A role for the AKT1 potassium channel in plant nutrition.

In plants, potassium serves an essential role as an osmoticum and charge carrier. Its uptake by roots occurs by poorly defined mechanisms. To determine the role of potassium channels in planta, we performed a reverse genetic screen and identified an Arabidopsis thaliana mutant in which the AKT1 channel gene was disrupted. Roots of this mutant lacked inward-rectifying potassium channels and displayed reduced potassium (rubidium-86) uptake. Compared with wild type, mutant plants grew poorly on media with a potassium concentration of 100 micromolar or less. These results and membrane potential measurements suggest that the AKT1 channel mediates potassium uptake from solutions that contain as little as 10 micromolar potassium.

Analysis of the acid and alkaline dissociation of earthworm hemoglobin, Lumbricus terrestris, by front-face fluorescence spectroscopy.

The steady-state fluorescence properties of the multisubunit hemoglobin isolated from the earthworm, Lumbricus terrestris, were studied by front-face fluorometry. Acid and alkaline dissociation of this high-molecular-weight hemoglobin were examined over the pH range 3.7-12.5 using different liganded states (oxy, CO, met). The relative intensity of the emission maximum at 320 nm (exc. 280 nm) is ligand-dependent increasing as follows: oxy less than deoxy less than CO less than met at pH 7.0. The intensity of the emission maximum of oxyhemoglobin at the alkaline acid end point, pH 10.5 (333 nm), is significantly greater than that observed at the acid end point, pH 4.18 (320 nm), suggesting different subunit dissociation. The spectra of oxyhemoglobin at pH 4.18 and the spectrum of carbonmonoxy hemoglobin at pH 7.0 in the presence of 1 M magnesium chloride were almost identical, indicating similar subunit dissociation. Difference spectrum (pH 9.0-7.2) of fluorescence emission (exc. 305) resulted in a maximum at 341 nm, indicative of tyrosinate formation. This suggests that tyrosine(s) may also be located at the subunit interface(s) of this hemoglobin. These studies indicate that several aromatic amino acid residues are associated with the critical sites of subunit interactions within this molecule. Analysis of the fluorescence spectra also suggests that the formation of different subunit species resulting from acid and alkaline dissociation cannot be ruled out.

Intrinsic fluorescence of carp hemoglobin: a study of the R----T transition.

The intrinsic fluorescence of hemoglobins is known to respond to ligand-induced changes in the quaternary structure of the protein. Carp hemoglobin is an interesting model to study the quaternary transition since its R----T equilibrium is pH-dependent and at low pH, in the presence of organic phosphate, it remains in the T or 'deoxy' quaternary structure, even when saturated with ligand. In this study, using front-face fluorometry, we show that the intrinsic fluorescence intensity exhibited by carp carboxyhemoglobin increases as the pH is lowered below 6.5 in the presence of inositol hexaphosphate. At low pH, carp methemoglobin is less affected by the addition of inositol hexaphosphate than is the CO derivative, while little or no change is observed in the met-azide derivative. We conclude: (1) the exact nature of the R to T state transition induced by inositol hexaphosphate differs for carp carboxy-, met- and met-azide hemoglobin derivatives; (2) the chromophores responsible for the changes observed with absorption spectroscopy may not be the same as those chromophores responsible for the fluorescence differences; and (3) alpha 46-Trp is tentatively assigned as one source of fluorescence emission. Furthermore, fluorescence properties of carp hemoglobin are compared to those of human hemoglobin.

A comparison of the intrinsic fluorescence of red kangaroo, horse and sperm whale metmyoglobins.

