ARYLAMINE N-ACETYLTRANSFERASE ACTIVITY OF THE HUMAN PLACENTA

The fetus is exposed to almost all of the substances found in the mate rnal circulation whether nutrients or foreign chemicals (''xenobiotics ''). The main route of exposure is the placenta. The placenta is metab olically active toward xenobiotics and the nature of the compounds rea ching the fetal circulation will, in part, depend on placental biotran sformation reactions. Arylamine N-acetyltransferase (NAT) catalyzes th e acetyl CoA-dependent N-acetylation of primary arylamine and hydrazin e substrates such as sulfamethazine, isoniazid, p-aminobenzoic acid as well as arylamine carcinogens such as 2-aminofluorene and benzidine. NAT activity is multigenically determined and can be attributed to two independently expressed proteins: NAT1 and NAT2. The acetylation capa city of the human placenta has not been investigated extensively. in t he current study we identified and characterized the NAT activity of h uman term placenta. The kinetic data show that the activity of NAT in human placenta predominantly reflects the NAT1 enzyme. The apparent af finity (19.1 +/- 0.97 mu M; mean +/- S.E.M., n = 22) and maximal veloc ity (3.84 +/- 0.32 nmol/min/mg; mean +/- S.E.M., n = 22) of p-aminoben zoic acid N-acetylation are similar to those measured in other tissues such as liver, blood lymphocytes, neutrophils and monocytes. In addit ion, there is evidence of NAT2 activity in some of the placental sampl es assayed, although the contribution of a small amount of NAT2 activi ty to the overall acetylation capacity of the placenta is likely to be small.