Molecular probe studies of contaminant transformation in reducing sediments: Hydride transfer and dehydrogenase activity

One way to investigate specific biogeochemical reduction processes in whole sediments is with molecular probes. The approach is to use the rates and products of reactions that are diagnostic of particular reducing agents (or reduction mechanisms) as chemical probes for the biogeochemical processes that influence contaminant fate. 2-Chloroacetophenone (2-CAP) could be used as such a probe because it reacts via two pathways to give distinctive products: electron transfer to give acetophenone and hydride transfer to give 2-chlorophenylethanol (2-CPE). Preliminary results with 2-CAP that indicate an enantioselective, temperature-sensitive hydride transfer pathway in slurries of anaerobic sediment was presented. Using horse liver alcohol dehydrogenase (HLADH) as a model enzyme, the major product was the one that is diagnostic for hydride transfer (2-CPE). However, the rate of 2-CAP reduction in the HLADH/NADH model system was too slow to give good quantitative kinetics, and a more effective model system is the NADPH-dependent alcohol dehydrogenase from Thermoanaerobium brockii (TBADH). The TBADH/NADPH model system gave 100% conversion of 2-CAP to 2-CPE with a maximum rate at pH 6.5 and well defined Michaelis-Menton curves as a function of both substrate concentrations (2-CAP and NADPH).