Expression of arylamine N-acetyltransferases in pre-term placentas and in human pre-implantation embryos.

Arylamine N -acetyltransferases (NATs) catalyse the acetylation from acetyl-CoA of arylamines and hydrazines. There are two human isoenzymes which show polymorphism, and both enzymes are involved in the activation and detoxification of environmental carcinogens and teratogens. The two human isoenzymes NAT1 and NAT2 show different tissue distribution, with human NAT2 being found in liver and intestine whilst human NAT1 is expressed in many tissues including erythrocytes, bladder, lymphocytes and neural tissue, as well as liver and intestine. It has been proposed that NAT1 has an endogenous role in the acetylation of the folate catabolite p -aminobenzoyl-L-glutamate (pABGlu) to produce the major urinary product, N -acetyl-pABGlu. The murine homologue of human NAT1 is known to be concentrated in the neural tube during development. We show here that human NAT1 but not human NAT2 is expressed in pre-implantation embryos at the blastocyst stage and show that NAT1 is also expressed in early human placenta at the earliest available stage, 5.5 weeks. We demonstrate that there is inter-individual variation in NAT1 expression. In view of the role of folate in protecting against neural tube defects, we propose that NAT1 is a candidate risk factor for susceptibility to neural tube defects.

Placental expression of arylamine N-acetyltransferases: evidence for linkage disequilibrium between NAT1*10 and NAT2*4 alleles of the two human arylamine N-acetyltransferase loci NAT1 and NAT2.

The study of placental xenobiotic metabolism is important for the determination of foetal exposure to environmental chemicals as placental metabolism influences the nature of chemicals reaching the foetus from its mother's blood. Arylamine N-acetyltransferases are drug metabolizing enzymes which N-acetylate hydrazines and arylamines, including carcinogenic arylamines and sulphonamide drugs. The two human arylamine N-acetyltransferase isoenzymes, NAT1 and NAT2, are encoded at multi-allelic loci. Here, we have determined N-acetyltransferase (NAT) activity in term placentas from normal, uncomplicated pregnancies. Both NAT1 and NAT2 enzyme activities were detectable. Placental NAT1 activity was at least 1000 fold greater than NAT2 activity. There was a 6 fold inter-placental variation in NAT1 activity. Mean placental NAT1 specific activity was 1.42 nmoles para-aminobenzoic acid N-acetylated.min-1.mg protein-1, which is comparable to NAT1 specific activities which have been measured in adult tissues. The NAT1, but not the NAT2, protein was detectable in placentas by Western blotting. Maternal and foetal NAT genotypes were determined from placenta, using placental blood clots and cord blood respectively, allowing NAT haplotype determination. There appeared to be linkage disequilbrium between NAT1* and NAT2* alleles such that the combination NAT1*10/NAT2*4 was found 3.5 times more frequently than would be expected.

Acetylation of arylamines by the placenta.

The N-acetylation of arylamines and hydrazines used as drugs may alter their pharmacological or toxicological activity. Arylamine N-acetyltransferase (NATs) are involved in drug metabolism, as they catalyse the N-acetylation of arylamine and mono-substituted hydrazine substrates. Placental metabolism regulates the nature of the chemicals which reach the developing fetus. The study of drug metabolism during pregnancy is important in determining the effect on the fetus of drugs administered to the mother and the maternal drug dose required, important if the treatment is to be effective. There are two forms of NAT in humans, NAT1 and NAT2, which are encoded at multi-allelic loci. There is inter-individual variation in both NAT1 and NAT2 activity, which has implications in drug dosage. Using a combination of enzyme activity measurements and Western blotting, this study has characterised the arylamine N-acetylation capabilities of placenta and cord blood. NAT1 activity in placenta and cord blood demonstrated inter-individual variation and the variation was in the range expected for adult NAT1 activity. The genotypes of both NAT1* and NAT2* were determined using DNA prepared using placental blood clots (maternal DNA) and cord blood (fetal DNA). The results indicate that placental NAT activity is an important factor when considering N-acetylation during pregnancy.