Several metmyoglobins (red kangaroo, horse and sperm whale), containing different numbers of tyrosines, but with invariant tryptophan residues (Trp-7, Trp-14), exhibit intrinsic fluorescence when studied by steady-state front-face fluorometry. The increasing tyrosine content of these myoglobins correlates with a shift in emission maximum to shorter wavelengths with excitation at 280 nm: red kangaroo (Tyr-146) emission maximum 335 nm; horse (Tyr-103, -146) emission maximum 333 nm; sperm whale (Tyr-103, -146, -151) emission maximum 331 nm. Since 280 nm excites both tyrosine and tryptophan, this strongly suggests that tyrosine emission is not completely quenched but also contributes to this fluorescence emission. Upon titration to pH 12.5, there is a reversible shift of the emission maximum to longer wavelengths with an increase greater than 2-fold in fluorescence intensity. With excitation at 305 nm, a tyrosinate-like emission is detected at a pH greater than 12. These studies show that: (1) metmyoglobins, Class B proteins containing both tyrosine and tryptophan residues, exhibit intrinsic fluorescence; (2) tyrosine residues also contribute to the observed steady-state fluorescence emission when excited by light at 280 nm; (3) the ionization of Tyr-146 is likely coupled to protein unfolding.

The differential effects of carbon monoxide and oxygen on the pressure dissociation of Lumbricus terrestris hemoglobin.

We have explored the subunit affinities of Lumbricus terrestris hemoglobin (LtHb) under a variety of conditions using high-pressure spectroscopy. While only small changes were observed for LtHb-oxy below 1.0 kbar, higher pressures resulted in a 1000 cm-1 red shift and 2-fold increase in fluorescence intensity with a concomitant 12-fold decrease in scattering intensity, all of which reached completion by approx. 2.2 kbar. In the presence of 1 M MgCl2 or at acidic pH (4.2), the curves shifted by 400 and 1000 bar corresponding to significant destabilization. At pH 9.1, the initial spectral parameters were almost equal to the final endpoints and were unaffected by pressure. While the pressure curve of the CO form was similar to the oxy form at pH 7.2, the midpoints of the other samples were shifted to higher pressures relative to their oxy counterpart, indicating tighter subunit contacts. This stabilization was unexpected based upon the sequence homology to vertebrate hemoglobins, and the minimal structural differences between these two liganded forms of human hemoglobin. These data indicate that the differences are the result of the additive nature of the interactions involved in subunit packing whose effects become significant in larger aggregates.

Alteration of tryptophan fluorescence properties upon dissociation of Lumbricus terrestris hemoglobin.

Fluorescence analysis has been used to study dissociation of the dodecameric 3.8 kDa Lumbricus terrestris hemoglobin. Since tryptophan intrinsic fluorescence has been used as a reporter group to study Lumbricus hemoglobin, it is of interest to study dissociation perturbed properties of the tryptophan residues. Shifts in the fluorescence emission maximum to longer wavelengths upon dissociation at pH 9.2 suggest that tryptophans buried at the subunit interface(s) become more exposed. Fluorescence lifetime and quenching studies are employed in this present investigation as a means to confirm the location of tryptophan residues at the subunit interfaces. Acrylamide titration (to 2.5 M) indicate only a fraction of the residues can be quenched at either pH. At pH 7.0, the Stern-Volmer plot has downward curvature, while at pH 9.2 there is slight upward curvature, again indicating a change in environment. The intrinsic fluorescence decay requires at least four exponentials at both pHs. The mean fluorescence lifetime of CO Lumbricus hemoglobin increases from 1.1 ns at pH 7 to 3.3 ns at pH 9.2. The lifetime data can be further interpreted as a decrease in the quenching of residues with a approximately 30 ps lifetime, and a concomitant increase in the longer lifetime components. This is consistent with interface tryptophans becoming exposed to solvent upon dissociation, and loss of quenching by intersubunit hemes. The overall results suggest that in the dodecamer, most of the tryptophans are located in a hydrophobic environment, not all of which are located at the subunit interface.

Potassium uptake supporting plant growth in the absence of AKT1 channel activity: Inhibition by ammonium and stimulation by sodium.

