METABOLISM OF CARBO-HYDRATES 537 



theses be started, which may go far to retrace the steps of the pre- 

 ceding katabolism in respect to a portion of the dextrose. 



Thus, lactic acid can be retransformed into dextrose, and this, of 

 course, into glycogen. The formation of fat from sugar may also 

 start from some of the stages displayed in the scheme, for it is 

 only a short step to obtain by the reduction of glyceric aldehyde its 

 alcohol glycerin. And from acetaldehyde fatty acids can be 

 derived. 



Not only does lactic acid afford a point of contact between the 

 metabolism of carbo-hydrates and that of fats a junction, so to 

 speak, where these two great metabolic currents cross each other, 

 and where material originating in the one may be shunted into the 

 other but it also affords a point of junction and interchange with 

 the current of protein metabolism. For example, certain of the 

 amino-acids, such as alanin, yield as a decomposition product a 

 compound called methylglyoxal (CH 3 .CO.CHO), which by the 

 assumption of a molecule of water can be changed into lactic acid. 

 It may also be one of the intermediate stages in the decomposition 

 of dextrose as a precursor of lactic acid, and one of the ways in 

 which the conversion of amino-acids into dextrose is accomplished 

 may be through this link. 



Pyruvic acid is another possible link. As has just been mentioned, 

 it probably forms a stage in the decomposition of dextrose, and has, in 

 addition, chemical relations on the one hand to certain of the amino- 

 acids, especially to alanin, and on the other to glycerin and even to 

 fatty acids. Thus 



CH 3 CH 



CH.NH 2 +O = CO + NH 3 

 COOH COOH 



Alanin (a-amino-pro- Pyruvic acid. 



CH 2 .OH CH 3 



CH.OH + 2O = 



H 2 .OH COOH 



Glycerin. Pyruvic acid. 



pionic acid. 



The more completely the various steps in the metabolism of the 

 three great groups of food substances are unveiled, the more- 

 clearly does it appear that, far from being independent circuits, 

 the three currents are constantly exchanging materials with each 

 other. 



It is to be supposed that in many of these transformations enzymes 

 are concerned, although comparatively little is definitely known as 

 to this. Normal blood itself has been credited with a ferment 

 which has the power of destroying sugar (glycolysis) . But with 

 rigid aseptic precautions the loss of sugar, even in several hours, is 

 small, and it is doubtful whether such a ferment exists. On the 

 other hand, Cohnheim stated that while no glycolytic ferment can 

 be demonstrated in the pancreas, and only an exceedingly weak 

 glycolytic action in muscular tissue (Brunton), by combining ex- 



