ACETYLATION OF P-AMINOBENZOYLGLUTAMATE, A FOLIC-ACID CATABOLITE, BY RECOMBINANT HUMAN ARYLAMINE N-ACETYLTRANSFERASE AND U937 CELLS

N-acetyl-p-aminobenzoylglutamate is a major urinary metabolite of foli c acid, It is formed by acetylation of p-aminobenzoylglutamate followi ng cleavage of the C-9-N-10 bond of folic acid. Using recombinant huma n type 1 (NAT1) and type 2 (NAT2) arylamine N-acetyltransferase, we ha ve shown that p-aminobenzoylglutamate is a specific NAT1 substrate. At an acetyl-CoA concentration of 50 mu M, the K-m for p-aminobenzoylglu tamate (pABG) acetylation by recombinant NAT1 was 130 +/- 13 mu M. For the human pro-monocytic cell-line U937, the apparent K-m was slightly higher (333 +/- 17 mu M). Inhibitor studies supported NAT1-dependent acetylation of pABG by U937 cell cytosols. These studies are the first to identify a potential endogenous substrate for human NAT1 and sugge st that this enzyme may be important in the cellular clearance of pABG .