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

Delta-aminolevulinate dehydratase (ALAD) is a phylogenetically conserved en zyme that is responsible for the synthesis of porphobilinogen in a key step involving the production of heme. ALAD is a metalloprotein that requires m agnesium or zinc for its activity, depending on the species. In humans, ALA D activity is zinc-dependent, and has been used extensively as a biomarker for lead (Pb) exposure and toxicity. ALAD activity in other multicellular e ukaryotes has been used in ecotoxicology as a biomarker for environmental l ead pollution. Because microorganisms are sensitive indicators of toxicity at the fundamental level of ecological organization, we hypothesized that b acterial ALAD can serve as a reliable biomarker for lead bioavailability in contaminated environments. In this study, ALAD activity in an environmenta l strain of Pseudomonas putida was investigated to evaluate potential inhib ition by Pb and other toxic metals. There was a statistically significant d ose-response relationship between ALAD activity in cells of P. putida, ATCC 700097 and [Pb] (Pearson correlation coefficient = -0.985; r(2) = 0.97, an d P < 0.001). The highest level of inhibition of ALAD activity was approxim ately 74% of the normal level when cells were incubated with [Pb2+] > 500 m uM The relationship between Pb and ALAD activity was statistically describe d 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 muM of Pb. A hi gher concentration of Pb was needed to produce a comparable level of ALAD i nhibition in P. putida cells seeded into natural freshwater, suggesting tha t Pb was not completely bioavailable in the water samples. In contrast to t he findings with P. putida, the ALAD activity in a known metal-resistant P, aeruginosa PU21 (Rip64) was not sensitive to Pb exposure. Therefore, the s ensitivity of ALAD to Pb in complex heterogenous ecosystems depends on the molecular diversity of ALAD in predominant species, and on the bioavailabil ity of Pb. (C) 2000 Elsevier Science Ltd. All rights reserved.