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

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Abstract
Rat albumin doubly labelled with Cl4 and I131 has been prepared with Cl4 glycine or [Cl4] leucine. The former was incorporated into plasma by injection into a living rat. The biosynthesis with [C14] leucine was by means of a liver perfusion. After removal of fibrin the plasmas were labelled with I131 and subjected to preparative electrophoresis in starch gel. The rate of catabolism of this doubly labelled albumin was estimated during perfusion through isolated rat livers both from the rate of formation of non-protein I131 and from the amount of C14O2, liver C14 and plasma free amino acid C14 finally present. When [Cl4]ieucine was the precursor and after 2 hours preliminary perfusion through a rat liver, which served to reduce the amount of nonnative protein present, the rates of catabolism of the albumin calculated on the basis of the 2 labels were found to be in fair agreement. With [Cl4]glycine as precursor the rate of catabolism as calculated from Cl4 was the higher. The rate of formation of C14O2 by the isolated liver from [Cl4]leucine-labelled albumin (0-02-0-054%/hour) observed in the present work has been considered in relation to the C14O2 rate from [C14]-lysine-labelled whole plasma, which was reported By Green and Miller (1960) to be 013%/hour. The rate of catabolism deduced from the present findings with doubly labelled albumin is not in accord with their view that the liver can catabolize fast enough to account for the whole of the turnover rate in vivo of the plasma.