An improved radioenzymatic assay for plasma norepinephrine using purified phenylethanolamine N-methyltransferase

Radioenzymatic assays have been developed for norepinephrine (NE) using either catechol O-methyltransferase (COMT) or phenylethanolamine N-methyltransferase (PNMT). Assays using PNMT are specific for NE but have been considered less sensitive than the more complex assay procedures employing COMT. An improved purification procedure for bovine PNMT has permitted development of a NE assay with substantially improved sensitivity (less than 0.5 pg), reproducibility, and decreased manipulative effort. PNMT was purified by sequential pH 5.0 treatment and dialysis and by column chromatographic procedures using DEAE-Sephacel, Sephacryl S-200 and Phenyl Boronate-agarose. Recovery of PNMT activity through the purification scheme was 50% while blank recovery was less than 0.001%. Norepinephrine can be directly quantified in 25 microliters of human plasma and a seventy-tube assay can be routinely completed within 4 h. The capillary to venous plasma NE gradient was examined in eight normotensive male subjects. Capillary plasma NE (211 +/- 21.7 pg/ml) was significantly lower than venous plasma NE (367 +/- 32.7 pg/ml) in all subjects (p less than 0.005). This difference suggests the concentration of NE in capillary blood may be a unique indicator of sympathetic nervous system activity in vivo.     

Low pH-induced conformational changes in vesicular stomatitis virus glycoprotein involve dramatic structure reorganization

Membrane fusion is the key step in the entry of enveloped animal viruses into their host cells. Fusion of vesicular stomatitis virus with membranes occurs at acidic pH and is mediated by its envelope glycoprotein, the G protein. To study the structural transitions induced by acidic pH on G protein, we have extracted the protein from purified virus by incubation with nonionic detergent. At pH 6.0, purified G protein was able to mediate fusion of either phospholipid vesicles or Vero cells in culture. Intrinsic fluorescence studies revealed that changes in the environment of Trp residues occurred as pH decreases. In the absence of lipidic membranes, acidification led to G protein aggregation, whereas protein-protein interactions were substituted by protein-lipid interactions in the presence of liposomes. 1,1'-Bis(4-aniline-5-naphthalene sulfonate) (bis-ANS) binding was utilized to probe the degree of exposure of hydrophobic regions of G protein during acidification. Bis-ANS binding was maximal at pH 6.2, suggesting that a hydrophobic segment is exposed to the medium at this pH. At pH 6.0, a dramatic decrease in bis-ANS binding was observed, probably due to loss of tridimensional structure during the conformational rearrangement. This hypothesis was confirmed by circular dichroism analysis at different pH values, which showed a great decrease in alpha-helix content at pH values close to 6.0, suggesting that a reorganization of G protein secondary structure occurs during the fusion reaction. Our results indicate that G protein undergoes dramatic structural changes at acidic pH and acquires a conformational state able to interact with the target membrane.     

The interaction of inositol pentaphosphate with the hemoglobins of highland and lowland geese.

We have studied the binding of inositol pentaphosphate (IPP) to the hemoglobins from two species of goose living at low and high altitudes, using the proton absorption method. Measurements were done at 25 and 37 degrees C in a pH range between 6.0 and 8.8. The bird hemoglobins show a high affinity and a binding stoichiometry of 1 IPP molecule/hemoglobin tetramer both in the ligated and unligated state, indicating the same binding site for IPP in oxy- and deoxyhemoglobin. The results indicate that the interaction of IPP with both geese hemoglobins is very similar. For the deoxyhemoglobins of both species the IPP-binding constant shows a strong pH dependence extending over a wide pH range (i.e. +/- 2 x 10(6) M at pH 8.8 and +/- 6 x 10(10) M at pH 6.0). The binding constant of IPP for the oxyhemoglobins shows a much weaker pH dependence (i.e. +/- 4 x 10(4) M at pH 8.8 and +/- 3 x 10(6) M at pH 6.0), indicating that the interaction of IPP with the goose hemoglobin is strongly dependent on the state of ligation of the protein. The IPP binding constants for the oxy- and deoxyhemoglobins are found to be in good agreement with the IPP-induced change in oxygen affinity of both hemoglobins as estimated from oxygen binding curves.     

