SLOW ACETYLATION IN MICE IS CAUSED BY A LABILE AND CATALYTICALLY IMPAIRED MUTANT N-ACETYLTRANSFERASE (NAT2-9)

Three N-acetyltransferase genes (NAT) were detected in inbred parenta l and congenic mice, Direct sequencing of NAT2 and liver cytosolic N- acetylation activity determinations with NAT2-specific (p-aminobenzoic acid) and NAT2-selective (2-aminofluorene) substrates have establishe d that the acetylator congenic A.B6 and B6.A mice are genotypically an d phenotypically identical to the parental B6 (''wild-type''; rapid ac etylator) and A (mutant; slow acetylator) mice, respectively, from whi ch they originated, The apparent K-M for p-aminobenzoic acid and therm al inactivation rates determined with liver cytosol from the mutant (A and B6.A) mice were 3-fold and one order of magnitude higher than the corresponding values with liver cytosol from the wild-type (B6 and A, B6) strains, Northern blotting and immunoblotting revealed hepatic NAT 2 mRNA and protein bands of equal size and intensity, regardless of th e NAT2 genotype or phenotype of the animals. Incubation of liver cyto sol from mutant A and B6.A mice at 37 degrees C for 6 hr resulted in v irtual cessation of p-aminobenzoate N-acetylation activity, whereas th e steady-state level of immunoreactive NAT2 remained unchanged, The re sults indicate that the amino acid change (N991) in mutant NAT2 from slow acetylator mice does not hinder the synthesis of hepatic NAT2 pro tein, but, rather, leads to production of a conformationally modified NAT2 molecule that resists degradation by tissue proteases but is labi le and catalytically impaired.