708 METABOLISM 



What evidence have we that such a process actually occurs in the body? 

 If we compare the intake of oxygen with the output of carbon dioxide 

 in the respired air, we shall find that usually there is less of the latter; 

 that is to say, the respiratory quotient, as this ratio is called, is usually 

 less than unity. During the extensive conversion of carbohydrate into 

 fat, however, which occurs during the fall months in hibernating animals, 

 the K.Q. has been found to rise as high as 1.4. The great excess of 

 C0 2 - output over 2 - intake which such a quotient indicates conforms 

 with the above equation. 



The entire dissimilarity in chemical structure between the molecules 

 of fat and carbohydrate suggests that the primary step in the conversion 

 must be a thorough breakdown of the carbohydrate chain into compara- 

 tively simple molecules, from which the fat molecules are then recon- 

 structed and the unnecessary oxygen set free. The problem is to ascer- 

 tain the chemical structure of these simpler molecules and the manner 

 of their union into fatty acid. 



Of the several suggestions which have been made, that of Smedleyss seems the most 

 likely. As will be seen from the following equations, the first step is the conversion of 

 glucose to pyruvic acid (page 600, No. 1 in equations). By enzymie action pyruvic 

 acid is converted into acetaldehyde (No. 2), which then condenses with another pyruvic- 

 acid molecule to form a higher ketonic acid (No. 3), from which by the loss of (XX, 

 as in the case of the production of acetaldehyde from pyruvic acid, an aldehyde is pro- 

 duced (No. 4). This higher aldehyde behaves like acetaldehyde in again combining with 

 pyruvic acid, forming a still higher ketonic acid; and so on until at last a long fatty- 

 acid chain is built up, thus : 



( 1 ) C^H^O,, + O, = 2CH 3 COCOOH + 2H 2 O 

 ( glucose ) ( pyruvic acid ) 



( 2 ) CH 3 COCOOH CH 3 CHO + CO 2 



(acetaldehyde) 



( 3 ) CII 3 CHO + CH 3 COCOOH CH 3 CH : CHCOCOOH -f H 2 O 



(unsaturated ketonic acid) 

 (4) CH 3 CH : CHCOCOOH i= CH^CH : CHCHO + CO 2 ; and so on. 



(higher aldehyde) 



(5) From the ketonic aldehyde formed at any stage, an unsaturated fatty acid (with 

 one less C-atom) is readily formed, and this by taking up H may become saturated: 

 CH 3 CH:CH CO COOH + O = CH 3 CH:CH COOH + CO,. 



During the butyric-acid fermentation of sugar a slightly different process may occur 

 namely, the lactic acid, which we know is readily produced from glucose, may break down 

 into acetaldehyde (and formic acid), and two such molecules condense to form /3-oxy- 

 butyric aldehyde; and this again condense to form higher fatty acids, thus: 



( 1 ) C H 12 O 6 r= 2CH 3 CHOHCOOH; 

 (glucose) (lactic acid) 



( 2 ) 2CH 3 CHOHCOOH = 2CH 3 CHO + H.COOH 



(acetaldehyde) 



(3) 2CH 3 CHO = CH 3 CHOHCH 2 CHO ; and so on. 



(/3-oxybutyric aldehyde) 