Kinetics and structural requirements for the binding protein of the Di-tripeptide transport system of Lactococcus lactis

The gene (dppA) encoding the binding protein of the di-tripeptide ABC transporter of Lactococcus lactis (DppA) was cloned under the control of the nisin promoter. Amplified expression ( approximately 200-fold increase) of the protein fused to a carboxyl-terminal six-histidine tag allowed the purification of DppA-(His)(6) by nickel-chelate affinity and anion-exchange chromatography. Ligand binding to DppA-(His)(6) elicited an electrophoretic mobility shift, a decrease in the intrinsic fluorescence, and a blue shift of the emission maximum. Each of these parameters detected conformational changes in the protein that reflect ligand binding, and these were used to determine the structural requirements of DppA-(His)(6) for binding peptides. The major features of peptide binding include (i) high affinity for di- and tripeptides, (ii) requirement of a free N-terminal alpha-amino group and an alpha-peptide bound contiguous with the N-terminal amino group, (iii) stereospecificity for L-isomers, and (iv) preference for dipeptides containing methionine or arginine, followed by hydrophobic tripeptides consisting of leucine or valine residues. Maximal binding affinity was detected at pH 6.0, and the K(d) for binding increased 1 order of magnitude for every unit increase in pH. This suggests that the ionization of protein residues (pK > 6.0) in or in close proximity to the binding site is critical in the binding mechanism.     

Study of pH- and temperature-induced transitions in F-protein (phosphofructokinase) by spectroscopy and microcalorimetry methods

The temperature- and pH-induced transitions in F-protein (phosphofructokinase (ATP:D-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11] have been studied by means of microcalorimetry and fluorescence and CD spectroscopy. An increase in pH from approx. 6.0 to approx. 8.0 causes a change in the protein state which seems to correspond to a shift of the dimer-tetramer equilibrium in favour of the tetramers. In the absence of phosphate, stability of the protein to temperature- and urea-induced denaturation at pH 6.0 is higher than that at pH 8.0. An addition of 150 mM phosphate results in a pronounced increase in the protein's stability in such a way that the protein becomes more stable at pH 8.0 than at pH 6.0. The shift of the denaturational heat capacity peak induced by the phosphate binding exceeds 25 degrees C at pH 8.0 and 9 degrees C at pH 6.0.     

Folate binding in intestinal brush border membranes: evidence for the presence of two binding activities

Binding of [(3)H]folic acid by isolated rat jejunal brush border membranes (BBMs) was analyzed by chromatography on small Biogel P-30 columns. Folic acid binding to BBMs exhibited a prominent pH effect with a sharp maximum at pH 5.5 to 6.0. After acid treatment to strip the BBMs of bound folate, the membranes demonstrated a wider pH optimum (5.5 to 7.5) of folate binding and a higher binding capacity. Scatchard analysis of binding experiments performed at pH 6.0 revealed the existence of two components: one with a high affinity (kd = 12 to 25 nM) and low capacity (V(max) for non-acidified BBMs = 0.259 to 0.264 pmol/mg protein, V(max) for acidified BBMs = 0.41 to 0.71 pmol/mg protein) and the other with a low affinity (kd = 1.1 to 5.1 microM and high capacity (V(max) for non-acidified BBMs = 0.93 to 1.93 pmol/mg protein, V(max) for acidified BBMs = 4.05 to 7.69 pmol/mg protein). Phosphatidylinositol-specific phospholipase C preferentially detached the high affinity component from jejunal BBMs. Phosphatidylinositol-specific phospholipase C-released folate binding protein was precipitated by antibodies to the high-affinity folate-binding protein from rat kidney. These data suggest the existence of two different folate-binding proteins in isolated rat jejunal BBMs. The high-affinity folate-binding protein shares epitopes with the folate-binding protein in the kidney.     

