Chemical-induced alterations in heme/porphyrin metabolism as bioindicators of sublethal stress in fish

Abstract

Selective inhibition of {dollar}\delta{dollar}-aminolevulinic acid dehydratase (ALAD) in mammals by lead (Pb) has served as an important biological marker of chemical exposure and injury. Water-borne exposure of fish to Pb was shown to inhibit ALAD in the blood, liver and kidney of several fish species. The relative susceptibility of ALAD to Pb inhibition in channel catfish (Ictalurus punctatus) and the biochemical characteristics of the fish hepatic enzyme were evaluated. Intraperitoneal injections of 0.5, 1.0 and 10.0 mg Pb/kg resulted in reduction of blood and liver ALAD activity but no change in hematocrit or zinc-protoporphyrin, which suggested that the fish heme biosynthetic pathway is less sensitive to Pb than mammals. Kinetic analyses of fish hepatic ALAD activities indicated a Km of 4 {dollar}\times{dollar} 10{dollar}\sp{lcub}-5{rcub}{dollar} M ALA and a Vmax of 2.57 {dollar}\pm{dollar}.218 nmol porphobilinogen (PBG)/hr/mg. The IC{dollar}\sb{lcub}50{rcub}{dollar} for Pb inhibition of fish liver ALAD activity was 40 times higher than those values found for mammals. Other in vitro studies demonstrated that, in contrast to mammals, fish hepatic ALAD was not activated by zinc and only moderately inhibited by EDTA. Western blot studies using rabbit polyclonal antibodies directed to mammalian ALADs suggested weak immunological cross-reactivity. In addition, a 10 KDa lead-binding protein (PbBP) in the liver of the channel catfish was isolated and partially characterized. These results provide further comparative biochemical data that tissue-mediating factors may play a role in the intracellular availability of Pb to sensitive biochemical processes in fish. Thus, the relative resistance of fish hepatic ALAD activity to Pb inhibition in comparison to mammals appears to be the result of two mediating factors (i) intrinsic differences in the fish hepatic enzyme; and (ii) the presence of a PbBP.