all four bionucleophiles. MFA-AMP, MFA-CoA, MFA-1-O-G, MFA-GSH, and mefenamic acid-taurine were also detected in rat in vitro hepatocyte MFA (100 μM) incubations, while mefenamic acid-glycine was not. These results demonstrate that MFA-AMP selectively reacts with the amino functional groups of glycine and lysine nonenzymatically, MFA-CoA selectively reacts nonenzymatically with the thiol functional groups of GSH and NAC, and MFA-GSH reacts with the thiol functional group of GSH nonenzymatically, all of which may potentially elicit an idiosyncratic toxicity in vivo.

The metabolism of the xenobiotic triacylglycerols, rac-1- and sn-2- (3-phenoxybenzoyl)-dipalmitoylglycerol, following intravenous administration to the rat

The metabolism of 3-phenoxybenzoic acid (3PBA) in the form of triacylglycerol conjugates was compared with that of non-esterified 3PBA. Three radiolabeled triacylglycerols (rac-1-(3-phenoxy-[ring-14C]-benzoyl)-2,3-dipalmitoylglycerol (1(3PBA)DPG), sn-2-(3-phenoxy-[ring-14C]benzoyl)-1,3-dipalmitoylglycerol (2(3PBA)DPG) and the "natural" tri-[1-14C]oleoylglycerol) were incorporated into rat VLDL. Nonesterified 3PBA was prepared in rat serum albumin solution. Each preparation was administered i.v. to rats and serial blood samples were taken during the subsequent 6 hr. Urine and faeces were collected and tissue residues determined at 6 hr and 48 hr after administration. Biphasic elimination of 3PBA was observed with half-lives of 18 min and 2 hr. The triacylglycerols showed a rapid first phase and a longer second phase half-life: trioleoylglycerol 26 hr, 1(3PBA)DPG 7.6 hr and 2(3PBA)DPG 17.3 hr. The majority (63-76%) of 3PBA (whether esterified or not) was eliminated within 24 hr in urine, which contained similar profiles of metabolites. The triacylglycerols gave rise to higher tissue residues than did non-esterified 3PBA, particularly in adipose tissue which alone was not significantly depleted of radioactivity between 6 and 48 hr. The results accord with the rapid association of the VLDL-(3PBA)DPG complexes with lipoprotein lipase of the capillary epithelium, followed by hydrolysis to 3PBA, metabolism and elimination but with a proportion being redistributed into adipose tissue, re-esterified and then eliminated relatively slowly.

Participation of a medium chain acyl-CoA synthetase in glycine conjugation of the benzoic acid derivatives with the electron-donating groups

Glycine conjugation of a series of benzoic acid derivatives was investigated in bovine liver mitochondria. Benzoic acids with chlorine, methyl, methoxy or ethoxy substituents in the para-or meta-positions of the benzene ring showed a high degree of glycine conjugation. In contrast, the acids with cyano, nitro, amino, or acetylamino groups were conjugated to a small extent with glycine. A medium chain acyl-CoA synthetase that activates carboxylic acids was purified from bovine liver mitochondria. The purified medium chain acyl-CoA synthetase accepted not only medium chain fatty acids but also aromatic and arylacetic acids as substrates. There was a good correlation between the activity of the purified medium chain acyl-CoA synthetase and glycine conjugation of ten benzoic acids with electron-donating substituents. These findings indicate that the purified medium chain acyl-CoA synthetase is a major enzyme for glycine conjugation of benzoic acids with electron-donating groups in bovine live mitochondria.

Species variations in the renal and hepatic conjugation of 3-phenoxybenzoic acid with glycine

1. 3-Phenoxy[14C]benzoyl-CoA has been chemically synthesized, purified and characterized by field-desorption mass spectrometry. Biological activity of the purified thioester was greater than 92%. 2. The two enzymic steps involved in the conjugation of 3-phenoxybenzoic acid (3PBA) with glycine have been investigated in hepatic and renal tissues from various mammalian species. 3. A 10- to 300-fold excess of acyl-CoA: glycine N-acyltransferase activity as compared with acyl-CoA synthetase activity was found in most tissue preparations, while the rate of the activating step matched that of the overall process. This suggests that formation of the acyl-CoA thioester (3PBA-CoA) is the rate-limiting step in the conjugation of 3PBA with glycine. 4. In most of the species tested, renal activities were higher than those of corresponding liver preparations. 5. The gerbil and ferret, which excrete 3-phenoxybenzoylglycine as the principal urinary metabolite of 3PBA, gave the highest 3PBA-CoA synthetase and glycine N-acyltransferase activities in vitro. By contrast, the hamster, which excretes only small amounts of the glycine conjugate of 3PBA, had the lowest enzymic activities in vitro. 6. In the mouse and rat there were differences between the patterns of metabolism found in vivo and in vitro, and possible reasons for this are discussed.

The metabolism of 3-phenoxybenzoic acid and its glucoside conjugate in rats

1. 3-Phenoxy[14C]benzoic acid administered orally to rats (0.76 to > 100 mg/kg) is extensively metabolized and rapidly eliminated mostly via the urine. 2. The major metabolic pathway involves 4'-hydroxylation followed by conjugation of the resulting phenol with sulphate. Only minor amounts of amino acid and glucuronic acid conjugation were observed. 3. 3-Phenoxy[14C]benzoyl glucoside, derived from the metabolism of the acid by corn leaves, was also rapidly absorbed by rats and eliminated as a mixture of metabolites very similar to that derived from 3-phenoxybenzoic acid. It was concluded that the glucoside conjugate was rapidly hydrolysed to the free acid in vivo.

Xenobiotic triglyceride formation

1. When 3-phenoxy[14C]benzoic acid was dosed orally (0.76 mg/kg) to rats, 1--3% of the administered radioactivity was found in the skin four days after dosing. Approximately 90% of this residue was unchanged 3-phenoxybenzoate, 10% was a neutral compound. 2. The residue in the skins of rats dosed with 193 mg of 3-phenoxybenzoic acid over 7 days (c. 100 mg/kg per day) contained 40% 3-phenoxybenzoate and 60% neutral metabolites. 3. The components were separated and purified by gel permeation, absorption and t.l.c. and analysed by mass spectrometry. 4. 2- and 3-(3-Phenoxybenzoyl)dipalmitins were major components of the neutral metabolite.