Affinity alkylation of hamster hepatic arylamine N-acetyltransferases: isolation of a modified cysteine residue

HG Cheon, LW Boteju and PE Hanna

Department of Pharmacology, University of Minnesota, Minneapolis 55455.

N-Acetyltransferases (NATs) play key roles in the detoxification and/or bioactivation of arylamines, arylhydroxylamines, arylhydroxamic acids, and hydrazines in mammalian tissues. In the present study, two hamster hepatic NATs (NAT I and NAT II) were separated, and each was purified greater than 2000-fold by sequential ammonium sulfate fractionation, DEAE anion exchange chromatography, Sephadex G-75 gel filtration chromatography, aminoazobenzene-coupled affinity chromatography, and DEAE anion exchange high performance liquid chromatography. Both NAT I and NAT II were purified to near-homogeneity. The molecular masses of NAT I and NAT II were estimated to be 30.5 kDa and 32.6 kDa, respectively. 2-(Bromoacetylamino)fluorene (Br-AAF) and bromoacetanilide were synthesized and evaluated as affinity labels for NAT I and NAT II. Whereas Br-AAF was a highly selective inactivator of NAT II, bromoacetanilide inactivated both NAT I and NAT II in a similar fashion. Inactivation of NAT II by both Br-AAF and bromoacetanilide, and inactivation of NAT I by bromoacetanilide, followed pseudo-first- order kinetics. Relative rate constants (k(obs)/[I]) for the two compounds indicate that Br-AAF is approximately 25 times more potent than bromoacetanilide as an inactivator of NAT II. Both acetylcoenzyme A (CoASAc) and 2-acetylaminofluorene protected NAT II from inactivation by Br-AAF, and CoASAc provided protection of both NAT I and NAT II activities from inactivation by bromoacetanilide, indicating that the inactivation by both bromoacetanilide and Br-AAF is active site directed. The irreversibility of the inactivation of NATs by Br-AAF and bromoacetanilide was demonstrated by the failure to recover transacetylase activities after gel filtration of enzyme preparations that had been preincubated with Br-AAF or bromoacetanilide. Preincubation of NAT II with CoASAc significantly reduced the incorporation of [14C]Br-AAF into the enzyme, providing further evidence that the labeling is active site directed. In addition, pretreatment of NAT II with N-ethylmaleimide completely prevented the labeling of NAT II with [14C]Br-AAF, which suggests that a cysteine thiol is the target nucleophile of Br-AAF. High performance liquid chromatography analysis of the hydrochloric acid hydrolysate of [14C]Br- AAF-labeled NAT II revealed that 70% of total radioactivity is associated with S-carboxymethyl-L-cysteine, indicating that Br-AAF reacts primarily with a cysteine residue at the active site. These studies provide direct evidence that hamster hepatic NAT II contains an essential cysteine residue at the active site, and they establish the potential utility of Br-AAF for determining amino acid sequences in the active site of hamster hepatic NAT II.

Volume 42, Issue 1, pp. 82-93, 07/01/1992
Copyright © 1992 by American Society for Pharmacology and Experimental Therapeutics