Differential Regulation of Phenylethanolamine N-Methyltransferase Expression in Two Distinct Subpopulations of Bovine Chromaffin Cells

Abstract: Chromaffin cells were isolated from bovine adrenal glands and fractionated into two distinct subpopulations by density gradient centrifugation on Percoll. Cells in the more dense fraction stored epinephrine (E) as their predominant catecholamine (81% of total catecholamines), contained high levels of phenylethanolamine N-methyltransferase (PNMT) activity, and exhibited intense PNMT immunoreactivity. This population of chromaffin cells was termed the E-rich cell population. Cells in the less dense fraction, the norepinephrine (NE)-rich cell population, stored predominantly NE (75% of total catecholamines). Although the NE-rich cells had only 3% as much PNMT activity as did the E-rich cells, 20% of the NE-rich cells were PNMT immunoreactive. This suggested that the PNMT-positive cells in the NE-rich cell cultures contained less PNMT per cell than did E-rich cells and may not be typical adrenergic cells. The regulation of PNMT mRNA levels and PNMT activity in primary cultures of E-rich and NE-rich cells was compared. At the time the cells were isolated, PNMT mRNA levels in NE-rich cells were ～20% of those in E-rich cells; within 48 h in culture, PNMT mRNA in both populations declined to almost undetectable levels. Treatment with dexamethasone increased PNMT mRNA levels and PNMT activity in both populations. In E-rich cells, dexamethasone restored PNMT mRNA to the level seen in freshly isolated cells and increased PNMT activity twofold. In NE-rich cells, dexamethasone increased PNMT mRNA to levels twice those found in freshly isolated cells and increased PNMT activity sixfold. Cycloheximide blocked the effects of dexamethasone on PNMT mRNA expression in NE-rich cells but had little effect in E-rich cells. Angiotensin II, forskolin, and phorbol 12,13-dibutyrate elicited large increases in PNMT mRNA levels in E-rich cells but had no effect in NE-rich cells. Our data suggest that PNMT expression is regulated differently in the two chromaffin cell subpopulations.     

Epinephrine: A Potential Neurotransmitter in Retina

Abstract: Dopamine (DA), norepinephrine (NE), and epinephrine (EPI) are present in rat retina. DA is the major catecholamine, whereas NE and EPI represent ∼5% of the DA content. DA is contained in a subpopulation of amacrine cells and has been the subject of numerous studies. We investigated the origin and properties of NE and EPI in retina. Following superior cervical ganglionectomy, there was a decrease in NE content, but no decrease in EPI or phenylethanolamine- N -methyltransferase (PNMT) activity. PNMT in retina has many of the substrate-specificity and inhibitor-sensitivity characteristics of other tissues. Enzyme activity is enhanced in newborn rats by treatment with dexamethasone. Exposure to a lighted environment increases retinal EPI in normal and superior cervical ganglionectomized rats. EPI content increased for more than 2 h in a lighted environment. We conclude that most of the NE is contained within the sympathetic neurons that innervate the eye from the superior cervical ganglion, whereas EPI is contained in retinal elements that are responsive to photic stimulation.     

Catecholamine Metabolism in Paraganglioma and Pheochromocytoma: Similar Tumors in Different Sites?

