666 METABOLISM 



iii the case of fat metabolism. It can also readily be produced during 

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

 such for example as alanine (CH 3 dINH 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 considered as probabilities on account 

 of the discovery thai enzymes exisl in various tissues which are capable 

 of converting methylgloxal into lactic acid: 



CH 3 CH, 



I I 



CO + II., -» IICOH 



0"<- | 

 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 CHNII.,COOH v 



(alanine) CH..COCHO -» C 6 H r ,0 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 hypothesis of 1 lie conversion of glucose into lactic acid as a stepping 

 stone in the metabolism of carbohydrate is difficult to test by direct ex- 

 periment because the lactic acid does not accumulate 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 si ones of the protein molecule, it has been found that 

 trlycocoll, alanine, and aspartic and glutamic acids increase the glucose 

 excretion when t>iven 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. 



