556 VETERINARY PHYSIOLOGY 



and. in the liver, linked to sulphuric acid or potassium 

 sulphate derived from the sulphur of the protein molecule, and 

 thus excreted as ethereal sulj^hates, the amount of which in 

 the urine is a fair measure of the putrefaction changes in the 

 intestine. 



The remaining sulphur of the protein is chiefly oxidised to 

 sulphuric acid, linked to bases and excreted as the inorganic 

 sulphates. 



A small part of the sulphur which in the protein exists as 

 cystin may escape oxidation and appear in the urine. In 

 some people this excretion of cystin is large, and cystin crystals 

 appear in the urine {neutral sulphur). 



The sulphates and the neutral sulphur are derived from the 

 sulphur of the proteins, and the amount excreted in the urine is 

 a measure of the amount of protein catabolised. 



The nucleo-proteins are first split into the protein moiety, 

 which is broken down as described above, and the nucleic acid 

 moiety, which is broken down, possibly by an enzyme, a 

 nuclease, and the phosphorus and purin parts then undergo 

 ciianges and are excreted in the urine. 



(a) The purins of nucleic acid, e.g. adenin, are amino-purins 

 i.e. they have an amidogen molecule attached. This is split 

 off, possibly by the action of an enzyme, and excreted as urea. 



The deaminised purins, such as hypoxanthin, are then 

 oxidised to uric acid and partly excreted in this form. In this 

 process an enzyme may play a part. About one-half of the uric 

 acid is split into two molecules of urea, probably by the action 

 of another enzyme (uricoclastase) in the liver, while the 

 connecting chain is oxidised to CO.,. 



(6) The phosphorus is oxidised to P0O5 and linked with 

 monobasic sodium and potassium and dibasic calcium and 

 magnesium. 



2. Regulation of the H— OH ions of the Blood. — The phos- 

 phates play an important part in regulating the Ch of the tissues 

 and in determining the reaction of the urine (Appendix III.). 



If the Ch of the blood is increased in acidosis (p. 481), the 

 Na„HPO, of the cells is changed to (NaH.PO J (p. 482) and 

 is turned out into the plasma to be excreted in the urine and 

 thus to carry off H ions. 



