Stereoselective (S)- and (R)-naproxen glucuronosyl transferases of rat liver

Stereoselective glucuronidation of naproxen, one of the 2-arylpropionic acids that are widely used as anti-inflammatory drugs, was investigated in chromatofocusing fractions of solubilized liver microsomes from 3-methylcholanthrene- (MC) and phenobarbital- (PB) treated rats. On chromatofocusing of solubilized microsomes of PB-treated rats, two naproxen glucuronosyltransferase (GT) fractions were separated. The fraction eluting at pH 8.7 preferentially conjugated (S)-naproxen (S/R ratio = 1.6) and the fraction eluting at pH 7.8 mostly conjugated (R)-naproxen (S/R ratio = 0.7). Chromatofocusing of solubilized microsomes from MC-treated rats also resulted in the separation of two naproxen GT fractions, eluting at pH 9.4 (S/R ratio 0.2) and at pH 8.7 (S/R ratio 0.8). These two fractions coincided with the elution of known MC-inducible GT activities assigned to a GT isozyme variously termed 4-nitrophenol GT or GT-I. Interestingly, kidney microsomes, known to contain a high constitutive expression of GT-I, preferentially glucuronidated (R)-naproxen (S/R ratio 0.2). The S/R ratio of 0.8, observed with the pI 8.7 fraction of MC-treated rat liver, may be explained by the presence of a mixture of naproxen GTs, consisting of (R)-naproxen GT (S/R ratio 0.2) and of (S)-naproxen GT (S/R ratio 1.6). The results suggest that naproxen is conjugated by at least 3 GT isozymes in rat liver; these have been operationally designated (S)-naproxen GTPB (S/R ratio 1.6), (R)-naproxen GTPB (S/R ratio 0.7), and (R)-naproxen GTMC (S/R ratio 0.2). The latter isozyme is probably identical to the previously characterized MC-inducible GT-I. Thus, (S)- and (R)-naproxen represent useful substrates to distinguish different GT isozymes.