A transferred-DNA insertion mutant of Arabidopsis that lacks AKT1 inward-rectifying K+ channel activity in root cells was obtained previously by a reverse-genetic strategy, enabling a dissection of the K+-uptake apparatus of the root into AKT1 and non-AKT1 components. Membrane potential measurements in root cells demonstrated that the AKT1 component of the wild-type K+ permeability was between 55 and 63% when external [K+] was between 10 and 1,000 microM, and NH4+ was absent. NH4+ specifically inhibited the non-AKT1 component, apparently by competing for K+ binding sites on the transporter(s). This inhibition by NH4+ had significant consequences for akt1 plants: K+ permeability, 86Rb+ fluxes into roots, seed germination, and seedling growth rate of the mutant were each similarly inhibited by NH4+. Wild-type plants were much more resistant to NH4+. Thus, AKT1 channels conduct the K+ influx necessary for the growth of Arabidopsis embryos and seedlings in conditions that block the non-AKT1 mechanism. In contrast to the effects of NH4+, Na+ and H+ significantly stimulated the non-AKT1 portion of the K+ permeability. Stimulation of akt1 growth rate by Na+, a predicted consequence of the previous result, was observed when external [K+] was 10 microM. Collectively, these results indicate that the AKT1 channel is an important component of the K+ uptake apparatus supporting growth, even in the "high-affinity" range of K+ concentrations. In the absence of AKT1 channel activity, an NH4+-sensitive, Na+/H+-stimulated mechanism can suffice.

Evaluation of 5-aminosalicylic acid (5-ASA) for cancer chemoprevention: lack of efficacy against nascent adenomatous polyps in the Apc(Min) mouse.

Recent experimental and epidemiological evidence suggests that nonsteroidal anti-inflammatory drugs (NSAIDs) are effective in the prevention of colorectal cancer. However, the toxicity associated with the long-term use of most classical NSAIDs has limited their usefulness for the purpose of cancer chemoprevention. Inflammatory bowel disease (IBD) patients, in particular, are sensitive to the adverse side effects of NSAIDs, and these patients also have an increased risk for the development of intestinal cancer. 5-Aminosalicylic acid (5-ASA) is an anti-inflammatory drug commonly used in the treatment of IBD and may provide protection against the development of colorectal cancer in these patients. To directly evaluate the ability of 5-ASA to suppress intestinal tumors, we studied several formulations of 5-ASA (free acid, sulfasalazine, and Pentasa) at multiple oral dosage levels [500, 2400, 4800, and 9600 parts/million (ppm)] in the adenomatous polyposis coli (Apc) mouse model of multiple intestinal neoplasia (Min). Although the ApcMin mouse is not a model of colitis-associated neoplasia, it is, nonetheless, a useful model for assessing the ability of anti-inflammatory agents to prevent tumor formation in a genetically preinitiated population of cells. We used a study design in which drug was provided ad libitum through the diet beginning at the time of weaning (28 days of age) until 100 days of age. We included 200 ppm of piroxicam and 160 ppm of sulindac as positive controls, and the negative control was AIN-93G diet alone. Treatment with either piroxicam or sulindac produced statistically significant reductions in intestinal tumor multiplicity (95% and 83% reductions in tumor number, respectively; P < 0.001 versus controls). By contrast, none of the 5-ASA drug formulations or dosage levels produced consistent dose-progressive changes in polyp number, distribution, or size, despite high luminal and serum concentrations of 5-ASA and its primary metabolite N-acetyl-5-ASA. Thus, 5-ASA does not seem to possess direct chemosuppressive activity against the development of nascent intestinal adenomas in the ApcMin mouse. However, because intestinal tumor development in the ApcMin mouse is driven by a germline mutation in the Apc gene rather than by chronic inflammation, we caution that these findings do not definitively exclude the possibility that 5-ASA may exert a chemopreventive effect in human IBD patients.

Hemoglobin oxygen affinity is increased in erythropoietic protoporphyria.