Pheochromocytoma (PHEO) and paraganglioma (PGL) are catecholamine-producing neuroendocrine tumors that arise respectively inside or outside the adrenal medulla. Several reports have shown that adrenal glucocorticoids (GC) play an important regulatory role on the genes encoding the main enzymes involved in catecholamine (CAT) synthesis i.e. tyrosine hydroxylase (TH), dopamine β-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT). To assess the influence of tumor location on CAT metabolism, 66 tissue samples (53 PHEO, 13 PGL) and 73 plasma samples (50 PHEO, 23 PGL) were studied. Western blot and qPCR were performed for TH, DBH and PNMT expression. We found a significantly lower intra-tumoral concentration of CAT and metanephrines (MNs) in PGL along with a downregulation of TH and PNMT at both mRNA and protein level compared with PHEO. However, when PHEO were partitioned into noradrenergic (NorAd) and mixed tumors based on an intra-tumoral CAT ratio (NE/E >90%), PGL and NorAd PHEO sustained similar TH, DBH and PNMT gene and protein expression. CAT concentration and composition were also similar between NorAd PHEO and PGL, excluding the use of CAT or MNs to discriminate between PGL and PHEO on the basis of biochemical tests. We observed an increase of TH mRNA concentration without correlation with TH protein expression in primary cell culture of PHEO and PGL incubated with dexamethasone during 24 hours; no changes were monitored for PNMT and DBH at both mRNA and protein level in PHEO and PGL. Altogether, these results indicate that long term CAT synthesis is not driven by the close environment where the tumor develops and suggest that GC alone is not sufficient to regulate CAT synthesis pathway in PHEO/PGL.     

Kinetic evidence for two interconvertible forms of the folate transport protein from Lactobacillus casei.

Lactobacillus casei cells contain a folate transport protein which exhibits a high affinity for folate. The dissociation constant (KD) for folate derived from binding parameters at the steady state (at 0 degree C) is 0.4 nM at pH 7.5 and 0.1 nM at pH 6.0. In the present study, folate binding to this protein at pH 7.5 (and 0 degree C) was shown to follow second-order kinetics and to proceed with an association constant (k+1) of 4.9 X 10(7) liter/mol per min. K+1 was not affected by preincubation conditions which alter the energetic state of the cell. Measurements on the extent of binding showed further that (at 0 degree C) essentially all unoccupied folate-binding sites reside at or are readily accessible to the outer surface of the membrane. In contrast, after saturating the binding site with [3H]folate, the first-order rate constant (k-1) for dissociation of the bound substrate (at 0 degree C) was found to vary substantially with the conditions employed. k-1 was 0.028/min in freshly harvested cells, but it increased by 2.8-fold in cells preincubated at 23 degrees C for 60 min and by 5.4-fold in isolated membranes. In addition, the faster rate observed in preincubated cells (k-1 = 0.077/min) returned to a slower rate after brief exposure of the cells to pH 6.0 (k-1 = 0.041/min), glucose (k-1 = 0.050/min), or both (k-1 = 0.012/min). k-1 was twofold lower at pH 6.0 than at pH 7.5 and was less dependent on the preincubation conditions, although it also increased substantially (5.5-fold) when the cells were converted to plasma membranes. The proposed explanation for these results is that folate transport protein of L. casei exists in two forms which can be distinguished by the accessibility of the binding site to the external medium and whose amounts are dependent upon the presence of bound folate, the pH, and the energetic state of the cell. It is suggested that these forms are transport proteins with binding sites oriented towards the inner and outer surfaces of the membrane.     

Hyaluronan binding to link module of TSG-6 and to G1 domain of aggrecan is differently regulated by pH

The physiological functions of hyaluronan (HA) in the extracellular matrix of vertebrate tissues involve a range of specific protein interactions. In this study, the interaction of HA with the Link module from TSG-6 (Link_TSG6) and G1 domain of aggrecan (G1), were investigated by a biophysical analysis of translational diffusion in dilute solution using confocal fluorescence recovery after photobleaching (confocal FRAP). Both Link_TSG6 and G1 were shown to bind to polymeric HA and these interactions could be competed with HA(8) and HA(10) oligosaccharides, respectively. Equilibrium experiments showed that the binding affinity of Link_TSG6 to HA was maximal at pH 6.0, and reduced dramatically above and below this pH. In contrast, G1 had maximum binding at pH 7.0-8.0 and moderate to strong binding affinity over a much broader pH range (5.5-8.0). The K(D) determined for Link_TSG6 binding to HA showed a 100-fold increase in binding affinity between pH 7.4 and 6.0, whereas G1 showed a 75-fold decrease in binding affinity over the same pH range. The sharp difference observed in their pH binding suggests that pH controls the physiological function of TSG-6, with a low affinity for HA at neutral pH, but with increased affinity as the pH falls below pH 7. TSG-6 and aggrecan interact with HA through structurally homologous domains and the difference in pH-dependent binding can be understood in terms of differences in the presence and topographical distribution of key regulatory amino acids in Link_TSG6 and in the related tandem Link domains in aggrecan G1.     

