Or (TC-R, CD320), the TCB12 complex is internalised into 
the lysosomes as illustrated 15900046 in Figure 1. Inside the lysosome, TC is degraded and the liberated B12 is transported into the cytoplasm by the lysosomal membrane transporter 1 (LMBRD1). Intracellular B12 serves as cofactor for the mitochondrial methylmalonyl-CoA mutase (MUT) and the cytosolic methionine synthase (MS) that acts in coordination with methylenetetrahydrofolate reductase (MTHFR) for the conversion of homocysteine (HCY) to methionine. B12 depletion will cause an increase in the metabolites methylmalonic acid (MMA) and HCY as their enzymatic conversion are reduced. The consequences ofB12 Deslorelin biological activity deficiency in humans include megaloblastic anaemia and neurological abnormalities [1], while the Calyculin A consequence in model animals such as rodents to our knowledge is unsettled. Several researchers have suggested use of the B12 transport system to deliver B12-conjugated drugs and nano particles into cells [5?], especially in the case of life-long treatments such as those for diabetic patients. This strategy may lead to a B12 overload of the cells if the B12-conjugate is cleaved to form conjugate and metabolic active B12 inside the cell or to interference with B12 metabolism if this is not the case. However, at present we do not know to what extent an overload with active or inactive B12 influences the B12 metabolism. Since mouse TC recognises both B12 and the biologically inert B12 analogue Cbi [2]. we were able to study an overload of the B12 transport system induced by B12 or Cbi. We report that infusion of Cbi leads to depletion of B12 from mice tissues and that overload with B12 leads to alterations in the metabolism of HCY.Materials and Methods Ethics statementAll animal experiments were carried out in accordance with provisions for the animal care license provided by the Danish National Animal Experiments Inspectorate. The protocol was approved by the Danish National Animal Experiments Inspectorate (provision no 2010/561?855). Surgery was performed underOverload of the B12 Transport System in Miceml) for 3 days post operation. 27 days after insertion of the osmotic minipumps, mice were anaesthetised using isoflurane. Following, they were sacrificed by exsanguination followed by cutting of the thorax and heart.Urine and blood collectionThe mice were weighed just after insertion of the pumps (day 1) and on days 5, 12, 19, and 27. Prior to urine collection, each mouse received an intraperitoneal injection of 250 mL 0.9 NaCl to increase urine output and were placed in individual, metabolic cages for 24 h with free access to water. 1 mL 20 Na-azide was added to each urine collection tube to prevent bacterial growth. Following urine collection, blood was sampled from the sublingual vein (,50 mL total volume) [9]. Blood samples were heparinised, centrifuged at 4,000 g for 8 min at room temperature, and the plasma fraction was collected. Urine and plasma were stored at 220uC. On the day of sacrifice (day 27), mice were anaesthetised with isoflurane and blood was collected from the inferior caval vein. One aliquot (,200 mL) of blood from each mouse was transferred to a dry EDTA tube for haematological analysis. The remaining blood was collected in heparinised tubes, and plasma was prepared as described above.Figure 1. Simplified overview of B12 metabolism. A simplified overview of vitamin B12 (B12) metabolism in mice with focus on the analysed parameters. Through binding to the transcobalamin re.Or (TC-R, CD320), the TCB12 complex is internalised into the lysosomes as illustrated 15900046 in Figure 1. Inside the lysosome, TC is degraded and the liberated B12 is transported into the cytoplasm by the lysosomal membrane transporter 1 (LMBRD1). Intracellular B12 serves as cofactor for the mitochondrial methylmalonyl-CoA mutase (MUT) and the cytosolic methionine synthase (MS) that acts in coordination with methylenetetrahydrofolate reductase (MTHFR) for the conversion of homocysteine (HCY) to methionine. B12 depletion will cause an increase in the metabolites methylmalonic acid (MMA) and HCY as their enzymatic conversion are reduced. The consequences ofB12 deficiency in humans include megaloblastic anaemia and neurological abnormalities [1], while the consequence in model animals such as rodents to our knowledge is unsettled. Several researchers have suggested use of the B12 transport system to deliver B12-conjugated drugs and nano particles into cells [5?], especially in the case of life-long treatments such as those for diabetic patients. This strategy may lead to a B12 overload of the cells if the B12-conjugate is cleaved to form conjugate and metabolic active B12 inside the cell or to interference with B12 metabolism if this is not the case. However, at present we do not know to what extent an overload with active or inactive B12 influences the B12 metabolism. Since mouse TC recognises both B12 and the biologically inert B12 analogue Cbi [2]. we were able to study an overload of the B12 transport system induced by B12 or Cbi. We report that infusion of Cbi leads to depletion of B12 from mice tissues and that overload with B12 leads to alterations in the metabolism of HCY.Materials and Methods Ethics statementAll animal experiments were carried out in accordance with provisions for the animal care license provided by the Danish National Animal Experiments Inspectorate. The protocol was approved by the Danish National Animal Experiments Inspectorate (provision no 2010/561?855). Surgery was performed underOverload of the B12 Transport System in Miceml) for 3 days post operation. 27 days after insertion of the osmotic minipumps, mice were anaesthetised using isoflurane. Following, they were sacrificed by exsanguination followed by cutting of the thorax and heart.Urine and blood collectionThe mice were weighed just after insertion of the pumps (day 1) and on days 5, 12, 19, and 27. Prior to urine collection, each mouse received an intraperitoneal injection of 250 mL 0.9 NaCl to increase urine output and were placed in 
individual, metabolic cages for 24 h with free access to water. 1 mL 20 Na-azide was added to each urine collection tube to prevent bacterial growth. Following urine collection, blood was sampled from the sublingual vein (,50 mL total volume) [9]. Blood samples were heparinised, centrifuged at 4,000 g for 8 min at room temperature, and the plasma fraction was collected. Urine and plasma were stored at 220uC. On the day of sacrifice (day 27), mice were anaesthetised with isoflurane and blood was collected from the inferior caval vein. One aliquot (,200 mL) of blood from each mouse was transferred to a dry EDTA tube for haematological analysis. The remaining blood was collected in heparinised tubes, and plasma was prepared as described above.Figure 1. Simplified overview of B12 metabolism. A simplified overview of vitamin B12 (B12) metabolism in mice with focus on the analysed parameters. Through binding to the transcobalamin re.