Microbial δ-aminolevulinate dehydratase as a biosensor of lead bioavailability in contaminated environments

Abstract

Delta-aminolevulinate dehydratase (ALAD) is a phylogenetically conserved enzyme that is responsible for the synthesis of porphobilinogen in a key step involving the production of heme. ALAD is a metalloprotein that requires magnesium or zinc for its activity, depending on the species. In humans, ALAD activity is zinc-dependent, and has been used extensively as a biomarker for lead (Pb) exposure and toxicity. ALAD activity in other multicellular eukaryotes has been used in ecotoxicology as a biomarker for environmental lead pollution. Because microorganisms are sensitive indicators of toxicity at the fundamental level of ecological organization, we hypothesized that bacterial ALAD can serve as a reliable biomarker for lead bioavailability in contaminated environments. In this study, ALAD activity in an environmental strain of Pseudomonas putida was investigated to evaluate potential inhibition by Pb and other toxic metals. There was a statistically significant dose–response relationship between ALAD activity in cells of P. putida, ATCC 700097 and [Pb] (Pearson correlation coefficient=−0.985; r 2=0.97, and P<0.001). The highest level of inhibition of ALAD activity was approximately 74% of the normal level when cells were incubated with [Pb 2+]>500 μM. The relationship between Pb and ALAD activity was statistically described by log[ Pb]=3.68−1.41[ ALAD Activity ]. In protein extracts of P. putida, ALAD activity was reduced by up to 85% in response to 500 μM of Pb. A higher concentration of Pb was needed to produce a comparable level of ALAD inhibition in P. putida cells seeded into natural freshwater, suggesting that Pb was not completely bioavailable in the water samples. In contrast to the findings with P. putida, the ALAD activity in a known metal-resistant P. aeruginosa PU21 (Rip64) was not sensitive to Pb exposure. Therefore, the sensitivity of ALAD to Pb in complex heterogenous ecosystems depends on the molecular diversity of ALAD in predominant species, and on the bioavailability of Pb.