110 



THE CELL AND PROTOPLASM 



these 2 carbon atoms was entirely unknown. 

 Although it seemed probable that they 

 might be split off as acetic acid, there ex- 

 isted no experimental evidence to support 

 this view. This has recently been fur- 

 nished by investigations which superficially 

 bear no relation to the problem of 3-oxy- 

 dation. 



Fatty acid decomposition by micro-or- 

 ganisms has been known to occur in a large 

 number of cases. In addition to organ- 

 isms which oxidize fatty acids in the pres- 

 ence of air, either completely or incom- 

 pletely, some special types are known 

 which carry out the degradation of such 

 compounds under anaerobic conditions. 

 Here, four groups stand out clearly. First, 

 a group of organisms exists that will bring 

 about a decomposition of fatty acids con- 

 comitant with a reduction of nitrates. The 

 second group consists of bacteria which re- 

 duce sulfate to hydrogen sulfide while de- 

 composing the organic compound (Baars 

 1930). 



Both these processes can readily be con- 

 ceived as a step-wise dehydrogenation of 

 the fatty acid with either nitrate or sulfate, 

 instead of oxygen, fulfilling the function of 

 final hydrogen acceptor. In general, the 

 oxidation of the fatty acid seems to go to 

 completion in these cases. However, Baars 

 observed that acetate was formed during 

 the oxidation of butyrate, and later disap- 

 peared through further oxidation. From 

 experiments with propionate, under the in- 

 fluence of a strain incapable of oxidizing 

 acetate, where the end products agreed 

 quantitatively with the equation 



CH3CH2COOH + 

 I H2SO4 



CH3COOH + 



COa + fHaS 



the conclusion was drawn that the complete 

 dehydrogenation of fatty acids would pro- 

 ceed through the intermediate formation of 

 fatty acids with only one carbon atom less. 

 Obviously, this would be contradictory to 

 the Knoop-Dakin scheme. 



But the inference is not necessarily cor- 

 rect. It would be entirely conceivable that 

 also a 3-carbon compound could be dehy- 



drogenated by means of a (3-oxidation. 

 Whereas the Knoop-Dakin scheme in the 

 case of butyric acid would require the for- 

 mation of two molecules of acetic acid, it is 

 clear that in the case of propionic acid, 

 which has only three carbon atoms, one of 

 the products must be split off as carbon 

 dioxide. The evidence presented by Baars 

 does not, therefore, justify his conclusion, 

 and the formation of the theoretically re- 

 quired amount of acetic acid might equally 

 well be interpreted as in agreement with a 

 breakdown through [3-oxidation. 



The third group of organisms comprises 

 the bacteria causing the methane fermen- 

 tation of fatty acids. In this fermenta- 

 tion, the fatty acids are converted into 

 carbon dioxide and methane, and the mech- 

 anism of this process has long been 

 puzzling. To the recent studies of Barker 

 (1936) we owe at least the first elucidation 

 of this curious process. It appears that it 

 is essentially similar to the previously dis- 

 cussed decompositions; the only difference 

 is that here the acceptor function is taken 

 over by carbon dioxide, which is conse- 

 quently reduced to methane. This could 

 be proved by studying the fermentation of 

 ethyl alcohol, which agrees quantitatively 

 with the equation 



2CH3CH2OH + CO2 -^ 2CH3COOH + CH4, 



all four compounds having been deter- 

 mined. If, now, this organism reduces 

 carbon dioxide in the presence of butyric 

 acid, it was observed that for every mole- 

 cule of butyric acid decomposed not quite 

 2 molecules of acetic acid were recovered. 

 Inasmuch as it could be shown that acetic 

 acid was also oxidized, it seems very prob- 

 able indeed that the process initially can 

 be represented by the equation (Barker 

 1936, p. 413) 



2CH3CH2CH2COOH + 2H2O + 



C02->4CH3C00H + CH4. 



The last group of organisms capable of 

 decomposing fatty acids in a still different 

 manner is that of the purple bacteria. Par- 

 ticularly Gaffron (1933, 1935) and Muller 

 (1933) have shown that we are here deal- 



