Purification of intrinsic factor receptor from pig ileum using as affinity medium human intrinsic factor covalently bound to Sepharose

Vitamin B12 absorption and malabsorption

Vitamin B12 is assimilated and transported by complex mechanisms that involve three transport proteins, intrinsic factor (IF), haptocorrin (HC) and transcobalamin (TC) and their respective membrane receptors. Vitamin deficiency is mainly due to inadequate dietary intake in vegans, and B12 malabsorption is related to digestive diseases. This review explores the physiology of vitamin B12 absorption and the mechanisms and diseases that produce malabsorption. In the stomach, B12 is released from food carrier proteins and binds to HC. The degradation of HC by pancreatic proteases and the pH change trigger the transfer of B12 to IF in the duodenum. Cubilin and amnionless are the two components of the receptor that mediates the uptake of B12 in the distal ileum. Part of liver B12 is excreted in bile, and undergoes an enterohepatic circulation. The main causes of B12 malabsorption include inherited disorders (Intrinsic factor deficiency, Imerslund-Gräsbeck disease, Addison's pernicious anemia, obesity, bariatric surgery and gastrectomies. Other causes include pancreatic insufficiency, obstructive Jaundice, tropical sprue and celiac disease, bacterial overgrowth, parasitic infestations, Zollinger-Ellison syndrome, inflammatory bowel diseases, chronic radiation enteritis of the distal ileum and short bowel. The assessment of B12 deficit is recommended in the follow-up of subjects with bariatric surgery. The genetic causes of B12 malabsorption are probably underestimated in adult cases with B12 deficit. Despite its high prevalence in the general population and in the elderly, B12 malabsorption cannot be anymore assessed by the Schilling test, pointing out the urgent need for an equivalent reliable test.

Gastric intrinsic factor: the gastric and small intestinal stages of cobalamin absorption. a personal journey

Intrinsic factor (IF) was first identified as a component of the gastric mucosa that reacted with an extrinsic factor, later discovered to be vitamin B12 (VB12). IF has been extensively characterized, and its cloned cDNA used to produce sufficient IF to produce high quality antibodies, and to elucidate its 3-dimensional structure bound to cobalamin (Cbl, VB12). The absorption of the IF-Cbl complex involves internalization by endocytosis, incorporation into multivesicular/lysosomal bodies, release of Cbl by lysosomal proteolysis and pH effects, with subsequent binding to transcobalamin (TC). Hereditary IF deficiency is rare, consistent with the need for IF to absorb Cbl, a vitamin essential for cell replication. When mutations occur, they are most often associated with loss of function, but some mutations occur outside the coding region. The IF-mediated intestinal uptake of Cbl has been harnessed for use as a transporter for peptides, proteins and even nanoparticles. Nanoparticle (NP) technology has produced Cbl-coated NPs that can incorporate peptides (insulin, IgG) that can be absorbed orally to function as hormones and antibodies in rodent models, but these systems are not yet ready for clinical use.

Plasma membrane receptor for beta-lactoglobulin and retinol-binding protein in murine hybridomas

The aim of the present work was to study the binding of [125I]-BLGA (beta-lactoglobulin variant A) to the plasma membrane fraction of hybrid cells. This binding increased as a function of time with on-rate and off-rate constant at 4.47 +/- 0.18 x 10(6) M-1 min-1 and 0.17 +/- 0.07 min-1, respectively (n = 3). The saturation study showed a single binding site type corresponding to a Kd at 8.26 +/- 2.98 nM and 14.02 +/- 2.61 x 10(12) sites per mg of the plasma membrane protein (n = 3). Competitive of binding BLGA was observed with BLGA, complexed with retinol and also with RBP (retinol-binding protein). Gel filtration of [125I]-BLGA incubated with Triton X-100 solubilized membrane showed the formation of a ligand-receptor complex. Cross-linking of the tracer to plasma membrane showed a complex with a M(r) at 69 kDa, suggesting a receptor M(r) of 51 kDa, as seen by autoradiography of SDS-PAGE.

Retinol free and retinol complexed beta-lactoglobulin binding sites in bovine germ cells

A high affinity specific binding site for bovine beta-lactoglobulin (BLG) was identified in bovine germ cell plasma membrane enriched fractions. Binding was found to be reversible and pH-dependent with maximum binding occurring at pH 5. The on-rate and off-rate constants were 2.26 +/- 0.8 x 10(5) M(-1) min(-1) (n = 3) and 0.016 +/- 0.004 min(-1) (n = 3), respectively. Scatchard analysis showed a single class of binding sites, with 12.38 +/- 4.62 x 10(12) sites per mg of membrane protein (n = 3) and a dissociation constant (K(D)) estimated at 26.43 +/- 2.68 nM. There was inhibition of iodinated-BLG (variant A) (125I-BLGA) binding to germ cell plasma membrane enriched fractions in the presence of unlabelled BLG variant A, BLG variant B, retinol complexed BLGA and human retinol-binding protein. Inhibition was observed neither with BSA nor with lactoferrin. 125I-BLGA incubated with a Triton X-100 solubilized plasma membrane fraction formed a high molecular mass complex in Superose 12B gel filtration. This receptor complex disappeared in the presence of unlabelled BLGA and in the presence of 10 mM EDTA. The results suggest that germ cell plasma membrane may contain a receptor which is capable of binding either retinol free or retinol complexed BLGA.

