Acetanilide 4-Hydroxylase

Acetanilide 4-hydroxylase (A4H) is a microsomal monooxygenase activity that can be followed by means of the model substrate acetanilide. In the HPLC assay developed for this activity, the conversion of acetanilide to 4-hydroxyacetanilide was followed. 

The enzyme activity was obtained using a microsomal suspension prepared from mouse liver. 

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Pegram et al. (1995) showed that induction of acetanilide-4-hydroxylase by 2,3,7,8-TCDD was age-dependent, as it was significantly greater in old than in young mice. 

Pretreatment with the Type I substrate, ethylmorphine, resulted in 100% mortality in both rats and mice, and aminopyrine pretreatment resulted in 100% and 64% mortality in rats and mice, respectively, exposed to disulfoton (Pawar and Fawade 1978). Nickel chloride, cobalt chloride, or cycloheximide decreased the levels of cytochrome bs, cytochrome c reductase, and total heme in rats (Fawade and Pawar 1983). These electron transport components were further decreased in rats pretreated with these inhibitors and given a single dose of disulfoton. Data from this study suggests an additive effect, since disulfoton also decreases the activities of these components. Evidence of an additive effect between disulfoton and these metabolic inhibitors was suggested by the decrease in ethylmorphine N-demethylase and acetanilide hydroxylase activities when rats were given an inhibitor followed by disulfoton. In another experiment, these inhibitors decreased the activity of delta-aminolevulinic acid synthetase, but this decrease was reversed when disulfoton was administered. 

Figure 4. Hydroxylation of 4-suhstituted acetanilides with microsomal aryl hydroxylases...

In the following years cytochrome P450 was also found as an integral part of the 1 l 3-hydroxylase system in adrenal mitochondria Its involvement in the liver microsomal monooxygenation of codeine, acetanilide and cyclohexane was established using the technique of the photochemical action spectrum (Fig. 2). 

Phenylalanine hydroxylase occurs only in mammalian liver (that is, in the rat, guinea-pig, rabbit, d<, chicken, and human) (see also 259). No activity has been observed in (rat) lung, kidney, brain, or muscle. The system is quite speciOc for L-phenylalanine. Tjrro-sine is not formed from n-phenylalanine, nor are the corresponding p-phenols formed from N-acetyl- or N-chloroacetyl-L-phenylalanine, L-phenylalanine ethyl ester, DL-phenylglycine, phenylserine, phenylpyruvic acid, phenylethylamine, benzoic acid, hippuric acid, cinnamic acid, or mandelic acid (768), or from aniline, acetanilide, tryptophan, kynurenine, anthranilic acid, or phenylacetate (557). This specificity is a distinguishing character of the enzyme, which occurs in the same tissue as the nonspecific aromatic hydroxylase described above. 

Aromatic hydroxylase Rat liver Acetanilide p-Hydroxyacetanilide Posner et at. (1956)... 

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