SNF472, a novel inhibitor of vascular calcification, could be administered during hemodialysis to attain potentially therapeutic phytate levels

BACKGROUND

Cardiovascular calcification (CVC) is a major concern in hemodialysis (HD) and the loss of endogenous modulators of calcification seems involved in the process. Phytate is an endogenous crystallization inhibitor and its low molecular mass and high water solubility make it potentially dialyzable. SNF472 (the hexasodium salt of phytate) is being developed for the treatment of calciphylaxis and CVC in HD patients. We aimed to verify if phytate is lost during dialysis, and evaluate SNF472's behaviour during dialysis.

METHODS

Dialyzability was assessed in vitro using online-hemodiafiltration and high-flux HD systems in blood and saline. SNF472 was infused for 20 min and quantified at different time points.

RESULTS

Phytate completely dialyzed in 1 h at low concentrations (10 mg/l) but not when added at 30 or 66.67 mg/l SNF472. In bypass conditions, calcium was slightly chelated during SNF472 infusion but when the system was switched to dialysis mode the calcium in the bath compensated this chelation.

CONCLUSION

Phytate dialyses with a low clearance. The administration of SNF472 as an exogenous source of phytate allows to attain supra-physiological levels required for its potential therapeutic properties. As SNF472 is infused during the whole dialysis session, the low clearance would not affect the drug's systemic exposure.

A moderate increase in dietary zinc reduces DNA strand breaks in leukocytes and alters plasma proteins without changing plasma zinc concentrations

BACKGROUND

Food fortification has been recommended to improve a population's micronutrient status. Biofortification techniques modestly elevate the zinc content of cereals, but few studies have reported a positive impact on functional indicators of zinc status.

OBJECTIVE

We determined the impact of a modest increase in dietary zinc that was similar to that provided by biofortification programs on whole-body and cellular indicators of zinc status.

DESIGN

Eighteen men participated in a 6-wk controlled consumption study of a low-zinc, rice-based diet. The diet contained 6 mg Zn/d for 2 wk and was followed by 10 mg Zn/d for 4 wk. To reduce zinc absorption, phytate was added to the diet during the initial period. Indicators of zinc homeostasis, including total absorbed zinc (TAZ), the exchangeable zinc pool (EZP), plasma and cellular zinc concentrations, zinc transporter gene expression, and other metabolic indicators (i.e., DNA damage, inflammation, and oxidative stress), were measured before and after each dietary-zinc period.

RESULTS

TAZ increased with increased dietary zinc, but plasma zinc concentrations and EZP size were unchanged. Erythrocyte and leukocyte zinc concentrations and zinc transporter expressions were not altered. However, leukocyte DNA strand breaks decreased with increased dietary zinc, and the level of proteins involved in DNA repair and antioxidant and immune functions were restored after the dietary-zinc increase.

CONCLUSIONS

A moderate 4-mg/d increase in dietary zinc, similar to that which would be expected from zinc-biofortified crops, improves zinc absorption but does not alter plasma zinc. The repair of DNA strand breaks improves, as do serum protein concentrations that are associated with the DNA repair process. This trial was registered at clinicaltrials.gov as NCT02861352.

There is no 'Conundrum' of InsP6

Indirect assays have claimed to quantify phytate (InsP6) levels in human biofluids, but these have been based on the initial assumption that InsP6 is there, an assumption that our more direct assays disprove. We have shown that InsP6 does not and cannot (because of the presence of an active InsP6 phosphatase in serum) exist in mammalian serum or urine. Therefore, any physiological effects of dietary InsP6 can only be due either to its actions in the gut as a polyvalent cation chelator, or to inositol generated by its dephosphorylation by gut microflora.

Dietary fiber intake increases the risk of zinc deficiency in healthy and diabetic women

Phytic acid is a major determinant of zinc bioavailability. Little is known about phytic acid intakes or indices of zinc bioavailability in type 2 diabetes mellitus (DM), a condition that predisposes to zinc deficiency. The aim of this cross-sectional study was to measure and explore the relationships among phytic acid intake, zinc bioavailability, and molecular markers of zinc homeostasis in 20 women with DM compared to 20 healthy women. The phytate/zinc, (calcium)(phytate)/zinc, and (calcium + magnesium)(phytate)/zinc molar ratios were used to indicate zinc bioavailability. Plasma zinc concentrations and zinc transporter (ZnT1, ZnT8, and Zip1) gene expression in mononuclear cells were measured. Participants with DM consumed 1,194 ± 824 mg/day (mean ± SD) phytic acid, an amount similar to the intake of healthy women (1,316 ± 708 mg/day). Bread products and breakfast cereals contributed more than 40 % of the phytic acid intake in each group. A positive relationship was observed in all participants between phytic acid and dietary fiber (r = 0.6, P < 0.001) and between dietary fiber and the (calcium)(phytate)/zinc ratio (r = 0.5, P < 0.001). Compared to the healthy group, the messenger RNA ratio of ZnT1 (zinc export) to Zip1 (zinc import) was lower in participants with DM, which may indicate perturbed zinc homeostasis in the disorder. The plasma zinc concentration was not predicted by age, body mass index, health status, zinc bioavailability, or zinc transporter expression. Healthy and diabetic women consume phytic acid in amounts that are likely to decrease the bioavailability of dietary zinc. Recommendations to consume greater amounts of dietary fiber, much of which is associated with phytate, increase the risk of zinc deficiency.

