666 METABOLISM 



in the case of fat metabolism, and it can also readily be produced during 

 the intermediary breakdown of certain of the protein building-stones, 

 such for example as alanine (CH 3 CHNH 2 COOH). 



These chemical possibilities regarding the nature of the substances 

 that serve as stepping stones between the above sugar-forming sub- 

 stances and sugar itself may be translated later into probabilities on 

 account of the discovery that the enzymes exist in various tissues which 

 are involved in converting methylglyoxal into lactic acid: 



CH 3 CH 3 



! I 



CO + H 2 -+ HCOH 



I O <- | 

 CHO COOH 



(methylglyoxal) (lactic acid) 



These enzymes are called glyoxalases, and since the reactions which 

 they mediate are undoubtedly reversible in character, it is probable that 

 the conversion into sugar of lactic acid and alanine to take those two 

 as among the commonest of the sugar precursors of the animal body 

 occurs according to the following equation: 



CH 3 CHNH 2 COOH v 



(alarinc) CILCOCHO -H C 6 H ]2 O 6 



CH,CHOHCOOH /* 



(lactic acid) (methylglyoxal) (hexose) 



The unique position of methylglyoxal, besides explaining the known 

 resolutions of protein and fat and carbohydrate in intermediary metab- 

 olism, is also of importance in explaining the synthetic production of 

 glucose from fructose (or levulose). Fructose will first of all become 

 converted into methylglyoxal radicles, and these will then become syn- 

 thesized into glucose. 



The theory of the conversion of glucose into lactic acid as a stepping 

 stone in the metabolism of carbohydrate meets with one objection; 

 namely, that the lactic acid is not produced from carbohydrate in the 

 organism, except in cases where there is oxygen deficiency or excess of 

 alkali in the tissue fluids. 



Coming now to the amino acids, which, it will be remembered repre- 

 sent the building stones of the protein molecule, it has been found that 

 glycocoll, alanine, and aspartic and glutamic acids. increase the glucose 

 excretion when given to phlorhizinized dogs, whereas leucine and tyro- 

 sine have no such action. By the method described above, it is possible 

 to determine the exact proportion of the carbon of each of those amino 

 acids which becomes converted to glucose. This is shown in the accom- 

 panying table. 



