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.