METABOLISM OF CARBO-HYDRATES 



545 



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 (Mandel and 

 Lusk), and this, of course, into glycogen. Pyruvic acid can be 

 changed into lactic acid by reduction, and dextrose can in this way 

 be again produced. There is evidence also that under certain 

 conditions pyruvic acid can yield dextrose in the organism by a 

 different reaction, being changed into acetaldehyde, which can 

 then undergo transformations leading back to dextrose (Ringer). 

 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, the amino-acid, 

 alanin, yields as a decomposition product a compound called 

 methylglyoxal (CH 3 .CO.CHO), a ketonic aldehyde, 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. The presence of a ferment glyoxalase, 

 or probably more than one ferment which rapidly changes methyl- 

 glyoxal into lactic acid, has been demonstrated in tissue extracts 

 and in leucocytes. The same change is effected when blood con- 

 taining methylglyoxal is perfused through an excised surviving 

 liver. The reaction can be reversed for methylglyoxal like lactic 

 acid when given to an animal rendered diabetic by phlorhizin can be 

 shown to yield dextrose, possibly being first converted into glyceric 

 aldehyde. The conversion of methylglyoxal into lactic acid is also 

 a reversible reaction, for in vitro, at any rate, lactic acid readily 

 yields methylglyoxal (Dakin). 



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 



H.NH 2 + 

 COOH 



Alanin (a-amino-pro- 

 pionic acid. 



CH 3 



CO 4-NH a 

 COOH 



Pyruvic acid. 



CHo.OH 



* 



CH 



CH.OH + 2O = CO + aHJD 



CH 2 .OH 



Glycerin. 



COOH 



Pyruvic acid. 



35 