Inhibition of the hydration of CO2 catalyzed by carbonic anhydrase III from cat muscle

Using stopped flow methods, we have measured the steady state rate constants and the inhibition by N3- and I- of the hydration of CO2 catalyzed by carbonic anhydrase III from cat muscle. Also, using fluorescence quenching of the enzyme at 330 nm, we have measured the binding of the sulfonamide chlorzolamide to cat carbonic anhydrase III. Inhibition by the anions was uncompetitive at pH 6.0 and was mixed at higher values of pH. The inhibition constant of azide was independent of pH between 6.0 and 7.5 with a value of KIintercept = 2 X 10(-5) M; the binding constant of chlorzolamide to cat carbonic anhydrase III was also independent of pH in the range of 6.0 to 7.5 with a value Kdiss = 2 X 10(-6) M. Both of these values increased as pH increased above 8. There was a competition between chlorzolamide and the anions N-3 and OCN- for binding sites on cat carbonic anhydrase III. The pH profiles for the kinetic constants and the uncompetitive inhibition at pH 6.0 can be explained by an activity-controlling group in cat carbonic anhydrase III with a pKa less than 6. Moreover, the data suggest that like isozyme II, cat isozyme III is limited in rate by a step occurring outside the actual interconversion of CO2 and HCO3- and involving a change in bonding to hydrogen exchangeable with solvent water.     

A sensitive radioenzymatic assay for epinephrine forming enzymes

Epinephrine (E) is formed in the adrenal medulla by phenylethanolamine-N-methyltransferase (PNMT), and in other tissues. Enzymes other than PNMT may be able to synthesize E, but this has been difficult to investigate because most assays do not have E as their final product. This assay produces 3H-E from norepinephrine (NE) and 3H-S-adenosylmethionine. The 3H-E is isolated on alumina, 3H-S-adenosylmethionine is precipitated and the 3H-E is suspended in diethylhexyl phosphoric acid in toluene for scintillation counting. The assay is sensitive and linear over a wide range. E was formed by most tissues tested. Brain and adrenal contained an enzyme specific for NE, but cardiac ventricle contained an enzyme that methylated both NE and dopamine. Denervated tissues in adrenal medullectomized rats contained very little NE, but still had E and E forming enzyme present. This assay detects a non-neuronal E forming enzyme with activity in vitro and in vivo.     

Circadian Fluctuation of Catecholamines in the Cockroach Brain-Subesophageal Ganglion Complex detected by HPLC-ECD

Fluctuations of catecholamine contents in the cockroach brain-subesophageal ganglion (Br-SG) complex were examined by HPLC with electrochemical detector. The chromatographic system detected dopamine (DA), norepinephrine (NE), epinephrine (EP) and some putative metabolites as standard compounds. Da, NE and EP were detected in the Br-SG complex whereas those metabolites such as 2,5-dihydroxyphenylacetic acid (DOPAC) and 3-methoxy 4-hydroxy phenylglycol (MOPEG) were not detected from the tissue samples in a significant amount. The distribution of DA in the central nervous system was strongly biased toward cephalic ganglia, whereas EP was distributed more evenly over all ganglia. EP existed in both free and conjugated forms, the latter being predominant. Fluctuation patterns of these catecholamines were distinct; DA level kept constant throughout the day, at ca 200 ng/mg protein, NE showed a peak around AZT (artificial Zeitgeber time) 12, i.e., the light-off moment and the rhythm free-ran in constant darkness (DD), and both the free and the conjugated, i.e., acetylsulfate, forms of EP had peaks around mid-dark (AZT 18), in antiphase to the NE peak, and had a trough around AZT 12. Since both forms of EP showed the same fluctuation pattern, EP content in free form was regulated mainly by phenylethanolamine N -methyltransferase (PNMT) but not by hydrolysis of the conjugated EP. Since the enzymatic activities of monoamine oxidase (MAO), catechol O -methyltransferase (COMT) and aldehyde reductase (AR) were low, the fluctuation of these amines must be regulated by N -acetyltransferase (NAT), dopamine ß-hydroxylase (DBH) and PNMT.     