Determination of phytic acid by gas chromatography-mass spectroscopy: application to biological samples

A GC-MS method is reported for the determination of phytic acid based on purification by anion-exchange chromatography, enzymatic hydrolysis of phytic acid to myo-inositol and derivation to trimethylsilyl derivative, with scyllo-inositol as an internal standard. Analytical features of the method are: limit of detection 9 microg l(-1) phytic acid, linear working range 18-500 microg l(-1) phytic acid, and coefficient of variation 1.9%. The method has been successfully applied to a variety of biological samples: various rat organs (kidney, liver, brain and bone), human plasma and urine and kidney stones. A comparative study of sample treatments, including deproteization, lipid extraction and the presence of a chelator, is also reported. Phytic acid amounts found in rat organs ranged from 1.07 g kg(-1) for bone to 32.0 g kg(-1) for brain. Phytic acid in human plasma was of the order of 0.14 mg l(-1). In kidney stones, phytic acid was found in calcium containing stones.

Absorption and excretion of orally administered inositol hexaphosphate (IP(6) or phytate) in humans

A study of the pharmacokinetic profile (oral absorption and renal excretion) of inositol hexaphosphate or phytate (IP(6)) is presented. Seven healthy volunteers were following a IP(6) poor diet (IP(6)PD) in a first period, and on IP(6) normal diet (IP(6)ND) in a second one. When following the IP(6)PD they become deficient in IP(6), the basal levels found in plasma (0.07+/- 0.01 mg/L) being clearly lower than those found when IP(6)ND was consumed (0.26+/- 0.03 mg/L). During the restriction period the maximum concentration in plasma were obtained 4 h after the ingestion of a single dose of IP(6), observing almost the same renal excretion profiles for the three different commercial sources and doses. After the IP(6) restriction period, volunteers were on IP(6)ND, reaching normal plasma and urinary IP(6) values in 16 days. Thus, the normal plasma and urinary concentrations, can be obtained either by consumption of a IP(6)ND taking a long time or in a short period by IP(6) supplements.

Engineered erythrocytes: influence of P50 rightward shift and oxemia on oxygen transport to tissues

The red blood cell (RBC) membrane may be reversibly opened using a lysis-resealing continuous flow method. The technology was adapted to the internalisation of an allosteric effector of haemoglobin, Inositol-Hexaphosphate (IHP). This molecule, occupying the allosteric site of 2,3 Bis-Phosphoglycerate with a very large affinity, induces a rightward shift of the oxyhaemoglobin dissociation curve (ODC). From ODC parameters in human volunteers, the potential effect of P50 (oxygen pressure at 50% haemoglobin saturation) on oxygen exchangeable fraction (OEF%), for various oxygen partial pressures (oxemia) was evaluated. For hyperoxic or normoxic arterial oxygen pressure (paO2), rightward shift greatly improved OEF%. In optimised conditions, engineered erythrocytes were potentially able to deliver two to three times more oxygen than normal cells. For patients with decreased paO2, as observed in chronic obstructive pulmonary deficiency (COPD), the reduction in arterial oxygen saturation (saO2%) reduces the benefit of the treatment for paO2 values between 60 and 80 mmHg. Below 60 mmHg, the saO2% reduction cannot be compensated by a corresponding reduction in svO2%, particularly for organs with physiologically low svO2%. In these organs, deleterious effects could be observed for a very large rightward shift of the ODC. Such engineered cells have unique properties for oxygen transport improvement and may be used for the treatment of patients suffering from diseases associated with hypoxia and ischemia.

Internalization and distribution of inositol hexakisphosphate in red blood cells

Inositol hexakisphosphate (InsP6), an allosteric effector of haemoglobin, is able to modify the oxyhaemoglobin dissociation curve. The rightwards shift of the curve increases the in vivo oxygen delivery to tissues. Such an exogenous substance may be internalized into red blood cells (RBC) using a reversed lysis-resealing process following a hypoosmotic shock, resulting in InsP6-RBC with modified oxygen transport capacity. The efficacy of the process depends on various physicochemical parameters which can be fixed during the experimental protocol. The variability of InsP6 internalization from one sample to another appeared to be mainly due to the natural variation in osmotic fragility of RBC. This factor was also modified during the storage of RBC units before the lysis-resealing process. The separation of InsP6-RBC on a density gradient revealed a wide heterogeneity of internalized InsP6 concentration, varying with the degree of osmotic shock. The control of these various parameters will result in resealed InsP6-RBC in reproducible conditions suitable for in vivo use.

Low-oxygen-affinity red cells produced in a large-volume, continuous-flow electroporation system

BACKGROUND

Human red cells containing inositol hexaphosphate (IHP) have a lowered O2 affinity, though they are able to bind and carry about the same amount of oxygen as native cells. These modified cells therefore deliver oxygen more efficiently to the tissues, which is a property of potential clinical utility. Investigators set out to devise a system and procedure by which large volumes of IHP-containing red cells, suitable for transfusion, could be produced quickly and efficiently.

STUDY DESIGN AND METHODS

The encapsulation of IHP into human red cells by use of several variations of static electroporation was performed to define the conditions necessary for optimal IHP incorporation and cell survival. These conditions were used as a starting point for optimization of a flow electroporation system.

