Assessment of a radioisotopic assay for vitamin B12 using an intrinsic factor preparation with R proteins blocked by vitamin B12 analogues.

A competitive protein binding radioassay kit for serum vitamin B(12) has been assessed. Precision, linearity, sensitivity, and specificity have been found to be satisfactory. Falsely-normal assay results in patients with vitamin B(12) deficiency have not been observed.

Neutrophils autoinactivate secretory products by myeloperoxidase-catalyzed oxidation.

The neutrophil response to inflammatory stimuli involves the formation of reactive oxygen species and secretion of granule enzymes. In studying secretion of vitamin B12 binding protein by human neutrophils, we noted a major decrease in total recoverable activity from the extracellular fluid plus the stimulated cells (54% of resting cells). Recovery of B12 binding protein from neutrophils exposed to phorbol myristate acetate or opsonized zymosan was significantly enhanced on addition of heme enzyme inhibitors (azide, cyanide) or catalase or when halide-free medium was used. The changes in B12 binding protein recovery were attributable entirely to increases in extracellular fluid levels, and cell pellet content was unaffected. These data indicate extracellular destruction of functional B12 binding protein by the halide-dependent heme enzyme myeloperoxidase and H2O2. Kinetic studies demonstrated rapid secretion of B12 binding protein in the first two to five minutes, followed by its inactivation over the next 20 to 30 minutes. A cell-free extract of vitamin B12 binding protein was readily inactivated on exposure to purified myeloperoxidase, H2O2, and a halide. These findings document a functional interaction among products of the neutrophil specific granules (B12 binding protein), azurophil granules (myeloperoxidase), and metabolic burst (H2O2). They provide an interesting model for the modulation of the inflammatory response by oxidation of secretory products of neutrophils.

Inhibitory influence of methotrexate and vincristine on the release of cobalophilins from polymorphonuclear granulocytes.

The studies have evaluated the effect of methotrexate and vincristine on the release of cobalophilins (vitamin B12 binding proteins) from resting and functionally stimulated polymorphonuclear granulocytes (PMN). Methotrexate (2.5 micrograms/ml; 5.0 micrograms/ml; 20.0 micrograms/ml; and 50.0 micrograms/ml) and vincristine (0.3 microgram/ml; 0.6 microgram/ml; 2.4 micrograms/ml; and 6.0 micrograms/ml) inhibited the cobalophilins release from resting granulocytes. This effect increased with growing concentrations of these drugs. Stimulated PMN could be shown to release cobalophilins more actively than resting granulocytes. Methotrexate (2.5 micrograms/ml; 5.0 micrograms/ml and 20.0 micrograms/ml) and vincristine (0.3 microgram/ml; 0.6 microgram/ml and 2.4 micrograms/ml) inhibited the phagocytosis-activated release of cobalophilins irrespective of the time of PMN stimulation, i.e. before or after being incubated with latex particles.

Antibodies to transcobalamin II block in vitro proliferation of leukemic cells.

The plasma protein transcobalamin II (TCII) binds and delivers cobalamin (Cbl; vitamin B12) to all cells, which internalize the TCII/Cbl complex by receptor-mediated endocytosis. Congenital deficiency of TCII results in intracellular Cbl deficiency, one effect of which is to disrupt DNA synthesis, leading to megaloblastic anemia. We report here an in vitro culture system in which cell growth is dependent on delivery of Cbl to cells by TCII. Recombinant human holo-TCII was shown to support in dose-dependent manner the growth of the human erythroleukemic cell line K562 and the murine lymphoma cell line BW5147. Free Cbl also supported cell growth; however, at 100- to 1,000-fold higher concentrations than those effective in the presence of apo-TCII. To determine if cellular depletion of Cbl could be achieved by interfering with interactions between TCII/Cbl and its cell-surface receptor, several monoclonal antibodies raised against human TCII were studied. Three antibodies, found to compete for the same binding site on TCII, proved to be effective inhibitors of TCII/Cbl-dependent cell growth. Our results suggest that monoclonal anti-TCII antibodies that block the function of this protein may prove useful in antitumor therapies.