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The metabolism of homologous plasma proteins, labeled with lysine-epsilon-C(14), after oral administration to dogs has been investigated. The speed of the various processes involved is indicated by the maximum rate of C(14) O(2) 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-C(14) into tissues. There are no essential differences between the behavior of labeled plasma and that of an amino acid digest containing epsilon-C(14) labeled lysine when these two materials are given orally. At the end of 48 hours after labeled plasma feeding, a CO(2) elimination of 16 to 28 per cent of the fed C(14) is noted. In contrast, after 48 hours following labeled plasma by vein, a CO(2) 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.
Yuile et al. (1952) studied this question.