The metabolism of homologous plasma proteins, labeled with lysine-ϵ-C14, after oral administration to dogs has been investigated.

The speed of the various processes involved is indicated by the maximum rate of C14 O2 excretion which is attained within 1 to 4 hours, the prompt appearance of protein activity in the plasma and disappearance of non-protein activity from it, both virtually complete in 7 to 10 hours, as well as the rapid incorporation of a large percentage of the fed-C14 into tissues.

There are no essential differences between the behavior of labeled plasma and that of an amino acid digest containing ϵ-C14 labeled lysine when these two materials are given orally.

At the end of 48 hours after labeled plasma feeding, a CO2 elimination of 16 to 28 per cent of the fed C14 is noted. In contrast, after 48 hours following labeled plasma by vein, a CO2 elimination of only 2.5 per cent is recorded—almost a 10 to 1 ratio. We believe this, together with the data concerning plasma and tissue protein activity, represents a significant difference in the metabolic process. The evidence favors a complete breakdown of plasma protein to the amino acid level when given orally but not when given by vein.

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