Effects of long-term exposure to Cu 2+ and Cd 2+ on the pentose phosphate pathway dehydrogenase activities in the ovary of adult Bufo arenarum: possible role as biomarker for Cu 2+ toxicity

Abstract

The effects of copper and cadmium on metabolism through the pentose phosphate pathway were evaluated in Bufo arenarum toad ovary. The effects of the two metals on dehydrogenases from this pathway were evaluated by three experiments: (1) in samples obtained from control females with addition of the metals to the reaction mixture (in vitro), (2) in samples obtained from control females and after long-term exposure of females to 4 and 100 μg/L of Cu or Cd in the incubation media (in vitro after exposure to the metals in vivo), and (3) 14CO 2 production through the pentose phosphate pathway was evaluated after [U- 14C]glucose microinjection on ovulated oocytes (in vivo after microinjection of the metals). Results from (1) evidenced inhibition of both enzyme activities but only above 1.5 mM Cu and Cd added to the reaction mixture. In (2) both glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities decreased in samples from the ovaries of females exposed in vivo to Cu, in a concentration-dependent manner (up to 90% in females exposed to 100 μg/L Cu: 2.12±1.57 NADPH μmol/min μg protein×10 −5 vs 19.97±8.54 in control females). Cd treatment of the toads only rendered an inhibitory effect on 6-phosphogluconate dehydrogenase activity after exposure to 4 μg/L of the bivalent cation. (3) In vivo 14CO 2 evolution significantly decreased in oocytes coinjected with 6.3×10 −3 mM Cu (calculated intracellular final concentration of the metal injected) and radioactive glucose. Cu and Cd concentration in samples from exposed females were always under detection limit by particle-induced X-ray emission. The results presented here are in agreement with a role for both glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities determination as biomarkers of effect and exposure for Cu but not for Cd toxicity.