Cadmium-dependent enzyme activity alteration is not imputable to lipid peroxidation

The effect of cadmium on the liver-specific activities of NADPH-cytochrome P450 reductase (CPR), malic dehydrogenase (MDH), glyceraldehyde-3-phosphate dehydrogenase (GADPH), and sorbitol dehydrogenase (SDH) was assessed 6, 24 , and 48 h after administration of the metal to rats (2.5 mg/kg of body wei ght, as CdCl2, single ip injection). CPR specific activity increased after 6 h and afterward decreased significantly, while MDH specific activity incr eased up to 24 h and then remained unchanged. Both SDH and GADPH specific a ctivities reduced after 6 h, the former only a little but the latter much m ore, and after 24 and 48 h were strongly inhibited. In vitro experiments, b y incubating rat liver microsomes, mitochondria, or cytosol with CdCl2 in t he pH range 6.0-8.0, excluded cadmium-induced lipid peroxidation as the cau se of the reduction in enzyme activity. In addition, from these experiments , we obtained indications on the type of interactions between cadmium and t he enzymes studied. In the case of CPR, the inhibitory effect is probably d ue to Cd2+ binding to the histidine residue of the apoenzyme, which, at phy siological pH, acts as a nucleophilic group. In vitro, mitochondrial MDH wa s not significantly affected by cadmium at any pH, indicating that this enz yme is probably not involved in the decrease in mitochondrial respiration c aused by this metal. As for GADPH specific activity, its inhibition at pH 7 .4 and above is imputable to the binding of cadmium to the SH groups presen t in the enzyme active site, since in the presence of dithiothreitol this i nhibition was removed. SDH was subjected to a dual effect when cytosol was exposed to cadmium. At pH 6.0 and 6.5, its activity was strongly stimulated up to 75 muM CdCl2 while at higher metal concentrations it was reduced. At pH 7.4 and 8.0, a stimulation up to 50 muM CdCl2 occurred but above this c oncentration, a reduction was found. These data seem to indicate that cadmi um can bind to different enzyme sites. One, at low cadmium concentration, s timulates the SDH activity while the other, at higher metal concentrations, substitutes for zinc, thus causing inhibition. This last possibility seems to occur in vivo essentially at least 24 h after intoxication. The cadmium -induced alterations of the investigated enzymes are discussed in terms of the metabolic disorders produced which are responsible for several patholog ical conditions. (C) 2000 Academic Press.