Catabolism of plasma albumin by the perfused rat liver.

The liver is known to be a major site of plasmaprotein synthesis, but there is little information concerning its role in the catabolism of native plasma proteins. The ease with which the isolated perfused liver can be maintained under physiological conditions would appear to make it an ideal system for studying plasma-protein breakdown. However, the ability of the liver to take up particulate material is well known, and Gordon (1957) has recently shown that the perfused rat liver rapidly catabolizes small proportions of altered protein molecules present in preparations of labelled plasma protein. Perfusion experiments, which are necessarily of short duration, can therefore be used to assess the physiological role of the liver in protein breakdown only if the labelled protein used is free from altered molecules. Rapidly catabolized components can be removed by preliminary injection of the labelled protein solution into a living animal; after the required period of 'screening' the plasma of this animal is used as the labelled protein solution to be tested (McFarlane, 1956). Using a screening period of 48 hr. in the rat, Gordon (1957) found in three experiments that the perfused rat liver catabolized about one-tenth of the homologous albumin broken down in the whole animal. Such data can be regarded as having physiological significance only if it is possible to establish a minimum catabolic rate which is unaffected by more prolonged screening of the labelled albumin. In the present investigation the effect of the length of screening upon the rate of homologous albumin catabolism by the perfused rat liver has been studied. Observations regarding the time taken for the release of diffusible label from [1311]albumin by the perfused liver are also recorded and in addition the capacity of the liver for breaking down native and denatured albumins is compared.