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Blog Archive

Wednesday, September 7, 2011

What VOCS cause the odor of feces ?

What makes up the smell of feces ?
There has to be a good chance that there is no overwhelmingly agreed list of what volatile organic compounds make up the odor of feces, given how our research systems are set up. It's the kind of research which would probably be overlooked. This paper from 1987 was an attempt to define what VOCs made the 'fecal odor' and gave an interesting list. However case studies like these should not be regarded as correct or generally agreed by researchers. It's not until the evidence is overwhelming that conclusions are accepted.

1987 paper says the methyl sulfides methanethiol, dimethyl disulfide and dimethyl trisulfide 
The paper came to the conclusion that the odor of feces was mainly caused by the methyl sulfides methanethiol, dimethyl disulfide, and dimethyl trisulfide. We have been unable to confirm if these make good FMO3 substrates but a suspicion is that they are. Dr Cashman said in his interview in the blog that dimethyl disulfide was a good FMO3 substrate. A guess would be the 2 other methyl sulfides are too. This is not evidence of anything, merely pointing out that they are probably good FMO3 substrates.

Only Trimethylamine has no alternative route
It should be remembered that enzymes are set up to deal with changing chemical compounds of certain structures, but that usually if they cannot be metabolized by that enzyme then they can go through another enzyme less suited to metabolizing them. The only exception for FMO3 is trimethylamine, which is why researchers currently think only this causes an odor in someone with a compromised FMO3 enzyme.

Gas-chromatographic and mass-spectrometric analysis of the odor of human feces.Moore JG, Jessop LD, Osborne DN.
Department of Medicine, Salt Lake Veterans Administration, Medical Center, Utah 84148.
Gas-chromatographic and mass-spectrometric analysis of human fecal samples was performed to identify the volatile compounds responsible for fecal odor. The compounds identified with fecal odor were the methyl sulfides methanethiol, dimethyl disulfide, and dimethyl trisulfide. Skatole and indole, the benzopyrrole volatiles believed to be responsible for fecal odor, in fact elaborated a napthalenelike "mothball" odor in the crystalline state as well as after purging from feces. A small amount of hydrogen sulfide gas was also identified in fecal samples. The components responsible for fecal odor are complex and may be influenced by dietary and endogenous contributions. However, the major components are methyl sulfide compounds rather than skatole and indole as is currently believed.


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