Whole blood and hemolysates from seven normal and three erythropoietic protoporphyria patients were compared in terms of their hemoglobin function. The oxygen affinity (P50) of the erythropoietic protoporphyria hemolysates compared to normals (13.1 +/- 0.2 vs 17.5 +/- 0.3 mmHg; P < 0.001) and erythropoietic protoporphyria erythrocytes compared to normals (23.4 +/- 0.6 vs 27.1 +/- 0.5 mmHg; P < 0.001) were increased while oxygen-binding cooperativity (n-value of the Hill equation) were similar. We conclude that hemoglobin function in erythropoietic protoporphyria patients is altered, but without pathophysiologic consequences. Because hemoglobin in which protoporphyrin IX substitutes for heme has a low oxygen affinity, our findings of a higher than normal affinity in erythropoietic protoporphyria red cells and hemolysates may indirectly support the findings by others that protoporphyrin IX binds to hemoglobin at non-heme-binding sites. In addition, based on the effect of other abnormal hemoglobins, this shift in P50 will decrease any tendency for anemia in erythropoietic protoporphyria patients.

Stopped-flow front-face fluorometer: a prototype design to measure hemoglobin R----T transition kinetics.

Stopped-flow techniques are successfully used to study the kinetics of the R----T transition of hemoglobin (Hb). We have previously used front-face fluorometry to demonstrate that (i) the intrinsic fluorescence of Hb primarily originates from beta 37 Trp; (ii) the intrinsic fluorescence is sensitive to the R----T transition; and (iii) the emission of the fluorescent probes bound to specific sites on the Hb molecule (beta 93 Cys) is sensitive to the R----T transition. These findings suggested that a stopped-flow front-face fluorometer could probe R----T transitions at specific sites, such as the aromatic amino acids and sites selectively binding extrinsic fluorophores. We have developed a prototype instrument using as the core a Gibson-Durrum stopped-flow apparatus on line with a digital data analysis system using a modified Marquardt algorithm. Excitation (470 nm) and emission light (520 nm) were selected by narrow band pass filters. To study the R----T transition, a solution of purified oxy Hb A covalently bound to the fluorescent probe 5-iodoacetamidofluorescein (Hb A-AF) (1.0 g%) was mixed rapidly with deoxygenated buffer (pH 7.35, 0.05 M potassium phosphate) containing 2 mg/ml of sodium dithionite. The hemoglobin, at a final concentration of 0.5 g% after mixing, is essentially completely tetrameric. A first-order reaction was observed with a rate constant near 8 s-1, similar to the oxygen dissociation rate reported for oxy Hb A.(ABSTRACT TRUNCATED AT 250 WORDS)

Conformational studies of hemoglobins using intrinsic fluorescence measurements.

Front face fluorometry allows for the detection of intrinsic fluorescence in intact hemoglobins, and we have been able to assign the origin of this emission in Hb A to the tryptophan at beta 37 (Hirsch, R. E., Zukin, R. S., and Nagel, R. L. (1980) Biochem. Biophys. Res. Commun. 93, 432-439). We now report that this fluorescence in hemoglobin is sensitive to the R leads to T conformational transition. Comparison between liganded states of Hb A suggest a difference in the R structures of HbCO and HbO2. The T state induced by inositol hexaphosphate in the presence of methemoglobin A appears to be different from the T state of deoxyHb A. In agreement with chemical data our fluorescence studies of Hb H indicate that no R to T transition occurs in this hemoglobin, secondary to ligand binding. A slight conformational difference exists between deoxyHbH and COHbH. Hb Beth Israel, a low oxygen affinity mutant, shows unique fluorescence properties suggesting significant differences between the R and T forms relative to those of Hb A.

Spectral properties of chlorophyll a monolayers in the presence of an exogenous electron donor and acceptor.

