Mpm. Portela et Aom. Stoppani, REDOX CYCLING OF BETA-LAPACHONE AND RELATED O-NAPHTHOQUINONES IN THE PRESENCE OF DIHYDROLIPOAMIDE AND OXYGEN, Biochemical pharmacology, 51(3), 1996, pp. 275-283
Lipophilic o-naphthoquinones (beta-lapachone, CG 8-935, CG 9-442, CG 1
0-248, and mansonones A, C, E, and F), catalyze the oxidation of dihyd
rolipoamide (DHLA) by oxygen, whereas rho-naphthoquinones (alpha-lapac
hone and menadione) are scarcely active. The greatest effects correspo
nded to beta-lapachone and its analogues. Quinol production was demons
trated by (a) the absorption spectrum of the reduced quinone, and (b)
the effect of pH variation on the rate of quinone-catalyzed DHLA oxida
tion. Superoxide dismutase (SOD) inhibited the rate of cytochrome c re
duction and decreased the apparent rate of oxygen consumption by sever
al DHLA/o-naphthoquinone systems. SOD also inhibited the rate of quino
l oxidation by oxygen, after quinone reduction by a stoichiometric amo
unt of DHLA. Catalase enhanced the effect of SOD, but in its absence c
atalase was inactive. It is concluded that quinone-catalyzed oxidation
of DHLA implies a free-radical mechanism in which the quinol and supe
roxide radicals play an essential role.