Endocytosis and breakdown of proteins by sinusoidal liver cells

We studied the uptake of modified forms of bovine pancreatic ribonuclease A (labeled with 125-iodine) by rat liver in vivo. On one hand, these experiments were intended to investigate a possible role of sinusoidal cells in the uptake of plasma proteins; on the other hand, the effect of well-defined modifications of the enzyme on the role of uptake might give us a key to the factors determining life time of plasma proteins. We used nephrectomized rats in most experiments to avoid uptake by the kidneys. Preparations of ribonuclease oligomers prepared by cross-linking with dimethyl-suberimidate enabled us to study a possible relation between uptake and molecular size. This modification does not lead to changes in charge of the protein. Monomer, dimer and polymer fractions were isolated by gel filtration on Sephadex G-75. Of the 11 amino groups in ribonuclease A, 9, 8 and 6 remained unaltered in the monomer, dimer and polymer fraction, respectively. The maintenance of biological activity, the stability of disulfide bonds, and the unchanged susceptibility to endoproteases of the cross-linked products established that gross conformational changes had not occurred. At 1 h after injection, 1% of themonomer, 7% of the dimer and 19% of the polymer were recovered per g of liver protein. Combination of autoradiography, subcellular fractionation, and the determination of labeled ribonuclease derivatives in the spleens showed that the dimer and polymer fractions were mainly present in the lysosomes of sinusoidal cells.