RESULTS

When fresh human red cells in a 35 mM IHP solution are subjected to three exponential pulses of field strength of 2.98 +/- 0.064 kV per cm per pulse and pulse length of 2.0 +/- 0.2 msec per pulse while flowing through a cooled electroporation chamber, the condition of the resultant cells, according to the criteria used here, is optimized. After storage for 24 hours in plasma at 37 degrees C, the cells show more than 85-percent survival (in vitro) and hematologic indices nearly identical to those of unpulsed control cells. The p50 value of these cells, however, has doubled to 50.4 +/- 2.0 torr. The processing time for 1 unit of blood is 90 minutes.

CONCLUSION

These data indicate that the system described here can efficiently produce low-oxygen-affinity red cells in volumes that are useful in clinical applications.

[3H]phytic acid (inositol hexaphosphate) is absorbed and distributed to various tissues in rats

To understand the mechanism of antineoplastic action of phytic acid, we investigated the absorption and distribution of myo-[inositol-2-3H(N)] hexakisphosphate in rats. The radioactivity was measured in urine, feces, blood, gastrointestinal tract contents and various organs and tissues at 1 and 24 h after intragastric administration. Of the total radioactivity, 79.0 +/- 10.0% was absorbed and at least 26.6% was degraded during the 24-h period following ingestion. The absorption was rapid; 11.0 +/- 2.6% of the radioactivity was detected in the wall of the stomach (4.4 +/- 3.7%) and upper small intestine (6.6 +/- 1.9%), 6.5 +/- 2.6% in the skeletal muscle and 4.0 +/- 1.5% in the skin after 1 h. Much of the radioactivity after 24 h was in the liver (4.0 +/- 0.9%), kidneys (2.2 +/- 1.1%), muscle (18.1 +/- 3.4%) and skin (10.1 +/- 3.3%). Analysis of plasma and urine demonstrated that most of the radioactivity was due to myo-inositol and small amounts of inositol monophosphate (InsP1). Gastric epithelial cells, however, contained inositol and various inositol phosphates (InsP1-6). Our data suggest that soluble InsP6 when administered in drinking water is rapidly absorbed through the stomach and upper small intestine, becomes quickly dephosphorylated within the mucosal cells and is distributed to various organs as inositol and InsP1.

Isolation and characterization of the triply oxidized derivative of a cross-linked hemoglobin

Hemoglobin A, cross-linked between Lys 99 alpha 1 and Lys 99 alpha 2, was used to obtain a partially oxidized tetramer in which only one of the four hemes remains reduced. Because of the absence of dimerization, asymmetric, partially oxidized derivatives are stable. This is evidenced by the fact that eight of the ten possible oxidation states could be resolved by analytical isoelectric focusing. A triply oxidized hemoglobin population HbXL+3 was isolated whose predominant component was (alpha + alpha +, beta + beta 0). This triferric preparation was examined as a possible model for the triliganded state of ferrous HbA. The aquomet and cyanomet derivatives were characterized by their CD spectra and their kinetic reactions with carbon monoxide. CD spectra in the region of 287 nm showed no apparent change in quaternary structure upon binding ligand to the fourth, ferrous heme. The spectra of the oxy and deoxy forms of the cyanomet and aquomet derivatives of HbXL+3 differed insignificantly and were characteristic of the normal liganded state. Upon addition of inositol hexaphosphate (IHP), both the oxy and deoxy derivatives of the high-spin triaquomet species converted to the native deoxy conformation. In contrast, IHP had no such effect on the conformation of the low-spin cyanomet derivatives of HbXL+3. The kinetics of CO combination as measured by stopped-flow and flash photolysis techniques present a more complex picture. In the presence of IHP the triaquomet derivative does bind CO with rate constants indicative of the T state whether these are measured by the stopped-flow technique or by flash photolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

IHP entrapment into human erythrocytes: comparison between hypotonic dialysis and DMSO osmotic pulse

Three different blood units were treated separately by the hypotonic dialysis (HD) and the dimethylsulphoxide osmotic pulse (DMSO) method, in order to load the erythrocytes with inositol hexaphosphate. A detailed comparison between the two loading techniques was performed by monitoring the red cell distribution patterns on discontinuous Percoll density gradients, the RBC oxygen affinity and the amount of the main intracellular organic phosphates with the 31P-NMR. The results obtained showed that: (1) The HD loading produces a redistribution of the RBC fractions with a concomitant smoothing of the relative differences among distinct fractions (2) only a minor portion of erythrocytes (from 8.5 to 24.9% of total RBCs) are loaded with IHP after the DMSO treatment. All of these cells move to the lightest fraction (d = 1.080 g/ml). (3) Both HD and DMSO IHP-loaded cells show an increase in P50 (basal vs. after loading, means +/- SD: 25.8 +/- 3.0 vs. 52.5 +/- 3.2 mm Hg) correlated to the IHP incorporation (mean intracellular IHP concentration: 4.2 mmol/l RBC). (4) probably the IHP incorporation efficiency could be probably improved at least by increasing the IHP concentration during the treatment.

Plasma exchange in the treatment of Refsum's disease (heredopathia atactica polyneuritiformis)

Five cases of heredopathia atactica polyneuritiformis (HAP--Refsum's disease) were treated by serial plasma exchanges. In all patients a reduction in calorie intake and body weight had been associated with a rise in plasma phytanic acid, followed by an exacerbation of the ataxia and neuropathy. Lowering the plasma phytanic acid by plasma exchange produced a rapid clinical improvement. The main indication for plasma exchange in HAP is a severe or rapidly worsening clinical condition. A lesser indication is failure of dietary management to reduce a high plasma phytanic acid level.

