ANTHRACYCLINE ANTIBIOTIC BLOCKADE OF SV40 T-ANTIGEN HELICASE ACTION

We previously showed that anthracycline antibiotics potently block SV4 0 large T antigen helicase; in the present study, we describe the kine tics and the structure-activity characteristics of this process. The c oncentration vs effect data for helicase blockade were fitted by the H ill equation to yield nearly parallel log-concentration effect curves for a series of active anthracycline antibiotics. The effective concen tration for 50% helicase blockade (EC50) values ranged from 0.34 mu M for daunorubicin to 40.8 mu M for 3'-deaminodaunorubicin. Clinically i nactive 3'-N-acyl anthracyclines produced no blockade. The Hill consta nts for the blockade ranged from 1.1 to 1.6 for the entire series of a ctive anthracyclines, indicating no positive cooperativity and suggest ing that a single molecule of bound drug is sufficient to block helica se action. The EC50 values for several clinically effective anthracycl ines showed a relationship to the average DNA binding constants for th ese drugs, and Lineweaver-Burk analysis of the blockade kinetics indic ated non-competitive inhibition. The kinetics of the blockade indicate d that the anthracycline, DNA, and helicase form a ternary complex tha t is irreversible under the reaction conditions. This mechanism may be central to the cytotoxic and anti-cancer activities of anthracycline antibiotics and may be useful in understanding the enzymatic mechanism of DNA helicase action. (C) 1998 Elsevier Science Inc.