The uptake of R-type cobalamin-binding protein by isolated rat liver cells

Area of Science:

• Biochemistry
• Cell Biology
• Hepatology

Background:

• Cobalamin (Vitamin B12) is essential for human health.
• R-type cobalamin-binding proteins transport Vitamin B12 in circulation.
• Understanding cellular uptake mechanisms is crucial for nutrient absorption research.

Purpose of the Study:

• To investigate the uptake mechanism of R-type cobalamin-binding proteins by isolated rat liver cells.
• To determine the role of protein sialylation in Vitamin B12 uptake.
• To elucidate the cellular pathway involved in the internalization and degradation of these proteins.

Main Methods:

• Incubation of isolated rat liver cells with radiolabeled [57Co] cyanocobalamin-bound R-type proteins (granulocyte-binding protein, transcobalamin I, and III).
• Treatment with neuraminidase to desialylate proteins.
• Use of inhibitors (EGTA, asialo-orosomucoid, chloroquine, colchicine) to probe uptake pathways.
• Measurement of vitamin uptake and intracellular degradation of binding proteins.

Main Results:

• High uptake (>80%) of cobalamin-bound granulocyte-binding protein and transcobalamin III by liver cells within 1 hour.
• No significant uptake of Vitamin B12 bound to native transcobalamin I; desialylation enabled uptake.
• Uptake of iodinated R-type binding protein led to 100% internalization and subsequent proteolytic degradation.
• EGTA and asialo-orosomucoid completely blocked uptake and degradation, while chloroquine and colchicine partially inhibited it.

Conclusions:

• Rat liver cells internalize R-type cobalamin-binding proteins via the asialoglycoprotein receptor.
• The uptake process involves endocytosis followed by lysosomal proteolysis of the binding protein.
• Desialylation is critical for the uptake of plasma-derived R-type cobalamin-binding proteins.