Long-term plasma exchange in a case of Refsum's disease

Refsum's disease (Heredopathia atactica polyneuritiformis) is caused by accumulation of phytanic acid in all body tissues due to an inherited failure of alpha-oxidation of branched chain fatty acids. Plasmapheresis has been reported to be beneficial by removal of phytanic acid from the blood. We describe a patient with Refsum's disease in whom long-term plasmapheresis did not improve clinical, biochemical or electrophysiological parameters.

Stable rightward shifts of the oxyhemoglobin dissociation curve induced by encapsulation of inositol hexaphosphate in red blood cells using electroporation

Rightward shifts of 50-100% of the P50 values in the oxygen dissociation curve of intracellular hemoglobin are obtained after encapsulation of inositol hexaphosphate in mouse and dog red blood cells (RBC) by electroporation. Life spans of mouse RBC-myo-inositol hexaphosphate in circulation are unchanged from the normal RBC values.

Binding of inositol hexakisphosphate to the oxygenated derivative of dromedary (Camelus dromedarius) and human hemoglobin: 31P-NMR study

Binding of inositol hexakisphosphate (IHP) to the oxygenated derivative of dromedary (Camelus dromedarius) and human hemoglobin (Hb) was investigated by 31P-NMR. The results obtained show that dromedary Hb binds, with different affinity, two IHP molecules per tetramer at distinct sites, while human Hb binds only one IHP molecule per tetramer.

A child with Refsum's disease: successful treatment with diet and plasma exchange

A child with Refsum's disease presented with cardiac failure, marked muscle wasting, weakness and inco-ordination. Management with multiple plasma exchanges and dietary restriction of phytanic acid intake has reversed the disabling features of the disease, although levels still remain higher than target values. Low phytanic acid intake is being achieved by restriction of total fat to 10 to 12 g/day, while allowing free amounts of fruit and green vegetables.

Effect of alpha-naphthylisothiocyanate on blood clearance of 99mTc-phytate in rats

The effect of alpha-naphthylisothiocyanate (ANIT) on blood clearance of 99mTc-phytate (99mTc-P) in rats was examined, and blood clearance test of 99mTc-P was compared with the cases of serum transaminase test, serum bilirubin test or histological test of the liver. The disappearance rate of 99mTc-P from blood decreased with the increase in dose of ANIT and with the passage of time after the ANIT administration. Changes of the blood clearance of 99mTc-P after ANIT treatment in rats may be influenced by the disorder in the hepatocytes rather than in the bile ductule cells. The blood clearance test of 99mTc-P in rats showed a sensitive reaction for the acute hepatic dysfunction induced by ANIT equally to the serum transaminase test, the serum bilirubin test or the histological test of the liver.

Modification of partial pressure of oxygen (P50) in mammalian red blood cells by incorporation of an allosteric effector of hemoglobin

Internalization of inositol hexaphosphate (IHP) in mammalian red blood cells (RBC) produces a modification of the hemoglobin-oxygen affinity, leading to a rightward shift of the dissociation curve. The process of incorporation, based on an osmotic shock, has been tested on RBC of different species. Two dialysis protocols have been defined to transform RBC, the first one for small volumes in a cellulose bag and the other for larger volumes using a commercially available dialysis device. Different optimal conditions must be used for each species. Most of the cellular characteristics of the transformed RBC having encapsulated IHP are similar to those of native cells. For several species, such modified RBC could be reinfused and used for physiological studies.

Blood clearance of 99mTc-phytate for evaluation of hepatic dysfunction in rats

Organ distribution pattern, hepatic uptake ratio, and blood clearance were examined by giving 99mTc-phytate (99mTc-P) to the normal rats. At the same time, the relation between the severity of hepatic function and blood clearance or hepatic uptake ratio of 99mTc-P was studied by using the rats with acute hepatic dysfunction experimentally induced by carbon tetrachloride (CCl4) administration. Furthermore, a comparative discussion on blood clearance test of 99mTc-P was made with serum transaminase test and with histological test of the liver. It appeared appropriate to administer 99mTc-P at the dose of 500 micrograms/kg in order to obtain an effective blood clearance curve. A major part of 99mTc-P intravenously administered was take up into the liver, while the remainder of small amount into the spleen, kidneys, lung, and so on. Little was recognized in the thyroid gland. The hepatic uptake ratio of 99mTc-P reached the maximum 15 minutes after the administration. The disappearance rate of 99mTc-P from blood decreased with the increase in dose of CCl4 and with the passage of time after the CCl4 administration. The blood clearance test of 99mTc-P showed a sensitive reaction for the acute hepatic dysfunction induced by CCl4 equally to or higher than the serum transaminase test or histological test of the liver.

Long-term physiological effects of enhanced O2 release by inositol hexaphosphate-loaded erythrocytes

A continuous lysing and resealing procedure with erythrocytes permitted incorporation in these cells of inositol hexaphosphate (InsP6), a strong allosteric effector of Hb. This leads to significant rightward shifts of the HbO2 dissociation curves with in vitro P50 (partial pressure of O2 at 50% Hb saturation), values increasing from 32.2 +/- 1.8 torr for control erythrocytes to 86 +/- 60 torr (pH 7.40; PCO2 40 torr at 37 degrees C; 1 torr = 1.333 X 10(2) Pa). The shape of the dissociation curve was still sigmoidal, although the Hill coefficient was decreased. The life span of InsP6-loaded erythrocytes equaled that of control erythrocytes. The long-term physiological effects of the InsP6-loaded erythrocytes on piglets were increased O2 release and reduced cardiac output. The reduced O2 affinity of the InsP6-loaded erythrocytes was still effective 20 days after transfusion in awake piglets. The electrolyte concentration appeared stable over the 5-day observation period except for a transient, but significant, hyperkalemia immediately after transfusion. The reductions in the O2 affinity of Hb reported here are large compared with previously reported values. Introduction of InsP6 into viable erythrocytes improves tissue oxygenation when, for any reason, normal blood flow is impaired.

