Immunomodulation and drug acetylation: influence of the immunomodulator tilorone on hepatic, renal and blood N-acetyltransferase activity and on hepatic cytosolic acetyl coenzyme A content.

The biochemical alteration responsible for immunomodulator enhancement of drug acetylation in vivo was probed ex vivo and in vitro in the rat. Rat liver or kidney cytosol, obtained by differential centrifugation, or whole blood served as the source of N-acetyltransferase (NAT). Addition of tilorone (0.5-8.0 mM) to incubation mixtures containing procainamide (PA, 0.6 mM) and acetyl coenzyme A (AcCoA, 0.42 mM) resulted in the inhibition of N-acetylprocainamide formation, while lower concentrations of tilorone had no effect. Pretreatment of rats with tilorone (50 mg/kg) administered orally 48 hr prior to sacrifice did not alter hepatic apparent Km and Vmax for NAT toward PA compared to control animals. Utilization of an AcCoA regenerating system in the incubation mixtures also resulted in no significant differences in the apparent Michaelis-Menten parameters obtained. Acetylation activity in kidney and whole blood also was not altered by immunomodulator pretreatment. Hepatic cytosolic AcCoA content was reduced significantly 48 hr after tilorone pretreatment (5.10 +/- 2.1 vs 11.97 +/- 2.2 nmol/mg protein) (P less than 0.05). These data indicate that an increase in NAT content or activity is not the biochemical alteration responsible for immunomodulator enhancement of drug acetylation, and that the required cofactor, cytosolic AcCoA, is decreased by immunomodulator pretreatment.