THE SEAT OF FORMATION OF UREA 689 



as necessary base, and may possibly represent ammonia from 

 blood which has passed through the kidney before going to 

 the liver. It would be interesting to know whether the effect 

 of alkalies on the urinary ammonia is due to decreased forma- 

 tion of ammonia or to its more complete conversion into urea ; 

 there are, however, no estimations of the ammonia contents 

 of the blood to show. 



Means by which the Liver converts Ammonia into Urea. We 

 have seen incidentally that many katabolic processes which 

 it used to be thought were carried out by the direct action of 

 protoplasm are now thought to be produced by ferments formed 

 within the cell. It is perhaps premature to generalise and 

 say that all katabolic processes are carried out by tissue fer- 

 ments, but the discovery of proteolytic, lipolytic, glycolytic 

 ferments, of various kinds of oxydases, and other ferments 

 in the tissues, almost justifies such a view. The formation 

 of urea from ammonia, however, cannot be looked upon as a 

 process of breaking down but rather as a synthesis. According 

 to Hofmeister's view it is an oxidative synthesis, and according 

 to the views of Schmiedeberg and Drechsel it is a synthesis 

 followed by dehydration. At one time it was thought that 

 ferments could not produce a synthesis ; this is now known 

 not to be the case. The kidney, for instance, contains a ferment 

 which synthesises glycin and benzoic acid with the loss of water 

 into hippuric acid. But even more important was the discovery 

 that some zymolyses are reversible, and that one and the same 

 ferment could both break down and build up. Thus Croft 

 Hill showed that yeast maltase could both hydrolyse maltose 

 to glucose and condense glucose back to maltose. Similarly 

 a lipase has been formed capable of converting fat into fatty 

 acid and fatty acid back into fat. It is therefore, a priori, 

 probable that one and the same ferment in the liver hydrolyses 

 glycogen into dextrose and condenses dextrose into glycogen. 

 It would certainly be premature to say that it is probable that 

 in the body all the hydrolyses are reversible, but it is possible 

 that a very large number of them are. Jacoby has shown 

 that liver juice contains a ferment which can hydrolyse urea 

 into some ammonium salt, just as urease, the ferment of the 

 Bacillus ureae, converts urea into ammonium carbonate in 

 the urine. We have no experimental evidence that the action 



2x 



