OBSERVATIONS ON THE BOUND FORM OF VITAMIN B12 IN HUMAN SERUM

Limitations of protein-coated charcoal in the separation of free from bound vitamin B12 in serum

The effect of haemoglobin and albumin-coated charcoal on the concentration of vitamin B12 binding proteins in serum has been investigated. As commonly employed, coated charcoal removes a significant amount of transcobalamin II (TCII) from serum, but does not affect transcobalamin I and III (TCI and III). Increasing the protein coat up to about 10 mg of protein per 25 mg of charcoal reduces the adsorption of TCII, but increasing the protein concentration beyond this has little added effect. The amount of adsorption is proportional to the amount of coated charcoal employed, but even small amounts adsorb some TCII. These results indicate that protein-coated charcoal is not the ideal way of separating free from bound vitamin B12 in serum; it cannot reliably be used for the measurement of the concentration of apo-TCII but can be employed for the measurement of apo-TCI and III.

Determination of hematocrit based on diffusion of an inert molecular probe from agarose gels into whole blood

Whole blood hematocrit was determined by an approach which depends on the diffusion of an inert probe, to which red blood cells are impermeable, from a small agarose gel into a stirred, much larger blood sample. Blood cells influence the diffusion rate of the probe by, on the average, physically blocking a fraction of the gel surface. The blocking effect increases with the hematocrit. Cyanocobalamin (B-12) was found to be a suitable probe because it did not penetrate, bind to, or lyse blood cells and was not bound by plasma solutes. The loss of B-12 from gels in contact with blood was monitored by determination of the absorbance change at 540 nm of gels which had been quickly rinsed. The visible spectrum of B-12 in agarose gels was identical to the spectrum in water. Beer's Law was obeyed in 1-mm thick agarose gels over a concentration range of 0.1-0.8 mM. Based on the results from 48 blood samples covering the hematocrit range 25-69, a least-squares line was generated with a slope, -3.46 X 10(-3) delta A/hematocrit unit, a Y intercept of 0.295, and a correlation coefficient of 0.971. The precision of the technique was +/- 9.7%. The assay was insensitive to mean corpuscular volume and sample volume as long as the latter was 50-fold larger than the gel volume. The diffusion coefficient for B-12 in 1% agarose gels was found to be 1.4 +/- 0.2 X 10(-6) cm2 sec-1.

Vitamin B12 binders (transcobalamins) in serum

The measurement of plasma or serum vitamin B12 (B12) binders is important in the diagnosis of congenital megaloblastic anemias, the myeloproliferative disorders and perhaps in other malignancies. Two classes of binders circulate in plasma, Transcobalamin II (TC II) and R-binders. The latter have been divided by some authors into transcobalamin I (TC I) and transcobalamin III (TC III), and the validity of this division is discussed. R-binders and TC II differ in their apparent molecular weight on gel filtration chromatography, by which method they can be separated reliably. However, the technique is time-consuming and cumbersome and a variety of rapid separation methods have been described in the literature. These are discussed and compared to the standard gel filtration separation method. TC I and III are separable on the basis of their differing charges, and the methods which have been described for accomplishing this are compared and critically reviewed. There has been some controversy in the literature as to whether plasma or serum should be used to measure circulating B12 binders. Leukocytes, which contain R-binders, release these in vitro and this release is greatest when blood is allowed to clot and the serum separated. Consequently, the use of plasma, sometimes with agents added to prevent leukocytic release of binder, has been advocated. Yet, none of the agents used eliminates artifact totally, and this aspect, too, is reviewed. Lastly, several techniques for the purification of B12 binders have been described. Some techniques result in pure binder preparations, while others result in preparations which are free of other binders but contaminated with non-B12 binding proteins. Both approaches have advantages and disadvantages, and these are discussed.

Newer aspects of pernicious anemia

Although readily treatable with vitamin B12, pernicious anemia continues to captivate investigative endeavors of those interested in the pathophysiology and pathogenesis of this disorder. Notable advances have been made in understanding properties of intrinsic factor, vitamin B12-binding proteins, structure and de novo synthesis of vitamin B12, mechanism of action of vitamin B12-dependent enzymes in man, and metabolic consequences of reduced activities of these enzymes in pernicious anemia. Similarly, newer morphological observations have given information regarding pathogenesis of some of the cytological abnormalities found in megaloblasts, and recent cytochemical studies have shed light on abnormalities of nuclear and cytoplasmic constituents in vitamin B12-deficient cells. Both cellular and humoral factors may contribute to immune-mediated processes in pernicious anemia, although as yet, it has not been established with certainty that pernicious anemia is an autoimmune disorder. As we look ahead, it will be important to define the process or processes responsible for atrophic gastritis, which is the pathophysiological basis of pernicious anemia. Likewise, advances in biophysics used in the study of cell membranes, cell surface phenomena, and metallic ion transport may find applicability in the study of pernicious anemia and perhaps provide further insights into metabolic abnormalities responsible for the development of megaloblastosis.