Electron spin resonance (ESR) studies of skeletal protein interactions in human erythrocyte membranes exposed to polyanions and in membranes prepared from inositol hexaphosphate (IHP)-incorporated low-affinity erythrocytes

Previous biophysical investigations, including those from our laboratories, have reported that polyphosphates weaken RBC membrane skeletal protein-protein interactions and decrease hemoglobin affinity for oxygen. We have additionally demonstrated that low-affinity intact RBC's may be produced by inositol hexaphosphate (IHP) incorporation via an osmotic pulse method. In the present electron spin resonance (ESR) study, IHP was shown to cause a concentration-dependent increase in the segmental motion of ghost membrane skeletal proteins, but no alterations in spin-labeled terminal sialic acid. Pyrophosphate and inositol hexasulfate were significantly less effective in altering the physical state of skeletal proteins than was IHP. Additional ESR studies of both the interaction of IHP with membrane skeletal proteins in the presence of hemoglobin and of membranes obtained from osmotic pulse-treated intact cells were performed. The results of all these studies are discussed in terms of previous biophysical investigations of the effects of polyphosphates on membranes and of possible molecular events that occur during the osmotic pulse procedure.

Testing the two-state model: anomalous effector binding to human hemoglobin

Three allosteric states are required to describe the relaxation of (carbon monoxy) hemoglobin following flash photolysis. Combined absorbance and fluorescence probes were used. The absorbance signals consist of a component corresponding to ligand recombination and a component for the R-T transition. The fluorescence of 8-hydroxy-1,3,6-pyrenetrisulfonate (HPT), an analogue of 2,3-diphosphoglycerate, shows rates and amplitudes correlated with the absorbance transients. Measurements were made at pII 6, 6.5, and 7.0 at CO partial pressures of 0.1 and 1 atm. The fractional photolysis was varied in each case to change the initial distribution of the R states. Data show an immediate absorbance change due to ligand dissociation, while the changes in the ligand isosbestic and the fluorescence signals occur with time constants of 80 microseconds (at pH 6.5). The signals then show a biphasic return to equilibrium, characteristic of the allosteric system. The measurements provide three independent probes of the kinetics of the substates of hemoglobin. Although the ligand binding data can be generally represented by a two-state model, the fluorescence data require T states with different affinities for HPT.

Reactions of oxy- and methemoglobin with tryptophan metabolites, 3-hydroxyanthranilic acid and 3-hydroxykynurenine

It was found that 3-hydroxyanthranilic acid and 3-hydroxykynurenine have the capacity of oxidizing and reducing oxy- and met-hemoglobin. However, tryptophan metabolites such as kynurenine and anthranilic acid in which hydrxyl group is not involved did not oxidize and reduce these hemoglobins. The oxidation of oxyhemoglobin with 3-hydroxyanthranilic acid and 3-hydroxykynurenine was much accelerated in the presence of myo-inositol hexakisphosphate or superoxide dismutase, but was much suppressed in the presence of catalase. Deoxyhemoglobin was not oxidized by these compounds. On the other hand, the reduction of methemoglobin with 3-hydroxyanthranilic acid and 3-hydroxykynurenine proceeded both under aerobic and anaerobic conditions, though the rate of reduction was much faster under aerobic conditions than under anaerobic conditions. The reduction of methemoglobin with these compounds was accelerated by myo-inositol hexakisphosphate, but was partially suppressed by superoxide dismutase under aerobic conditions. On the basis of these results, the paradoxical effects of 3-hydroxyanthranilic acid and 3-hydroxykynurenine are discussed in relation to the mechanism of oxidation and reduction of hemoglobin.

Quaternary structure has little influence on spin states in mixed-spin human methemoglobins

A key feature of the Perutz stereochemical model for cooperativity in hemoglobin is a strong coupling between quaternary structure and the spin state of the heme iron [Perutz, M. F. (1979) Annu. Rev. Biochem. 48, 327-386]. While this coupling appears to be present for carp azide methemoglobin, it should also be present for all liganded forms of human methemoglobin that exhibit a thermal high-spin in equilibrium low-spin equilibrium. To test this hypothesis, we have measured the changes in spin equilibria upon conversion of six mixed-spin forms of human methemoglobin from the R (high-affinity) to the T (low-affinity) quaternary structure by addition of inositol hexaphosphate. These experiments were done with a sensitive superconducting magnetic susceptibility instrument on solutions at 20 degrees C in 20 mM maleate buffer, pH 6. The data show zero or small increases in high-spin content upon switching from R to T, changes that are equivalent to a relative stabilization of the high-spin form by only 0-300 cal mol-1 heme-1. These changes in energy are far less than the 1200 cal mol-1 heme-1 predicted from the Perutz stereochemical model [Cho, K. C., & Hopfield, J. J. (1979) Biochemistry 18, 5826-5833]. That is, these data do not support a view that the low affinity of the T state is due to restraints acting through the iron-proximal histidine linkage. The mechanistic implications of these results and the differences between species and ferric ligands are discussed.

