Tissue locations for the turnover of radioactively labeled rat orosomucoid in vivo.

Tissues involved in the turnover of rat serum orosomucoid were identified by methods designed to cause lysosomal trapping of radiolabel at the sites of glycoprotein degradation. 125I-, [3H]Raffinose-, and [1-14C]glucosamine-labeled orosomucoid exhibited serum half-lives of 20, 20, and 27 h when injected intravenously into rats. As expected, the asialo derivative of [3H]raffinose-labeled rat orosomucoid was lost very rapidly from the circulation and recovered quantitatively in the liver within 30 min. At 50 h after injection of [3H]raffinose-asialo-orosomucoid the liver retained 38% of the radioactivity while the remainder was found in the gastrointestinal tract and urine. Chromatography of the urine on Bio-Gel P-4 revealed a single radioactive product that eluted similar to raffinose-lysine. The same material was found in the liver. This ability of the [3H]raffinose label to resist metabolic disposal was used to evaluate tissue catabolism of native rat orosomucoid. Comparison of the tissue radioactivity in experiments using 125I- and [3H]raffinose-labeled derivatives of the nondesialylated glycoprotein showed kidney, liver, and muscle to be most active in 3H accumulation. However, the [3H]raffinose metabolites excreted in the urine was markedly different from those produced from asialo-orosomucoid and in contrast there was minimal loss of label to the gastrointestinal tract from the native substrate. Leupeptin, an inhibitor of lysosomal thiol cathespins, was administered continuously to rats by a subcutaneous osmotic pump. At 24 h after injection of 125I-orosomucoid, leupeptin-treated rats showed a net 16% increase in tissue radioactivity above sham-operated animals and a corresponding decrease occurred in the radioactivity associated with the gastrointestinal tract and urine. Tissues that exhibited increases in radioactivity were kidney, muscle, liver, and hide. The different behavior of labeled native and asialo-orosomucoids suggests that the hepatic galactose receptor system plays, at most, a limited role in maintaining homeostasis of the native glycoprotein.