148 BIOLOGICAL CHEMISTRY 



if these amino acids are absent from the diet the body proteins 

 are broken down to furnish the required amino acid. If that 

 amino acid can be resynthesised it need not be present in the 

 diet, but if it cannot be resynthesised it must be present in 

 the diet, otherwise the proteins of the tissues are destroyed. 

 We must therefore aim at a supply of amino acids that are 

 necessary to the body but cannot be synthesised in the body.* 

 Applying the law of mass action to these processes the 

 equation for the union of two molecules of amino acid to 



C *C 



form a molecule of dipeptid, is K == -- v where K is the 



^z 



equilibrium constant and C*, C y and C z the concentrations 

 of the reacting substances x, y and z respectively. As the 

 formation of protein is due to a series of such unions we see 

 that deficiency of one substance will bring the whole process 

 of synthesis to a standstill, and as these represent reversible 

 reactions the destruction of one substance will cause the 

 breakdown of protein until equilibrium is re-established. 



In the process of deamidisation it is recognised that deamidi- 

 sation mainly affects the amino acids present in excess. 

 This likewise follows from the law of mass action. The rate 



of reaction depends upon the concentration, or ~~ = kC r 



where k is the velocity constant and C*, the concentration of 

 the substance x which is being deamidised. We therefore 

 see that the greater the concentration of any amino acid the 

 more rapidly it must be deamidised. The amino acids which 

 are present in minimal amounts will be only slowly deamidised 

 and they will be rapidly taken up by the synthesis of protein, 

 hence they are used most economically. 



The hypothesis of Voit is thus shown to mean that the 

 amino acids present in excess are rapidly deamidised, but 

 that those which are present in minimal amounts are formed 

 into tissue proteins. The requirements of the body for certain 

 amino acids will cause a breakdown of protein in the intervals 

 between absorption and the excess amino acids thus set free 

 will be deamidised. Thus there is a greatly increased excretion 

 of nitrogen after a meal rich in protein, followed by a low 

 nitrogen excretion when tissue proteins must be hydrolysed 

 to furnish special amino acids as they are required. 



This discussion assumes that there are not specialised 

 enzymes for deamidisation of special amino acids. As all the 



* T. B. Osborne and L. B. Mendel, Joum. Biol Chem., 1914, vol. 

 17 P. 325- 



