Comparative detoxication. 5. Conjugation of aromatic acids in reptiles: formation of ornithuric acid, hippuric acid and glucuronides

A new perspective on the importance of glycine conjugation in the metabolism of aromatic acids

A number of endogenous and xenobiotic organic acids are conjugated to glycine, in animals ranging from mosquitoes to humans. Glycine conjugation has generally been assumed to be a detoxification mechanism, increasing the water solubility of organic acids in order to facilitate urinary excretion. However, the recently proposed glycine deportation hypothesis states that the role of the amino acid conjugations, including glycine conjugation, is to regulate systemic levels of amino acids that are also utilized as neurotransmitters in the central nervous systems of animals. This hypothesis is based on the observation that, compared to glucuronidation, glycine conjugation does not significantly increase the water solubility of aromatic acids. In this review it will be argued that the major role of glycine conjugation is to dispose of the end products of phenylpropionate metabolism. Furthermore, glucuronidation, which occurs in the endoplasmic reticulum, would not be ideal for the detoxification of free benzoate, which has been shown to accumulate in the mitochondrial matrix. Glycine conjugation, however, prevents accumulation of benzoic acid in the mitochondrial matrix by forming hippurate, a less lipophilic conjugate that can be more readily transported out of the mitochondria. Finally, it will be explained that the glycine conjugation of benzoate, a commonly used preservative, exacerbates the dietary deficiency of glycine in humans. Because the resulting shortage of glycine can negatively influence brain neurochemistry and the synthesis of collagen, nucleic acids, porphyrins, and other important metabolites, the risks of using benzoate as a preservative should not be underestimated.

Evolutionary trend in the metabolic oxidation of chlorpromazine

The metabolism of the widely used antipsychotic drug chlorpromazine has been investigated in man, five mammalian and four non-mammalian species, and 7-hydroxylation was found to be the major metabolic route. All of the previously reported major oxidized metabolites were excreted by man, rhesus monkey and capuchin monkey, while sulfoxidation was extensive in rat and N-demethylation in rabbit. Lizard and tortoise did not form sulfoxides, and tortoise was unable to N-demethylate the drug. Toad, pigeon and cat had limited capability for these two metabolic pathways.

Water-soluble metabolites of p-nitrophenol and 1-naphthyl N-methylcarbamate in flies and grass grubs. Formation of glucose phosphate and phosphate conjugates

Metabolites isolated from houseflies dosed with 1-napththol or p-nitrophenol were identified as the phosphate and glucose phosphate conjugates of these phenols by titrations, hydrolysis, ionophoresis, i.r. spectra and mixed melting point. [(3)H]Carbaryl (1-naphthyl N-methylcarbamate) was metabolized by houseflies, blowflies and grass grubs to water-soluble metabolites which had chromatographic and ionophoretic behaviour similar to those of the conjugates of 1-naphthol with glucose, sulphate, phosphate and glucose 6-phosphate.

Detoxications in peripatus. Sulphate, phosphate and histidine conjugations

Phenols were detoxified in the Onycophoran Peripatoides novaezealandiae by conjugation with sulphuric acid and phosphoric acid, but no evidence for a glycoside detoxication could be found. [(14)C]Benzoic acid was metabolized in 24h to N(2)-benzoyl-l-histidine, which was identified by electrophoresis, chromatography and dilution analysis. Similar conjugates were formed with p-aminobenzoic acid and p-nitrobenzoic acid. In longer-duration experiments further unidentified metabolites were formed, two of which appeared to result from the further metabolism of the histidine conjugate.

The fate of benzoic acid in various species

1. The urinary excretion of orally administered [(14)C]benzoic acid in man and 20 other species of animal was examined. 2. At a dose of 50mg/kg, benzoic acid was excreted by the rodents (rat, mouse, guinea pig, golden hamster, steppe lemming and gerbil), the rabbit, the cat and the capuchin monkey almost entirely as hippuric acid (95-100% of 24h excretion). 3. In man at a dose of 1mg/kg and the rhesus monkey at 20mg/kg benzoic acid was excreted entirely as hippuric acid. 4. At 50mg/kg benzoic acid was excreted as hippuric acid to the extent of about 80% of the 24h excretion in the squirrel monkey, pig, dog, ferret, hedgehog and pigeon, the other 20% being found as benzoyl glucuronide and benzoic acid, the latter possibly arising by decomposition of the former. 5. On increasing the dose of benzoic acid to 200mg/kg in the ferret, the proportion of benzoyl glucuronide excreted increased and that of hippuric acid decreased. This did not occur in the rabbit, which excreted 200mg/kg almost entirely as hippuric acid. It appears that the hedgehog and ferret are like the dog in respect to their metabolism of benzoic acid. 6. The Indian fruit bat produced only traces of hippuric acid and possibly has a defect in the glycine conjugation of benzoic acid. The main metabolite in this animal (dose 50mg/kg) was benzoyl glucuronide. 7. The chicken, side-necked turtle and gecko converted benzoic acid mainly into ornithuric acid, but all three species also excreted smaller amounts of hippuric acid.

The metabolism of hexachlorocyclohexanes and pentachlorocyclohexenes in flies and grass grubs

1. gamma-Hexachlorocyclohexane, gamma-pentachlorocyclohexene and delta-pentachlorocyclohexene were converted by houseflies and grass grubs into metabolites that had chromatographic properties identical with those of S-2,4-dichlorophenylglutathione. 2. The metabolism of gamma-hexachlorocyclohexane and the pentachlorocyclohexene isomers was negligible in newly emerged blowflies, but increased over the next 10 days. 3. The metabolism of both gamma-hexachlorocyclohexane and the pentachlorocyclohexene isomers was inhibited by simultaneous dosage with tetrabromophenolphthalein ethyl ester or Bromophenol Blue in both grass grubs and flies, but only the metabolism of pentachlorocyclohexenes in blowflies was stopped by simultaneous dosage with bis-(N-dimethylaminophenyl)methane. NN-Di-n-butyl-p-chlorobenzenesulphonamide had no effect on the metabolism of pentachlorocyclohexenes by blowflies. 4. The use of these inhibitors and colorimetric assays leads to the conclusion that a pentachlorocyclohexene is not a major intermediary metabolite of gamma-hexachlorocyclohexane in these insects.

The conjugation of phenols with phosphate in grass grubs and flies

1. Houseflies, blowflies and New Zealand grass grubs were dosed with 1-naphthol, 2-naphthol or p-nitrophenol. 2. The corresponding monoaryl phosphates were identified in extracts of insects or excreta along with the beta-glucosides and ethereal sulphates of the phenols. 3. No diaryl phosphates or glucosiduronates were detected but an unidentified metabolite of [(14)C]1-naphthol was present in extracts of flies dosed with [(14)C]1-naphthol.

Comparative detoxication. The detoxication of aromatic acids by invertebrates: detection of agmatine conjugates in scorpions

1. Benzoic acid and p-nitrobenzoic acid were converted in the scorpion Palamnaeus into N(1)-benzoylagmatine and N(1)-p-nitrobenzoylagmatine. 2. Some benzoyl- and p-nitrobenzoyl-arginine were also formed and these compounds were probable precursors of the agmatine derivatives. 3. p-Nitrobenzoyl- and p-aminobenzoyl-arginine were formed in millipedes dosed with the aromatic acids. 4. Woodlice excreted p-amino- and p-nitro-benzoic acid as their glycine conjugates and no conjugation could be found in the crab Gecarcinus and a freshwater crayfish Astacus.