Pyridine aerobic degradation

The aerobic degradation of several azaarenes involves reduction of the rings at some stage, and are discussed in Chapter 10, Part 1. Illustrative examples include the degradation of pyridines (3-alkyl-pyridine, pyridoxal) and pyrimidines (catalyzed by dihydropyrimidine dehydrogenases). Reductions are involved in both the aerobic and the anaerobic degradation of uracil and orotic acid. 

As for the aerobic degradation of pyridines, hydroxylation of the heterocyclic ring is a key reaction in the anaerobic degradation of azaarenes by Clostridia. Whereas in Clostridium barkeri, the end products are carboxylic acids, CO2, and ammonium, the anaerobic sulfate-reducing Desulfococcus niacinii degraded nicotinate completely to CO2 (Imhoff-Stuckle and Pfennig 1983), although the details of the pathway remain incompletely resolved. 

Rhee S-K, GM Lee, J-H Yoon, Y-H Park, H-S Bae, S-T Lee (1997) Anaerobic and aerobic degradation of pyridine by a newly isolated denitrifying bacterium. Appl Environ Microbiol 63 2578-2585. 

Consequently, it appears likely fliat flie alcohols, ketones, o-cresol, ethyl acetate, and pyridine will degrade rapidly in soil if rapidly is defined as having a half-life of 10 days or less. Most ofthe benzene derivatives, F-11, and the chlorinated aliphatic hydrocarbons may be relatively persistent in soil. Analogous information was not located for diethyl ether, hexane, decane, or tetrahydrofuran. ATSDR for example, found that there was little information available for the degradation of n-hexane in soil. It was suggested that n-hexane can degrade to alcohols, aldehydes, and fatty acids under aerobic conditions. 

In summary, two general pathways are now accepted as the reaction basis of pyridine degradation by bacteria. One involves (i) hydroxylation reactions, followed by reduction, e.g., on Bacillus strain 4 and the other (ii) (aerobic) reductive pathway(s) not initiated by hydroxylations, e.g., on Nocardia strain Zl [348], Two review articles, one by Kaiser [320] and the other by Fetzner [326] gave the complete microbial metabolic pathways for several nitrogen compounds carried out in the presence of a variety of microorganisms, some of them previously studied by Professor Lingens [349], The complete degradation pathways of pyridine are shown in Fig. 29. 

Figure 29. Pathways of aerobic bacterial degradation of pyridine.

---