Purification by cobalamin-Sepharose affinity chromatography and intrinsic factor-binding activity of an extramembrane proteolytic product from pig ileal mucosa

We have purified a cobalamin-binding protein obtained by papain digestion of pig intestine by cobalamin-AH-Sepharose affinity chromatography, with a purification factor of 17,300, a yield of 63% and a cobalamin-binding activity of 11,260 pmol/mg of protein. The protein contained 3.8% carbohydrate and was O- and N-glycosylated. Its molecular mass was 69 kDa on SDS/PAGE and its isoelectric point was 5.1. It had a binding activity for both [57Co]cobalamin and [57Co]cobalamin-intrinsic factor in native PAGE autoradiography and it inhibited the binding of intrinsic factor to the intact intestinal receptor with an IC50 of 49.31 nmol/l in a radioisotope assay. In conclusion, the purified protein shared a binding activity for both cobalamin and intrinsic factor-cobalamin complexes and could correspond to the extracellular domain of the ileal intrinsic factor receptor.

Gastric intrinsic factor and its receptor

Cbl metabolism has been the subject of many studies since the existence of Cbl was suspected in the first decades of the twentieth century. These studies have confirmed the high complexity of the assimilation of Cbl by the organism. During absorption, Cbl is bound to two glycoproteins, Hc and IF, in a consecutive manner. Over the last few years, it has been demonstrated that Cbl bound to Hc in the stomach is only transferred to IF after the action of pancreatic trypsin. It is also possible that Hc-bound biliary Cbl is transferred to IF in this way and that the Cbl in the Cbl-IF complex is absorbed in the terminal ileum, thus constituting an enterohepatic cycle. Knowledge concerning the sites of synthesis and secretion of IF is becoming more detailed due to the use of immunocytochemistry and in situ hybridization techniques. It is now certain that in man, IF is not only localized in gastric parietal cells, but also in other foregut-derived cells. This observation may explain the multiple physiological stimuli involved in mediating IF secretion. Determination of the molecular structure of purified Cbl binders can be added to the significant progress made due to the application of molecular biology techniques to the field of isolation and structural characterization of cDNA encoding Cbl binders, and particularly IF. Studies of IF, Hc and TC in different species and those into the properties of acceptor fragments have allowed the distinction between the Cbl binding site on IF and the IF-Cbl binding site on the IFCR. The absence of experimental models cause difficulties in studying transcytosis of Cbl through the enterocyte. There are also problems in determining the structure of IFCR as it is difficult to obtain a large quantity of a molecule which denatures very quickly. Studies into IFCR expression in polarized cancerous cells of intestinal or renal origin, including the effects of different pharmacological agents, along with the results of immunochemical investigations are beginning to clarify the pathway involved in the transport of Cbl through the enterocyte.

Decreased activity of intestinal and urinary intrinsic factor receptor in Gräsbeck-Imerslund disease [corrected]

BACKGROUND/AIMS

The pathogenesis of inherited intestinal cobalamin malabsorption (Gräsbeck-Imerslund disease) remains unknown. The authors studied whether the disease corresponds to a defective expression and/or function of the intrinsic factor-cobalamin receptor in the ileum.

METHODS

Intrinsic factor-cobalamin receptor activity was measured using radioisotope assay and gel-filtration exclusion chromatography in ileal biopsy specimens and urine concentrates from 4 patients with Gräsbeck-Imerslund disease and 5 controls.

RESULTS

Receptor activity was 164 +/- 13 fmol/mg of protein in control biopsy specimens and < 2.6 fmol/mg protein in specimens from patients. The association constant was estimated to be 3.8 +/- 0.4 (nmol/L)-1 in controls. A dramatic decrease in receptor activity was also observed in urine concentrate from patients with an association constant of 1.9 and 3.3 (nmol/L)-1. Isoelectrofocusing of the cross-linked intrinsic factor-cobalamin receptor complex showed an isoelectric point at 4.8 in a patient as well as in control samples.

CONCLUSIONS

It is concluded that Gräsbeck-Imerslund disease is related to decreased intrinsic factor-receptor activity in intestinal mucosa; the receptor assay in urine can be helpful for diagnosis.

