METABOLISM OF PROTEINS 



573 



the amino-acids play a great part in protein metabolism, perhaps as 

 great a part as the dextrose does in the metabolism of the carbo- 

 hydrates. There is some evidence that serum-albumin is more 

 directly related to the proteins of the food than serum- globulin. 

 And it is said that during starvation the albumin is relatively 

 diminished, and the globulin relatively increased. It is, of course, 

 not at all improbable that the plasma-proteins have a double source 

 organ-proteins on the one hand, food-proteins on the other. In 

 any case, it is certain that serum-albumin and serum-globulin 

 cannot be interchangeable without far-reaching decomposition, for 

 their composition is very different. The globulin, e.g., yields glyco- 

 coll, but the albumin does not. That the plasma-protein mixture 

 maintains a very constant composition in the face of wide variations 

 in the composition of the food-protein is indicated by the following 

 experiment : 



A horse fed mainly on hay and oats was bled to the amount of 

 6 litres, and in the total protein of the serum the content of tyrosin and 

 glutaminic acid was determined. In order to eliminate the influence 

 of remains of the food in the digestive canal, nothing was given to the 

 animal for a week. Then 6 litres of blood were again removed, and the 

 tyrosin and glutaminic acid in the serum-protein again estimated. 

 The horse was now fed with gliadin (one of the prolamins or alcohol- 

 soluble proteins obtained from flour), a substance which contains 36-5 per 

 cent, glutaminic acid and 2-37 per cent, tyrosin that is, about the 

 same amount of tyrosin as the serum-protein, but about four times as 

 much glutaminic acid. The serum-protein was again analyzed for the 

 two amino-acids after this diet. The results of one experiment are 

 shown in the table : 



No increase in the glutaminic acid content of the serum-protein 

 occurred, although, owing to the loss of blood, much new serum-protein 

 must have been formed. If the amino-acids of the gliadin were used 

 without change to build up the new serum-protein, three-quarters of 

 the glutaminic acid must have been superfluous, and the nitrogen of 

 this portion may have been straightway changed into urea and excreted. 

 But the possibility that the glutaminic acid, or a portion of it, may- 

 have been changed into other amino-acids in the body cannot be 

 excluded. In the case of some of the amino-acjds it has been shown 

 that such a transformation occurs (p. 613) (Abderhalden). 



The high degree of independence of the food and body proteins is 

 still more clearly exhibited in the table from Abderhalden on p. 576, 

 in which the proteins of milk are compared with some of the proteins 

 which must be formed from them in the body of the suckling. The 

 numbers represent percentages of the weight of each protein. 