Chlorophyll a monolayers are studied at a nitrogen-water interface in the presence of a reducing or oxidizing agent: sodium ascorbate and benzyl viologen, respectively. Absorption spectra of the films are measured directly on the aqueous surface. With the aid of a computer, fourth derivative and difference spectra are determined. In the presence of ascorbate, a bathochromic shift of the absorption maximum to 693 nm can be induced as compared to 683 nm for a chlorophyll monolayer without any additives. In the presence of ascorbate, chlorophyll species at 676, 712 and 750 nm (present in a pure chlorophyll monolayer) are decreased or diminished. Illumination causes no change in the position of these absorption maxima; however, there is an increase of the absorbance of the main red absorption band. In the presence of benzyl viologen there is a hypsochromic shift of the red absorption maximum to 679 nm. Chlorophyll species at 670, 694, 712 and 740 nm (present in pure chlorophyll monolayers) are decreased or diminished upon addition of benzyl viologen. Upon illumination, there is a decrease in absorbance at 686 nm. It appears that the redox reagents induce the formation of specific chlorophyll aggregates, in the interfacial system, which might be analogous to the various chlorophyll species observed in green plant photosynthesis.

Conformational dynamics of two histidine-binding proteins of Salmonella typhimurium.

The Salmonella typhimurium periplasmic histidine-binding J-protein is one of four proteins encoded by the histidine transport operon. Mutant J-protein hisJ5625 binds L-histidine, but does not transport it. The tertiary structure and conformational dynamics of native and mutant J-protein have been compared using steady state fluorescence, fluorescence polarization, and fluorescence energy transfer measurements. The two proteins have different three-dimensional structures and exhibit different responses to histidine binding. Ligand-induced conformational changes were demonstrated in both J-proteins using fluorescence energy transfer (distant reporter method) between the single tryptophan residue per mole of protein and a fluorescein-labeled methionine residue. However, the conformational change of the mutant protein is qualitatively and quantitatively different from that of the wild-type protein. Moreover, the microenvironment of the tryptophan and its distance from the labeled methionine (44A for the wild type, 60A for the mutant J-protein) are different in the two proteins. In conclusion, these results indicate that the specific conformational change induced in the wild type J-protein is a necessary requirement for the transport of L-histidine.

Steady-state fluorescence emission from the fluorescent probe, 5-iodoacetamidofluorescein, bound to hemoglobin.

In the past, fluorescence emission from an extrinsic fluorophore bound to heme-proteins would only be studied with the removal of the heme since fluorescence from the fluorophore could not be detected using right-angle optics. Using front-face fluorometry, a significant steady state emission signal originating from the probe bound to hemoglobin is detected. This is the first report of the detection of extrinsic fluorescence of a probe bound to a heme-protein. We also demonstrate that the extrinsic probe, 5-iodoacetamidofluorescein, is covalently bound to hemoglobin, specifically at beta 93 Cysteine. Ligand binding results in a change in the fluorophore fluorescence intensity as predicted by hemoglobin crystallographic studies. Efficiency of energy transfer measurements are made.

The inhibition of hemoglobin C crystallization by hemoglobin F.

We have reported that circulating CC erythrocytes containing HbO2 C crystals exhibit little or no Hb F suggesting that Hb F may inhibit the crystallization of Hb C. We report now that Hb F inhibits in vitro crystallization of HbO2 and HbCO C when compared to the effect of Hb A in a wide range of mixture proportions. For example, while HbCO C solutions form tetragonal C crystals within 25 min, no crystals form within 2 h with 30% Hb F, whereas 550 crystals/mm3 form with 30% Hb A. Furthermore, an increase in the percent of Hb A is correlated with a greater number of orthorhombic crystal formation rather than the tetragonal morphology observed with 100% Hb C. We also report that Hb A2 (containing delta chains that exhibit 10 sequence differences with beta chains) and Hb Lepore Boston-Washington (a fusion mutant of delta and beta chains that contains only six of these differences) both inhibit Hb C crystallization. By comparing the sequences of the three inhibitory hemoglobins, we conclude that position Gln-87 in the gamma chains is, at least partially, the cause of the inhibitory effect of Hb F on the crystallization of Hb C.

Interaction of zinc protoporphyrin with intact oxyhemoglobin.

