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A MANUAL OF PHYSIOLOGY 



store of protein material proper to the circulating tissue blood 

 itself, and which confers on it certain chemical and physico- 

 chemical properties (e.g., the due degree of viscosity) necessary 

 for its function. Slowly accumulated, under ordinary conditions, 

 and slowly consumed, this protein store may, of course, be at the 

 disposal of the organs in an emergency for instance, in starva- 

 tion or may be rapidly recruited from the organ -proteins, as 

 after haemorrhage, just as in prolonged hunger the proteins of 

 skeletal muscle may be utilized to feed the heart. It is impossible, 

 with our present knowledge, to decide definitely between these 

 hypotheses. 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. That the plasma-protein mixture main- 

 tains a very constant composition in the face of wide variations in 

 the composition of the food-proteins 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 obtained 

 from flour, a protein 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 gluta- 

 minic 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 one amino-acid 

 may be changed into another in the body cannot be excluded, 

 although there is no evidence that it occurs (Abderhalden). 