Physiological effects of high-P50 erythrocyte transfusion on piglets

Rightward shifts of the O2 dissociation curve (ODC) were experimentally obtained in lysed and resealed erythrocytes following encapsulation of inositol hexaphosphate (IHP). This continuous lysing and resealing procedure led to in vitro P50 (Po2 at 50% hemoglobin saturation) increases up to 80 Torr (pH, 7.40; Pco2, 40 Torr; temp, 37 degrees C) for both human and pig erythrocytes. The Hill number of the transformed blood decreased when IHP was fixed on the hemoglobin, but the sigmoid shape of the ODC was maintained. The O2 hemoglobin binding capacity and the mean corpuscular hemoglobin content were found unchanged by the experimental procedure in human and pig erythrocytes. Isovolumic exchange transfusion of high-P50 erythrocytes in anesthetized and ambient air-ventilated piglets (n = 6) led to substantial in vivo P50 increases (range, 8-19 Torr). The rightward shift of the ODC was concomitant with an increase of the arterial Po2 and of the arteriovenous O2 content difference, 19 and 59% respectively above their control values. The mixed-venous Po2 (PVO2) remained unchanged. The cardiac output was shown to be inversely related to the P50 value. In spite of the O2-transport reduction (37%), O2 consumption was maintained due to enhanced O2 extraction.

Preparation of low-affinity red cells with dimethylsulfoxide-mediated inositol hexaphosphate incorporation: hemoglobin and ATP recovery using a continuous-flow method

Incorporation of IHP into red cells decreases oxygen affinity as a result of the binding of this compound to the 2,3-DPG site of hemoglobin. This investigation describes a continuous-flow method which utilizes the osmotic pulse technique to transport IHP into RBC. Using this procedure, it is possible to obtain a significant increase in P50 while maintaining in vitro cellular integrity. For example, IHP incorporation sufficient to cause an increase in the P50 of 20 mm Hg may be achieved with recovery of approximately 75% of the hemoglobin and with maintenance of ATP levels compatible with good viability. The continuous-flow method allows uniform treatment of large, unit-size volumes of red cells with a relatively small quantity of reagents. The final cell product is macrocytic/hypochromic with an increased number of stomatocytes.

Incorporation of inositol hexaphosphate into red blood cells mediated by dimethyl sulfoxide

Inositol hexaphosphate (IHP) binds to deoxyhemoglobin and markedly decreases the affinity of hemoglobin for oxygen. We introduce here a method for incorporating this polyphosphate into erythrocytes, thus preparing very low affinity cells for use in respiration research. The method uses dimethyl sulfoxide (DMSO) to facilitate entry of IHP. The cells are exposed to a high concentration of DMSO which is rapidly diluted with IHP solution. During this dilution the cells become leaky and IHP enters. The influence of several variables at each step of the process has been investigated and the data support a transient osmotic gradient mechanism for IHP incorporation.

Magnetic and spectral properties of carp carbonmonoxyhemoglobin. Competitive effects of chloride ions and inositol hexakisphosphate

We have extended our studies on the magnetic properties of carp carbonmonoxyhemoglobin and the dependence of these properties upon solution variables. Using an improved version of the superconducting magnetometer, we have found that the magnetic susceptibility of carp carbonmonoxyhemoglobin is sensitive to both inositol hexakisphosphate and chloride ion. The dependence upon chloride ion concentration is complex. At relatively low concentrations this anion reverses the effect of inositol hexakisphosphate, restoring paramagnetism. At higher chloride concentrations the protein is converted to a roughly diamagnetic state in the absence of inositol hexakisphosphate. Along with these susceptibility studies, we have examined the effects of these anions on other properties of carp carbonmonoxyhemoglobin. The positions of the Soret bands of human and carp methemoglobin derivatives are correlated with spin state; changes in the magnetic susceptibility of carbonmonoxyhemoglobin are similarly associated with alterations in this spectral band. We have also examined the effects of these anions on the proton nuclear magnetic resonance spectrum of carp carbonmonoxyhemoglobin. Both chloride and inositol hexakisphosphate alter the position of the proton resonances in the ring-current-shifted region of the spectrum.

A proton nuclear magnetic resonance investigation of proximal histidyl residues in human normal and abnormal hemoglobins. A probe for the heme pocket

Proton nuclear magnetic resonance spectroscopy at 250 MHz has been used to investigate the conformations of proximal histidyl residues of human normal adult hemoglobin, hemoglobin Kempsey [beta 99(G1) Asp leads to Asn], hemoglobin Osler [beta 145(HC2) Tyr leads to Asp], and hemoglobin McKees Rocks [beta 145(HC2) Tyr leads to Term] around neutral pH in H2O at 27 degrees C, all in the deoxy form. Two resonances that occur between 58 and 76 ppm downfield from the water proton signal have been assigned to the hyperfine shifted proximal histidyl NH-exchangeable protons of the alpha- and beta-chains of deoxyhemoglobin. These two resonances are sensitive to the quaternary state of hemoglobin, amino acid substitutions in the alpha 1 beta 2-subunit interface and in the carboxy-terminal region of the beta-chain, and the addition of organic phosphates. The experimental results show that there are differences in the heme pockets among these four hemoglobins studied. The structural and dynamic information derived from the hyperfine shifted proximal histidyl NH-exchangeable proton resonances complement that obtained from the ferrous hyperfine shifted and exchangeable proton resonances of deoxyhemoglobin over the spectral region from 5 to 20 ppm downfield from H2O. The relationship between these findings and Perutz's stereochemical mechanism for the cooperative oxygenation of hemoglobin is discussed.