In erythropoietic protoporphyria and lead poisoning, free protoporphyrin (PPIX) and zinc protoporphyrin (ZPP), respectively, accumulate in erythrocytes. That PPIX and ZPP bind to human hemoglobin A (Hb4) is established, but the site of binding is still a matter of controversy. We investigated the interaction of ZPP with intact, tetrameric oxy Hb4, using batch microcalorimetry, front-face fluorometry, absorption difference spectroscopy, oxygen equilibrium studies, and isoelectric focusing (IEF). In the presence of oxy Hb4 (pH 7.35, 0.05 M phosphate), the fluorescence emission maximum (excitation at 420 nm) of ZPP immediately shifts from 587 nm (ZPP alone) to 594 nm, as expected when binding to protein. The fluorescence intensity increases with time and is correlated with the ZPP:Hb4 mole ratio. A slow, time-dependent reaction is also observed with microcalorimetry: the rate of heat of reaction exhibits both a fast and a slow component. The heats of reaction range from -2.1 to -14.8 mcal depending upon the ZPP:Hb4 ratio of 4:1 (0.4 mM:0.1 mM) to 38:1 (3.8 mM:0.1 mM), respectively, and are typical of weak, noncovalent protein-ligand interactions. The optical difference spectra are a function of the ZPP:Hb4 molar ratio and also exhibit a slow increase in intensity over time. No time-dependent optical difference spectra are observed with ZPP or with Hb4 alone. The oxygen affinity of Hb4 in the presence of ZPP decreases with increasing mole ratio. During IEF, all ZPP separates from Hb4, consistent with a weak, noncovalent interaction at a non-heme pocket site. We conclude that ZPP binds to intact, tetrameric hemoglobin at non-heme pocket sites in a nonspecific, weak, noncovalent interaction.

Redox concerns in the use of acellular hemoglobin-based therapeutic oxygen carriers: the role of plasma components.

Within the past decade, most research efforts in the red blood cell substitute area have revolved about the development of acellular hemoglobin-based oxygen carriers (HBOC) as clinical replacements and/or augmentation of human blood's carrying and delivery function. A major requirement for all HBOC is the maintenance of the heme-Fe+2 in this reduced state for normal physiological behavior. Oxidation of hemoglobin results in the formation of methemoglobin (heme-Fe+3). MetHb is unable to bind oxygen thus effectively lowering the carrying capacity of the Hb-based substitute. In addition, met Hb gives rise to free radicals that have the potential to cause endothelial and surrounding tissue damage. Results of this study suggest that the normal endogenous reducing agents of human plasma have the capacity to provide redox protection and stability to specific acellular-types of HBOC. The effectiveness of these reducing agents may be related to the formal reduction potential of the HBOC being considered. The choice of buffer for HBOC storage is critical and specific to the HBOC product.

Estimation of 5-aminosalicylic acid and its metabolite in human serum by front-face fluorometry: a simple and sensitive method.

Salicylazosulfapyridine (SASP), commonly used in the treatment of inflammatory bowel disease, breaks down in the colon into sulfapyridine and 5-aminosalicylic acid (5-ASA), the active moiety of SASP. We report a sensitive method to measure 5-ASA and its known major metabolite acetyl 5-ASA (Ac-5-ASA) directly from the serum without any extraction procedure. Using front-face fluorometry, 5-ASA and Ac-5-ASA were detected at the excitation wavelength of 310 nm with emission maxima at 475 nm and 440 nm, respectively. Standard curves were obtained by adding known amounts of 5-ASA and Ac-5-ASA to several individual and pooled human sera. Presence of sulfapyridine (0 to 20 micrograms/ml) and SASP (0 to 15 micrograms/ml) in the serum did not interfere with the assays. Five microliters of acetic anhydride was added to the serum to convert all 5-ASA to Ac-5-ASA. The difference in the spectrum before and after addition of acetic anhydride represented the concentration of free 5-ASA. The values thus estimated were within 1% of the expected readings from the standard curves. This assay was compared with the organic extraction method for the determination of free and acetylated 5-ASA in sera of patients given olsalazine (azodisalicylate). The results demonstrate that direct analysis of the sera by front-face fluorometry enables us to measure 5-ASA and Ac-5-ASA at levels as low as 0.1 micrograms/ml in serum, making this method at least 10-fold more sensitive than the current available extraction methods.