Protein binding and stability of norepinephrine in human blood plasma. Involvement of prealbumin, alpha 1-acid glycoprotein and albumin

The binding of norepinephrine (NE) to plasma proteins of fresh human blood obtained from healthy volunteers was studied by ultrafiltration at different NE concentrations and incubation times at 37 degrees C. At 1.7 nM L-[3H]-NE binding was approximately 25%. The binding was rapid and was not influenced by the incubation time. [3H]-NE could be dissociated from its binding sites by acid precipitation and, after HPLC, showed to be unchanged NE. No difference in NE binding was found between plasma collected in EGTA-GSH or heparin solution. There was no degradation of NE when incubated in plasma at 37 degrees C for 10 h, even without the addition of antioxidants. Therefore, in the present study, binding represented interaction of unchanged NE with plasma proteins. The whole plasma binding was saturable over the range of 0.66 nM to 0.59 mM of NE. Scatchard plot of specific binding revealed high-affinity sites with a Kd of 5.4 nM and a Bmax of 3.9 fmoles.mg-1 protein, and low-affinity sites with a Kd of 2.7 microM and a Bmax of 3.3 pmoles.mg-1 protein. Electrophoretic characterization of NE-binding proteins showed that about 60% of bound NE was associated to albumin, and 20% to prealbumin. NE binding to pure human plasma proteins was also studied using ultrafiltration. Scatchard analyses revealed a single class of very high-affinity binding sites for prealbumin (Kd 4.9 nM), a single class of binding sites for alpha 1-acid glycoprotein (Kd 54 microM) and two classes of binding sites for albumin with high (Kd 1.7 microM) and low (Kd 0.8 mM) affinities respectively. The main results obtained in this study - a) reversibility of NE binding, b) stability of free and bound NE in plasma, c) involvement of the prealbumin as a specific binding protein - point out to a specific transport for NE in human blood plasma.     

Kinetic and pH studies on human phenylethanolamine N-methyltransferase

Phenylethanolamine N-methyltransferase (PNMT) catalyzes the conversion of norepinephrine (noradrenaline) to epinephrine (adrenaline) while, concomitantly, S-adenosyl-l-methionine (AdoMet) is converted to S-adenosyl-l-homocysteine. This reaction represents the terminal step in catecholamine biosynthesis and inhibitors of PNMT have been investigated, inter alia, as potential antihypertensive agents. At various times the kinetic mechanism of PNMT has been reported to operate by a random mechanism, an ordered mechanism in which norepinephrine binds first, and an ordered mechanism in which AdoMet binds first. Here we report the results of initial velocity studies on human PNMT in the absence and presence of product and dead end inhibitors. These, coupled with isothermal titration calorimetry and fluorescence binding experiments, clearly shown that hPNMT operates by an ordered sequential mechanism in which AdoMet binds first. Although the log V pH-profile was not well defined, plots of log V/K versus pH for AdoMet and phenylethanolamine, as well as the pK_i versus pH for the inhibitor, SK&F 29661, were all bell-shaped indicating that a protonated and an unprotonated group are required for catalysis.     

