The Conjugation of a - Naphthylacetic Acid in vivo

Aromatic and arylalkyl carboxylic acids commonly undergo metabolic conjugation prior to excretion, the conjugating agent being either a carbohydrate or an amino acid. Among mammalian species the carbohydrate is usually glucuronic acid, but in the case of the amino acids, one or more of glycine, glutamine or taurine may be used, and the particular pattern of amino acid conjugation depends upon the species. Perusal of theliterature suggests that the structure of the acid has an important bearing on the extent to which it is conjugated with either an amino acid or glucuronic acid. Thus, carboxylic acids of relatively simple chemical structure such as phenylacetic acid and its simple derivatives form mainly amino acid conjugates, the nature of which varies with species (James et al., 1972a) whereas other more complex carboxylic acids such as diphenylacetic acid (Miriam et al., 1927), indomethacin (Harman et al., 1964), iopanoic acid (McChesney & Hoppe, 1954) and iophenoxic acid (Wade et al., 1971) are conjugated with glucuronic acid. In order to investigate further the relationship between structure and pattern of conjugation we have extended our original studies on the conjugation of phenylacetic acid (James et al., 1972a) and p-chlorophenylacetic acid (James et al., 19726) to a-naphthylacetic acid and some of the results are presented here. a-[~arbonyl-~~C]Naphthylacetic acid was administered to animals and human volunteers, the routes of administration and doses being stated in Table 1. The urine was collected and analysed for the unchanged drug and its conjugates by paper and thin-layer chromatography, followed by radiochromatogram scanning, and by reverse isotope dilution. The ester glucuronide was not hydrolysed quantitatively by 8-glucuronidase and therefore was assayed by difference before and after mild alkaline hydrolysis. This treatment did not affect amino acid conjugates also present, and a high lability under mild alkaline conditions is a characteristic of ester glucuronides (Williams, 1947). All the species examined excreted 60-99 % of the 14C administered in the urine in 24h, with the exception of a low recovery (16 % of dose) for the bushbaby which was probably due to difficulties in obtaining a complete urine collection. With the exception of this species and the cat it can be seen from Table 1 that the major conjugate of a-naphthylacetic acid formed in seven species including man is the glucuronic acid conjugate. Small amounts of amino acid conjugates were also detected in these species with the exception of the fruit bat in which the only conjugate was a-naphthacetyl glucuronide. Thus, the glycine conjugate was detected in the marmoset, bushbaby, rat and rabbit while the glutamine conjugate was detected only in the marmoset, a New World monkey and the cynomologus monkey (Old World). The glutamine conjugation occurs with arylacetic acids and is believed to be restricted in its species occurrence to man and subhuman primates excluding prosimians (James et al., 1972a). Taurine conjugation has recently been found to be a reaction of certain arylacetic acids (James et al., 1971). In this investigation, man and other primate species formed small amounts of the taurine conjugate while in the cat it was a major metabolite. Two points regarding the fruit bat and cat should be noted. In the fruit bat there is a defect of hippuric acid synthesis (Bababunmi et al., 1973) and it is clear from these findings that this also extends to the conjugation of a-naphthylacetic acid. Secondly, the cat has a well recognized defect in its ability to conjugate various compounds with glucuronic acid (Cape1 et al., 1972) and this probably explains why the conjugation of a-naphthylacetic acid by this species is mainly with amino acids and is unlike the pattern seen in the other species. The question arises as to what factors govern the pattern of conjugation of a compound. Both phenylacetic acid andp-chlorophenylacetic acid are metabolized largely by conjugation with the amino acids glycine, glutamine and taurine, the pattern depending