Sickling as a function of oxygen delivery: effect of simulated transfusions of stored, fresh and inositol-hexaphosphate-loaded (low affinity) red cells

Low oxygen affinity red cells were prepared by incorporating inositol hexaphosphate (IHP) into red blood cells (rbc) by means of a liposomal transport system. The effect of in vitro simulated exchange transfusions on sickling was studied with buffered red cell suspensions containing 50% SS cells and 50% test cells. Test rbc were either stored cells with high oxygen affinity, fresh cells with normal affinity or IHP-loaded cells with decreased affinity. Oxygen equilibrium curves and percentage sickling as a function of PO2 were determined and the data analyzed in terms of percentage sickling as a function of oxygen delivery. Our simplified analysis shows that simulated exchange transfusion with stored and, to a lesser extent even with fresh blood, results in a decreased venous PO2 and increased sickling of the remaining SS cells. In contrast, transfusion with IHP-loaded cells results in higher venous PO2 values and less sickling throughout the range of oxygen delivery. Thus, the transfusion of IHP-loaded cells may result in less sickling of the remaining SS cells in addition to the normal dilutional effect.

Equilibrium aspects of the binding of myo-inositol hexakisphosphate to human hemoglobin as studied by 31P NMR and pH-stat techniques

The interaction of myo-inositol hexakisphosphate (P6-inositol) with human hemoglobin has been studied as a function of pH using pH-stat techniques and 31P NMR. With the pH-stat method the following data were obtained: the association constants for the P6-inositol/deoxyhemoglobin and P6-inositol/carboxyhemoglobin complexes at alkaline and acid pH respectively and the proton absorption curves associated with the protein/phosphate interaction for both complexes from pH 5.5 to pH 9. From these data affinities of P6-inositol towards deoxyhemoglobin (Hb) and carboxyhemoglobin (HbCO) have been calculated as a function of pH. The shape of the proton absorption curves was found to be strongly dependent on the ligation state of the hemoglobin molecule. The pH dependence of the 31P NMR spectra of P6-inositol bound to Hb or HbCO provides a monitor for the proton-binding behaviour of the phosphate groups of P6-inositol when present in the central cavity of the protein. It appears that this behaviour is only slightly dependent on the ligation state of the hemoglobin molecule. The NMR spectral data were interpreted in terms of a model which takes into account the electrostatic interaction between the phosphate groups within the P6-inositol molecule as well as the electrostatic interaction between the phosphate groups and positively charged groups on the protein. To account for the discrepancy between the pH-stat and 31P NMR results, i.e. a strong dependence of the proton-absorption curves and a weak dependence of the proton-binding behaviour of P6-inositol on the ligation state of the protein respectively, it is proposed that a conformational change takes place in HbCO upon P6-inositol binding.

31P NMR study of the kinetics of binding of myo-inositol hexakisphosphate to human hemoglobin. Observation of fast-exchange kinetics in high-affinity systems

The association and dissociation kinetics of the complexes of myo-inositol hexakisphosphate (P6-inositol) with deoxyhemoglobin (Hb) and carboxyhemoglobin (HbCO) have been investigated by 31P NMR between pH 6.8 and pH 5.5. These complexes represent high-affinity systems with binding constants varying between 10(5) M-1 and 2 X 10(9) M-1. 31P NMR spectra of P6-inositol were recorded in the presence of hemoglobin as a function of the P6-inositol/hemoglobin molar ratio. It appeared that the exchange of the polyphosphate molecule between the solution and the central cavity binding site is fast on the NMR time scale. This observation cannot be reconciled with a single-step binding mechanism of P6-inositol to hemoglobin. Analysis of the spectra revealed the occurrence of additional binding of P6-inositol to both Hb and HbCO. This binding was also observed in pH-state experiments performed at low ionic strength. 31P NMR experiments carried out with hemoglobin of which the alpha-chain N termini were carbamylated, strongly suggest that these termini constitute the additional binding site for P6-inositol. A model is proposed which accounts for the enhancement of exchange kinetics in these high-affinity systems. In this model a rapid migration is assumed for P6-inositol between the central cavity binding site and an entry/leaving site on the hemoglobin molecule. Based on this model 31P NMR linewidths and chemical shift patterns for this three-site exchange problem were calculated.

Interaction of organic phosphates with bovine hemoglobin. I. Oxylabile and phosphate-labile proton binding

Adult bovine hemoglobin solutions were studied with respect to the influence of organic phosphates (adenosine-5'-triphosphate, 2,3-diphosphoglycerate, myo-inositolhexaphosphate) on the oxylabile proton binding (fixed-acid Haldane effect). At alkaline pH (less than 7.5) this Haldane effect is increased by organic phosphates, whereas at acid pH (less than 6.5) the effect is decreased; in the neutral pH range no unequivocal trend was found. The influence on the Haldane effect is caused by binding of organic phosphates to both deoxy- and liganded hemoglobin. The binding of 2,3-diphosphoglycerate and myo-inositolhexaphosphate causes a massive proton uptake. In going from deoxy- to liganded hemoglobin in the presence of organic phosphates the proton release due to phosphate release from deoxyhemoglobin and the proton uptake due to phosphate binding by liganded hemoglobin were shown to account for the phosphate-induced part of the fixed-acid Haldane effect.

