Charles M. King

Role of acetyltransferases in the metabolism and carcinogenicity of aromatic amines

The genotoxicity of N-substituted aryl compounds is dependent on their conversion to reactive metabolites, frequently through the production of reactive N-acetoxyarylamines. This activation is accomplished by acetyltransferases that are widely distributed. In the rat, the production of N-acetoxyarylamines has been most clearly related to the induction of tumors in the mammary gland, but this pathway also appears to be an important factor in the production of tumors in the liver, Zymbal gland and gastrointestinal tract. Expression of rat acetyltransferases responsible for acetylation of the nitrogen and the oxygen of arylamine derivatives (i.e., acetyltransferases 1 and 2) in bacterial cells has now permitted experiments which demonstrate that these enzymes exhibit good affinities for and N-acetylation of the endogenous arylalkylamines derived from tryptophan, i.e., tryptamine, 5-hydroxytryptamine (serotonin) and 5-methoxytryptamine, the immediate metabolic precursor of melatonin. Evidence that these reactions are likely to reflect real biological potentials is bolstered by histological localization of acetyltransferase mRNAs with synthetic antisense oligodeoxynucleotide probes. The results of these studies in rat indicate that the expression of acetyltransferase in tissues of the central nervous, gastrointestinal, urinary and reproductive systems is highly regulated, as it is in other organs commonly associated with aromatic amine carcinogenicity. Similar experimental approaches have been successful with human liver, mammary gland, kidney and bladder preparations. These observations give evidence that genotoxic N-acetoxyarylamines are produced by acetyltransferases that can metabolize, and possibly modulate, the hormonal and neurotransmitter effects of endogenous arylalkylamines. These relationships may help explain the occasional induction of tumors in organs not usually considered as targets of aromatic amines, as well as raise the possibility that the production of N-oxidized endogenous substrates may represent a mechanism for tumor induction in the absence of exogenous carcinogens.

[35] N-acetyltransferase and arylhydroxamic acid acyltransferase

Publisher Summary This chapter presents procedures for the preparation of N -acetyltransferase and arylhydroxamic. N -acetyltransferase can be radioassayed using CoASAc as the acetyl donor. This method determines the extent to which the labeled acetyl group of CoASAc is transferred to the primary arylamine acceptor substrate to form an arylacetamide. N -Acetyltransferase can also be asssayed by calorimetric method using CoASAc as the acetyl donor. Acetylation is determined in this procedure from the decrease in the concentration of a primary aromatic amine in the incubation mixture; this decrease results from the transfer of the acetyl group of CoASAc to the acceptor amine. Arylhydroxamic acid N,O-acyltransferase assay determines the extent to which ring-labeled arylhydroxamic acids are incorporated into nucleic acid adducts as a consequence of the production of reactive N -acyloxyarylamines by N,O -acyl transfer. In enzyme purification, instability can be a major problem encountered in the purification of acyltransferases from the tissues of both the rabbit and rat, presumably caused by the oxidation of essential sulfhydryl groups. It is found useful to employ a chelating agent (pyrophosphate) as a buffer and a sulfhydryl protective agent (dithiothreitol) during purification.

METABOLISM OF ARYLAMINES: EVIDENCE FOR COMMON GENETIC CONTROL OF ARYLHYDROXAMIC ACID ACYLTRANSFERASE (AHAT) AND GENETICALLY POLYMORPHIC N-ACETYLTRANSFERASE (NAT) OF RABBIT LIVER1

Publisher Summary Rabbit and human liver N-acetyltransferase acetylate certain carcinogenic arylamines, such as aminofluorene (AF) and benzidine, and drug substrates, such as isoniazid and sulfamethazine (SMZ). In these species, there are large differences in the rates of acetylation of these compounds and individuals can be classified as either rapid or slow acetylators. Such compounds are termed polymorphic substrates. Genetic differences in the rate of N-acetylation are associated with individual susceptibility to toxicity from chronic administration of drugs such as isoniazid. N-Acetylation may be important in the metabolic activation of certain arylamines and may be regarded as an initial step in this pathway. N-Hydroxylation of the corresponding amide occurs in many species in vitro and in vivo. Enzyme activity has been found in several rat tissues that are susceptible to the induction of tumors by 2-acetylaminofluorene (AAF). Many similarities between arylhydroxamic acid acyltransferase (AHAT) and N-acetyltransferase (NAT) exist with respect to biochemical parameters, isolation and purification, tissue distribution, and species variation. This chapter presents evidence for common genetic control of these two enzymes in rabbit liver and for the possibility of their identity.