The catabolic rate of albumin doubly labelled with 131-I and 14-C in the isolated perfused rat liver.

The ability of the isolated perfused rat liver to catabolize plasma proteins has been investigated both with blood containing 131I-labelled isolated proteins (Gordon, 1957; Cohen & Gordon, 1958) and with blood containing whole plasma labelled with 14C (Green & Miller, 1960). Since the rate of catabolism obtained for albumin by the former method (1 1-1 4 mg. of albumin/hr.) is very much below the value which might be expected from the experiments with 14C-labelled whole plasma [approximately 7 mg. of protein/hr./300 g. rat as calculated from the data given by Green & Miller (1960), assuming that the catabolic rate of albumin in the liver is an average of that for the whole plasma], further work was needed to show whether the low results with [1311]albumin were due to the relatively slow catabolism of this protein in the liver compared with other plasma proteins, or to some discrepancy between the labels. In vivo the many different proteins in whole plasma are catabolized at different rates and a different proportion of each may be expected to be broken down by the liver. Thus in order strictly to compare the two labels, single proteins are required, preferably in fully native form, labelled both with 14C and with 131I. For the present work rat albumin doubly labelled in this way has been used. The results show that there is fair agreement between the rates of catabolism calculated from liberation of non-protein 131I and from the amounts of 14C appearing in the liver, the plasma free amino acids and in the carbon dioxide. However, although the method of isolation was selected because of its mildness, the rate of catabolism as measured by 131I of the doubly labelled albumin was found to be considerably above that previously found for 131I-labelled fully native protein. The present data cannot therefore be used directly to calculate the proportion of native albu in catabolized by the liver; however, they are considered to validate previous results obtained by means of ""'I-labelled proteins.