Labeled plasma proteins are produced by administering to dogs the amino acid lysine synthesized with heavy nitrogen. Such labeled proteins are apparently indistinguishable biologically from proteins of normal isotope concentration.

Labeled plasma proteins, as plasma, injected into normal dogs pass out of the blood stream at an initially rapid but constantly decreasing non-logarithmic rate. This outflow is balanced by a simultaneous inflow of plasma proteins from the tissues. Fifty per cent of the labeled protein is out of the blood stream in about 24 hours; 75 per cent in about 6 days.

Shock due to trauma of intestine or leg shows a dilution curve of labeled plasma protein not unlike that of the normal dog. If anything, dilution appears a little less rapid in shock. Since the usual shrinkage of plasma volume and plasma protein mass is present in these shocked dogs, these data are compatible with a decreased inflow of protein into the plasma during shock.

Methods are described which are suitable for the use of heavy nitrogen incorporated in the epsilon group of lysine and its subsequent analysis in body fluids.

These data may indicate that the plasma proteins are normally in constant and rapid exchange with a mobile pool of body protein.

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