Effect of lead ions on chick-embryo liver mitochondrial δ-aminolaevulinate synthase

Abstract

Pb2+ activated native chick-embryo liver mitochondrial δ-aminolaevulinate synthase (EC 2.3.1.37). This result contradicted with the inhibitory effect observed by earlier workers who used degraded enzyme preparations. Enzyme activation was biphasic. An initial activation phase was observed with Pb2+ concentrations up to 200 μM, and a secondary phase with concentrations from 200 μM to at least 2 mM. Maximum primary activation was 2.5-fold at 200 μM-Pb2+, with a further 2-fold activation observed at 2 mM-Pb2+. Primary activation was not affected by a 10-fold molar excess of dithioerythritol, but the secondary activation was abolished by dithioerythritol. Secondary-phase activation was lost upon increasing time of incubation of the enzyme with Pb2+. The implications of these findings are discussed with reference to lead poisoning and the mechanism of δ-aminolaevulinate synthase.