Uptake of transcobalamin II-bound cobalamin by isolated rat kidney tubule cells

Abstract

The uptake and intracellular processing of transcobalamin II-bound cobalamin by isolated rat kidney tubule cells were studied. The cells absorbed the complex in a temperature-and calcium-dependent process, which could be inhibited by monensin, an inhibitor of endocytosis. Cells, loaded with a mixture of ¹²⁵I- and ⁵⁷Co-labelled transcobalamin II-vitamin B12, released ¹²⁵I-labelled protein-degradation products, while keeping the ⁵⁷Co-labelled vitamin. Protein degradation was inhibited by chloroquine and monensin, which is further evidence for a process of endocytosis, followed by intralysosomal hydrolysis of the transport protein. Transcobalamin II-vitamin B12 uptake was not fully saturable and other proteins, for example, haemoglobin, inhibited the uptake in a concentration-dependent way. Apparently the uptake proceeds throμgh relatively unspecific protein-binding sites, probably involved in the reabsorption of filtrated proteins, althoμgh the affinity for transcobalamin II seems relatively high. Consequently, elevated urinary excretion of cobalamin is expected in patients with overflow proteinuria, and was indeed found in a patient with paroxysmal nocturnal haemoglobinuria.