Intrinsic factor covalently bound to Sepharose as affinity medium for the purification of a soluble intrinsic factor receptor from human urine

We have identified a soluble receptor for intrinsic factor (IF) in human urine. The purification of this protein by affinity chromatography required a preliminary purification of IF from hog pyloric mucosal extract. This was achieved by thermolabile cobalamin-ethanol-aminohexane Sepharose affinity chromatography with a 133-fold purification, a yield of 45% and a specific binding activity of 15720 pmol/mg protein. The purified Cbl-IF complex was coupled to epoxy-Sepharose with a yield of 23.8% and a specific activity of 1.2 nmol per mol of gel. The soluble IF receptor was purified form 200 ml of urine concentrate of pregnant women. Desorption was performed at pH 5.0 and in the presence of 5 mM EDTA. The soluble IF receptor was purified 17,200-fold with a yield of 52% and a IF binding capacity of 3260 pmol per mg of protein. A single protein with a Mr of 70,000 was found in silver-stained SDS-PAGE.

In vitro expression and secretion of functional mammalian intrinsic factor using recombinant baculovirus

Intrinsic factor was produced at levels of 1-2 mg per 1 (0.25 micrograms per 10(6) cells) by growth of recombinant baculovirus-infected Sf9 cells in spinner culture. The recombinant IF showed a binding affinity for cobalamin (2.6.10(-10) M) and for the intrinsic factor-cobalamin receptor (3.5.10(-10) M) nearly identical with native IF. Purification of the recombinant intrinsic factor could be accomplished by affinity chromatography, but final purification by gel chromatography (FPLC) was necessary to separate intrinsic factor from a 62 kDa protein secreted from uninfected Sf9 cells. This protein binds selectively to the cobalamin-Sepharose column, but demonstrates no cobalamin binding activity after elution. Microgram quantities of radiolabelled protein could be produced for metabolic and autoradiographic studies. The stability of intrinsic factor to pancreatic proteinases was nearly identical with human gastric intrinsic factor, both native and recombinant as produced in mammalian cells. Glycosylation of the intrinsic factor was demonstrated by lectin binding to the recombinant protein separated on SDS-PAGE, and by a shift in apparent molecular mass from 47 kDa to 43 kDa following treatment of Sf9 cells with tunicamycin. Most of the recombinant IF was produced by Sf9 cells in the first 48 h post infection.

Intrinsic factor receptor during fetal development of the human intestine

Intrinsic factor receptor activity was observed in mucosal homogenates from whole small intestine and colon of 10-19-week fetuses, whereas it was only detected in the distal part of the small intestine of a 25-week fetus. The receptor-intrinsic factor-cobalamin complex was eluted into the void-volume position when ileum mucosal extract was assayed for receptor activity by gel filtration after incubation with either fetal gastric extract or human gastric juice. The intrinsic-factor-binding capacity of intestinal mucosal extracts ranged from 2.6 to 30.5 fmol/mg and was correlated with the gestational age of six fetuses. The dissociation constant of the receptor for the intrinsic factor-cobalamin complex was estimated at 0.24-0.36 nM at pH 7.4. In conclusion, intrinsic-factor-receptor activity was detected in the whole intestine in 10-19-week fetuses, whereas it was only present in the distal ileum at the end of fetal development.

Binding assay and physicochemical characteristics of solubilized intrinsic factor receptor in ileal mucosal homogenates using phenyl-Sepharose to separate the saturated receptor from free intrinsic factor

A radioisotopic assay was set to determine the physicochemical properties of the solubilized intrinsic factor receptor in pig mucosal extracts. In this assay, phenyl-Sepharose was used to separate the receptor-intrinsic factor-labelled cobalamin complex from the free saturated intrinsic factor. The association constant (at pH 7.4) of the receptor-intrinsic factor complex was estimated at 3.4 +/- 0.3 nM-1. Adsorption of the apo-receptor to phenyl-Sepharose allowed its binding site to be made accessible to intrinsic factor with an association constant in order of 6 nM-1. The receptor binding activity obtained with five mucosal extracts was closely correlated with that obtained by gel filtration of the intrinsic factor-receptor complex (r = 0.99). The radioisotope assay was used to detect the unsaturated receptor (apo-receptor) in sucrose density ultracentrifugation and in superose 6 gel filtration. The sedimentation coefficient was 9.5 s. The apo-receptor was eluted in three peaks in gel filtration, corresponding to the formation of oligomers. The peak of the monomer was increased in presence of EDTA. Its molecular mass was estimated at 270 kDa and its Stokes radius at 5.9 nm. It was concluded that calcium is involved in the oligomerisation of the apo-receptor.