Tissue sites of catabolism of albumin in rabbits

The sites of albumin catabolism were determined in rabbits using [¹⁴C]sucrose-labeled rabbit albumin. The [¹⁴C]sucrose moiety is not degradable and accumulates in tissues degrading the protein. Albumin was labeled with [¹⁴C]sucrose, and the monomeric form was selected for injection into rabbits. The validity of the sucrose-labeled albumin as a tracer for native albumin was shown by the similar plasma decay kinetics of ¹²⁵I-labeled albumin derivatized with sucrose and ¹³¹I-labeled native albumin and by the similar decay kinetics for the biologically screened and unscreened preparations of [¹⁴C]sucrose-albumin. Two days after injection of [¹⁴C]sucrose-albumin, tissues were assayed for accumulated ¹⁴C-labeled degradation products soluble in trichloroacetic acid. All tissues catabolized albumin with no tissue of predominant importance; liver, kidney, and muscle were the largest contributors. Expressed in terms of activity per unit wet weight, adrenal, kidney, spleen, ovary, bone marrow, and liver were the most active. These most active tissues are those with fenestrated or discontinuous capillary beds, suggesting the exposure to high concentrations of albumin is an important determinant of their high rates of albumin degradation.