Enzymatic Study on the Microbial Degradation of Homocholine

Abstract

This research was conducted to investigate the enzymatic activities in the degradation pathway of homocholine by bacterial strains isolated from the soil. Screening of the homocholine oxidation activity in the isolated strains, by replica staining method and spectrophotometric assay, showed that NAD+ - dependent dehydrogenase enzymes were predominant in all isolates. Furthermore, dried cell reaction of Pseudomonas sp. strain A9 cells with homocholine in the presence and absence of NAD+ demonstrated that the enzymes responsible for the metabolism of homocholine were alcohol and aldehyde dehydrogenases that require NAD+ as electron acceptor. Moreover, in the cell free extract of Pseudomonas sp. strain A9 an inducible NAD+ - dependent homocholine dehydrogenase was detected. The crude preparation of this enzyme has broad substrate specificity. Although various buffering conditions and stabilizing reagent were applied to stabilize the enzyme activity, the enzyme was unstable in vitro and lost its activity soon after and during the purification processes. Furthermore, an inducible NAD+ - dependent 3- hydroxypropionate dehydrogenase activity was also detected in the cell free extract of Pseudomonas sp. strain A9. This result indicated the presence of 3-hydroxypropionate as an intermediate metabolite in the degradation pathway of homocholine by this strain. Thus, in Pseudomonas sp. strain A9, homocholine is oxidized to trimethylaminopropionaldehyde by a NAD+-dependent homocholine dehydrogenase and consequently, trimethylaminopropionaldehyde oxidized to β-alanine betaine by a NAD+ - dependent aldehyde dehydrogenase. Thereafter, cleavage of β-alanine betaine C-N bond yielded trimethylamine and 3-hydroxypropionate (C-3 moiety). Thereafter, 3- hydroxypropionate was further oxidized to malonate semi-aldehyde by a NAD+ - dependent 3-hydroxypropionate dehydrogenase.