string: The demonstration of high catalytic activity toward MPA, a phenolic substrate, is consistent with data from early studies indicating that rat intestinal microsomes are quite active ( 1 nmol/mg/min) in the glucuronidation of substrates with phenolic -OH groups, such as o-aminophenol, 4-nitrophenol, 1-naphthol and 2-naphthol, 4-methylumbelliferone, and morphine (Hietanen et al., 1972; Hietanen and Vainio, 1973; Koster et al., 1985; Vargas and Franklin, 1997).