Inactivation by acivicin of carbamoyl-phosphate synthetase II of human colon carcinoma

Abstract

The effect of the anti-tumor, anti-glutamine drug acivicin, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid, was determined on the activity of the rate-limiting enzyme of de novo pyrimidine biosynthesis, carbamoyl-phosphate synthetase II (glutamine-hydrolyzing) (EC 6.3.5.5), in human colon carcinoma. The synthetase II activity in human colon carcinoma was elevated 2- to 3-fold over values of the normal colon mucosa, and the substrate kinetic constants were similar for the enzyme in normal and neoplastic colon. The Km for glutamine was 17 microM (colon carcinoma) and 23 microM (normal mucosa), whereas the Km for ATP was 2.1 and 1.7 mM in tumor and mucosa respectively. The synthetase II activity in colon carcinoma was inhibited to a similar extent by UMP, UDP and UTP (36-41%). The three uracil nucleotides were also equally effective in inhibiting the enzyme from normal mucosa (39-46%). Both enzymes were activated by PRPP (63 and 57%) in mucosa and carcinoma respectively. Acivicin in vitro selectively inactivated the glutamine-dependent synthetase II from human colon carcinoma, and it did not affect the ammonia-dependent activity. The acivicin inactivation constant (Kinact) was 100 microM, and the minimum inactivation half-time (T) was 0.7 min. Acivicin most likely exerts its effect against human colon synthetase II by acting as an active site directed affinity analogue of L-glutamine.