Titration of the carboxyhemoglobin tetramer-dimer equilibrium by inositol hexaphosphate

The results of a series of light scattering experiments of the reaction of inositol hexaphosphate (at pH 7.0) over 6 orders of magnitude of concentration (10(-8) to 10(-2) M) with carboxyhemoglobin indicates that there is a shift in the tetramer-dimer equilibrium towards the tetramer, reaching a maximum effect at 0.1 mM inositol hexaphosphate. Raising the phosphate concentration beyond this latter value promotes dissociation to dimers. However, in this range some of the dissociation of carboxyhemoglobin was undoubtedly due to the increase in ionic strength from the inositol hexaphosphate ion. If the effect of ionic strength is allowed for by classical Debye-Hückel theory, one- and possibly two-phosphate binding sites per dimer can be detected. (Approximate association constant is 8000 M-1 for a single site at 0.1 ionic strength). The location of such sites is considered to lie near the dissociable plane of the hemoglobin tetramer and possibly to include half of the residues that bind inositol hexaphosphate in the tetramer.

The relationship between erythropoietin-dependent cellular differentiation and colony-forming ability in prenatal haemopoietic tissues

Levels of haem synthesis achieved by foetal liver erythroblasts responding to erythropoietin in vitro are similar in dissociated cell cultures and in cultures of organized tissues. Erythroid colony-forming cells reach maximum numbers on the sixteenth day of gestation. Their presence in foetal liver is associated with the period of most rapid production of erythrocytes, and with in vitro sensitivity to erythropoietin measured as enhanced haem synthesis. It is concluded that at least a proportion of erythroid colony-forming cells in the foetal liver are dependent on erythropoietin in situ and that these cells are separated from the earliest recognizable pro-erythroblast by 1-2 cell divisions. Populations of granulocyte-macrophage colony-forming cells change independently of erythroid colony-forming cell numbers.

Expression of congenital defects in the haemopoietic micro-environment. Erythroid and granulocyte-macrophage progenitor cells in pre-natal 'steel' (Slj/Slj) anaemic mice

The erythropietin sensitivities of dissociated cell cultures and explanted fragments of fetal livers of congenitally anaemic Slj/Slj mice, and their normal littermates, have been compared. The erythropoietin responsiveness of Slj/Slj foetal liver cells is deficient in both types of culture. The maximum liver complement of erythroid colony forming cells (CFUe) occurs on the 16th day of development when 'normal' livers contain approximately 6 X 10(5) erythroid colony forming cells/liver. In Slj/Slj fetuses the maximum reached is only 1 X 10(5). Granulocyte-macrophage colony forming cells (CFUc) in Slj/Slj fetal livers are also reduced to approximately 60% of normal numbers. Erythroid colony forming cells are also reduced in the spleen and femoral bone marrow of Slj/Slj mice in the 2-3 days preceding birth. Granulocyte-macrophage colony forming cells are rare in the femoral marrow of pre-natal Slj/Slj mice, but their production in the Slj/Slj pre-natal spleen appears unaffected.

A microplaque reduction assay for human and mouse interferon

A microplaque reduction assay for human and mouse interferons is described, using plastic disposable multicompartmental plates with flat-bottomed wells of 6 mm in diameter. The procedure is rapid and reproducible and lends itself to a considerable degree of automation. Its potential application in other areas of virology, such as rapid screening of samples for virus content or assay of neutralizing antibodies, is also discussed.

The molecular dissociation of ferrihemoglobin derivatives

Measurement of the dissociation constants of ferrihemoglobin by light scattering indicates that the quaternary structure is altered by the type of heme ligand. Fluoromethemoglobin and aquomethemoglobin, high spin derivatives with weak ligands, have tetramer-dimer dissociation constants of 80 and 50 muM, respectively. For low spin cyanmethemoglobin the dissociation constants were 1 muM (pH 6.0) and 3 muM (pH 9.0) under the general conditions of 0.1 ionic strength and 25 degrees. Of the ferrihemoglobins studied, alkaline methemoglobin (pH 9.0) has the lowest dissociation constant (0.2 muM). Dissociation constants of mixtures of alkaline and fluoromethemoglobin were significantly higher than that of the alkaline form alone. At pH 9.0 the 55 and 78% fluoride-bound derivatives had tetramer-dimer dissociation constants of 0.7 and 2 muM, respectively. The cyanmethemoglobin quaternary conformation was found to be less affected by pH than the fluoromethemoglobin and aquomethemoglobin conformations. Measurement of the dissociation constant (0.2 muM) for aquomethemoglobin-inositol hexaphosphate indicates stabilization of the tetramer by this organic phosphate. The extent of stabilization by inositol hexaphosphate does not appear to be that found for deoxyhemoglobin as suggested by Perutz (Perutz, M. F. (1972) Nature 237, 495-499) even though inducement of higher spin and iron-heme plane displacement may occur.