Metabolism of 14C-phytate in rats: effect of low and high dietary calcium intakes

The purpose of this study was to explore in rats the in vivo metabolism of phytate, using [U-14C]phytate and to assess the effects of calcium intake on phytate and myo-inositol metabolism. Labeled phytate was prepared from wheat following injection of the plants at the milk stage with [U-14C]myo-inositol. Groups of 30-day-old male rats were adapted for at least 6 weeks either to a low-calcium (2.9 mM/100 g dry diet) or a high-calcium (30.6 mM/100 g dry diet) cereal-based diet. They were then transferred to metabolic cages, continued on their respective diets and given an oral dose of [14C]phytate or [14C]myo-inositol. Appearance of radioactivity in expired air, feces and urine was monitored for 48 hours; the rats were then killed for determination of 14C-activity in liver, kidneys, blood and femur. The high-calcium diet significantly increased the output of radioactivity in feces (54% of dose) and reduced the appearance of radioactivity in expired air (25% dose) and in body tissues following 14C-phytate administration. The high-calcium intake did not affect the fate of [14C]myo-inositol (96% of dose). These results suggest that phytate or a derivative(s) is almost quantitatively absorbed (94% of dose) when calcium intake is low (Ca/P molar ratio; 0.21) and it is extensively oxidized to CO2 (60% of dose). Thus, a high-calcium intake (Ca/P molar ratio; 2.24) inhibits the utilization of [14C]phytate. Hence, it is suggested that the impact of dietary phytate on trace mineral bioavailability will depend upon the presence of factors, including excess calcium, that alter the absorption and utilization of phytate.

Functional properties of the glycosylated minor hemoglobins A1a-1,A1a-2 and A1b. EPR evidence for increased stability of the low affinity quaternary structure and decreased susceptibility to organic phosphate

Electron paramagnetic resonance (EPR) spectra of the glycosylated minor hemoglobins A1a-1, A1a-2, A1b and A1c and the major hemoglobin A0 in the nitrosyl form have been obtained in the absence and presence of inositol hexaphosphate. In the absence of inositol hexaphosphate, nitrosyl hemoglobins A1a-1, A1a-2 and A1b exhibited a triplet hyperfine structure centered at g = 2.009 which has been shown to be diagnostic of the low affinity (T) quaternary structure. Addition of inositol hexaphosphate to nitrosyl hemoglobins A0, A1c, A1b and A1a-2 developed a triplet hyperfine structure of the EPR spectra but the magnitude of the hyperfine was decreased in the order of hemoglobins A0, A1c, A1b and A1a-2. However, inositol hexaphosphate had essentially no effect on the EPR spectrum of nitrosyl hemoglobin A1a-1. The present results account qualitatively for the oxygen binding properties of these glycosylated minor hemoglobins in the framework of a two-state allosteric model.

Incorporation of inositol into intact red blood cells. II. Enhancement of gas transport in inositol hexaphosphate-loaded red blood cells

The gas-transport function of red blood cells which have incorporated inositol hexaphosphate is significantly improved by fusion with effector-loaded lipid vesicles. "Right-shifts" of the O2-binding curves of inositol hexaphosphate-loaded red blood cells with half-saturation pressures at 37 degrees C up to 98 mmHg are observed. The transformation of 56% of the intracellular haemoglobin into the low affinity state corresponds to an optimum increase of the O2-release capacity of 270%. The CO2 transport is also correspondingly enhanced. The treated red blood cells show no inositol hexaphosphate depletion during a reasonable time.

Incorporation of inositol hexaphosphate into intact red blood cells. I. Fusion of effector-containing lipid vesicles with erythrocytes

Fluid charged lipid vesicles loaded with an inositol hexaphosphate solution were used to transport this allosteric effector into human intact red blood cells. Rate and extent of uptake of the vesicles and of the effector by the red blood cells were measured as changes in the O2 half-saturation pressure and the 31P-NMR spectra of the intracellular inositol hexaphosphate-haemoglobin complex.

The CO and NO Bohr effect of human hemoglobin with and without inositolhexaphosphate

Using NO and CO as ligands the Bohr effect of human hemoglobin has been measured with and without inositolhexophosphate. It appears that in the absence and presence of inositolhexaphosphate hemoglobin shows a distinct ligand specificity with respect to the Bohr effect. Ligation with NO is accompanied by release of a larger number of Bohr effect. It is shown that this latter result is due to the fact that the number of protons taken up upon binding of inositolhexaphosphate to ligated hemoglobin is larger for HbNO than for HbCO. It is suggested that this additional proton uptake is partially due to a restoration of the saltbridge between His 146beta and Asp 94beta upon addition of IHP.

Reciprocal interaction of hemoglobin with oxygen and protons. The influence of allosteric polyanions

The interaction of three inositol esters, inositol hexaphosphate (IHP), inositol pentaphosphate (IPP), and inositol hexasulfate (IHS), with hemoglobin has been investigated. The proton uptake method was used to obtain the six binding constants for deoxy- and oxyhemoglobin. These data combined with oxygen binding curves over a range of cofactor concentrations were used to test theoretical and empirical equations relating the affinity of hemoglobin for oxygen and allosteric effectors. The Bohr and Haldane coefficients in the presence of the inositol esters are unequal at low, but not at high, concentration of the cofactors. The maximum value reached by both parameters increases with the number of negative charges of the polyanion. 2,3-Diphosphoglycerate (DPG) differs sharply from the inositol esters since even at high concentrations of this cofactor, the Haldane coefficient remains elevated. This is a reflection of the negligible affinity of DPG for fully oxygenated hemoglobin.