Acetylation of p-aminobenzoic acid by human blood

Abstract

Acetylation of p-aminobenzoic acid was studied in human blood cell lysates. The rate of acetylation with acetyl-coenzyme A was 3.4 nmoles per min per 0.5 ml lysate (corresponding to 0.5 ml blood with a 50% hematocrit). In the presence of a coenzyme A generating system, the rate was only 0.1 nmole per min per 0.5 ml lysate. N-Acetyltransferase (acetyl-CoA : arylamine N-acetyltransferase, EC 2.3.1.5) activity exhibited a temperature optimum within the range of 34–37° with a Q 10of 2 between 24–34°. Heating for 3 min at 50° caused 50 per cent inactivation of enzymatic activity. The pH activity profile showed an optimum at pH 6.0–6.5. p-Chloromercuribenzoate was a potent inhibitor causing 50 per cent inhibition at 9 μM. The apparent K m value for p-aminobenzoic acid was 0.4 mM and for acetylcoenzyme A, 0.3 mM. The enzyme activity with p-aminosalicyclic acid was about 50 per cent that obtained with p-aminobenzoic acid. Acetylation of sulfamethazine was either very low or in several individuals undetectable. Isonicotinic acid hydrazide at a concentration 10 times that of p-aminobenzoic acid did not interfere with acetylation of the latter, nor did trimethoprim, another compound with an amine moiety. Folic acid and amethopterin competitively inhibited the acetylation of p-aminobenzoic acid, with respective K i values of 0.01 mM and 0.06 mM. It is concluded that the activity of N-acetyltransferase in human blood may have important clinical implications.