The three transcobalamins in myeloproliferative disorders and acute leukaemia

The unsaturated vitamin B12 binding capacity of whole serum (UBBC) and of the three transcobalamins (TC) has been studied in patients with various haematological diseases including myeloproliferative disorders (MPD) and acute leukaemia. The binding capacity of TC I and TC III was increased in MPD; TC I being particularly high in chronic granulocytic leukaemia (CGL) and TC III especially raised in polycythaemia rubra vera (PRV) and in infectious leucocytosis. The binding capacity of both TC I and TC III correlated with blood neutrophil count and the ratio TC III/TC I was low in CGL and increased in PRV. TC II was increased in acute myelogenous leukaemia, during remission and blast cell crisis of CGL and in refractory anaemia with excess of myeloblasts but not in acute lymphoblastic leukaemia (ALL). TC II correlated inversely with blood neutrophil count. There is an inverse ratio between TC II and TC I at least in myelogenous leukaemia. These abnormalities are discussed in relation to granulocyte kinetics. TC III and TC I reflect probably the total body granulocyte pool and share some biochemical and immunological properties supporting the view that they have a common origin in the more mature stages of the granulocyte cell line while TC II probably originates partly in more primitive granulocytes.

Tissue vitamin B-12 assay by a radioisotope dilution technique

A method is described for the measurement of vitamin B-12 (B-12) in solid tissues by radioisotope dilution (RID) assay. The method is a modivication using double extraction of a serum B-12 RID assay which uses chicken serum as the B-12 binder. The method was developed and tested using human and bat liver specimens. Double extraction was shown to be more efficient than single extraction, and resulted in complete release of endogenous liver [57Co] B-12 administered to bats. Results using the RID assay in 16 humans and 17 bat liver specimens were compared with those obtained using the Lactobacillus leichmannii microbiological assay. Correlation was good, but the RID assay gave higher results using the microbiological assay appear to be due to inadequate extraction of B-12 from tissues.

Vitamin B12-binding proteins of r-type, cobalophilin

A gradient was developed for isoelectric focusing in the pH range 2-5. Cobalophilin (earlier called R proteins or vitamin B12-binding proteins of R-type) was isolated from saliva and amniotic fluid in homogeneous form. It was found to be a glycoprotein with a molecular weight of 59,300-69,100. The preparation from amniotic fluid contained 33% carbohydrate. Cobalophilin variants in plasma, serum, granulocytes, platelets, amniotic fluid, milk, saliva and gastric juice were characterized by isoelectric focusing. Most fluids and cells contained the same isoproteins, with pI values between 2.3 and 5.0. Isoproteins of presumably myelogenic origin (e.g. those in granulocytes and plasma) had pI values below 4.2, whereas those of glandular origin (in milk and saliva) had a pI range of 4.0-5.0. Serum contained more cobalophilin than plasma, owing to release of this protein from granulocytes during clotting. This phenomenon also changed the isoprotein pattern. Plasma and serum from newborn infants and from patients with leucocytosis, polycythaemia vera and chronic myelogenous leukaemia contained the same isoproteins as were found in plasma from healthy subjects. In addition to these, isoproteins with lower than 'normal' pI values were often found in chronic myelogenous leukaemia and occasionally in leucocytosis. It is concluded that cobalophilin from different fluids and cells is a single microheterogeneous protein with a variable carbohydrate composition. The distribution of cobalophilin in different body fluids and cells supports the suggestion that cobalophilin is an antimicrobial protein (Gullberg 1972) like lactoferrin and lysozyme.

THE VITAMIN B12 CONTENT OF HUMAN LIVER TISSUE OBTAINED BY ASPIRATION BIOPSY

It is possible to estimate the vitamin B12 content of liver specimens obtained by needle biopsy. The liver B12 content is not related to the serum levels of vitamin B12, bilirubin, alkaline phosphatase, glutamic-oxaloacetic transaminase, or albumin. It is reduced in a number of pathological conditions of the liver and the reasons for this are discussed, as also are the factors determining the content of B12 in the normal liver.