Studies of the pH dependence of the formation of binary and ternary complexes with liver alcohol dehydrogenase

The association of the coenzyme NAD+ to liver alcohol dehydrogenase (LADH) is known to be pH dependent, with the binding being linked to the shift in the pK of some group on the protein from a value of 9-10, in the free enzyme, to 7.5-8 in the LADH-NAD+ binary complex. We have further characterized the nature of this linkage between NAD+ binding and proton dissociation by studying the pH dependence (pH range 6-10) of the proton release, delta n, and enthalpy change, delta Ho(app), for formation of both binary (LADH-NAD+) and ternary (LADH-NAD+-I, where I is pyrazole or trifluoroethanol) complexes. The pH dependence of both delta n and delta Ho(app) is found to be consistent with linkage to a single acid dissociating group, whose pK is perturbed from 9.5 to 8.0 upon NAD+ binding and is further perturbed to approximately 6.0 upon ternary complex formation. The apparent enthalpy change for NAD+ binding is endothermic between pH 7 and pH 10, with a maximum at pH 8.5-9.0. The pH dependence of the delta Ho(app) for both binary and ternary complex formation is consistent with a heat of protonation of -7.5 kcal/mol for the coupled acid dissociating group. The intrinsic enthalpy changes for NAD+ binding and NAD+ plus pyrazole binding to LADH are determined to be approximately 0 and -11.0 kcal/mol, respectively. Enthalpy change data are also presented for the binding of the NAD+ analogues adenosine 5'-diphosphoribose and 3-acetylpyridine adenine dinucleotide.     

Hypoxic stress‐induced changes in adrenergic function: role of HIF1α

Sustaining epinephrine‐elicited behavioral and physiological responses during stress requires replenishment of epinephrine stores. Egr‐1 and Sp1 contribute by stimulating the gene encoding the epinephrine‐synthesizing enzyme, phenylethanolamine N‐methyltransferase (PNMT), as shown for immobilization stress in rats in adrenal medulla and for hypoxic stress in adrenal medulla‐derived PC12 cells. Hypoxia (5% O₂) also activates hypoxia inducible factor (HIF) 1α, increasing mRNA, nuclear protein and nuclear protein/hypoxia response element binding complex formation. Hypoxia and HIF1α over‐expression also elevate PNMT promoter‐driven luciferase activity in PC12 cells. Hypoxia may be limiting as HIF1α over‐expression increases luciferase expression to no greater extent than oxygen reduction alone. HIF1α inducers CoCl₂ or deferoxamine elevate luciferase as well. PC12 cells harboring a HIF1α expression construct show markedly higher levels of Egr‐1 and Sp1 mRNA and nuclear protein and PNMT mRNA and cytoplasmic protein. Inactivation of Egr‐1 and Sp1 binding sites in the proximal ?893 bp of PNMT promoter precludes HIF1α stimulation while a potential hypoxia response element (?282 bp) in the promoter shows weak HIF1α affinity at best. These findings are the first to suggest that hypoxia activates the proximal rat PNMT promoter primarily via HIF1α induction of Egr‐1 and Sp1 rather than by co‐activation by Egr‐1, Sp1 and HIF1α. In addition, the rise in HIF1α protein leading to Egr‐1 and Sp1 stimulation of PNMT appears to include HIF1α gene activation rather than simply prevention of HIF1α proteolytic degradation.     

pH and temperature resolve the kinetics of two pools of calcium bound to the sarcoplasmic reticulum Ca2+ -ATPase

Calcium bound to the sarcoplasmic reticulum Ca2+ -ATPase was removed by chelating free calcium ion with EGTA. The kinetic calcium binding reaction to the calcium-unbound ATPase was studied by varying the pH (6.0-8.8) and temperature (0-20 degrees C) at a saturating concentration of 50-100 microM [Ca2+]. At pH 6.0 and 0 degrees C, calcium sites of the enzyme at a rate of t1/2 approximately 10 s. By increasing the pH from 6.0 to 8.8, about half of the total calcium sites were converted from a slow binding state to a rapid binding state (less than 2s). The maximum level was reached at about pH 7.4, and the midpoint of the conversion was observed at about pH 6.7. On the other hand, the slow binding reaction to the other sites was not significantly affected by the pH increase. At pH 7.0 and 20 degrees C, about 90% of the total calcium sites rapidly (less than 2s) bound calcium. The present results suggest that pH and temperature resolve the kinetics of two pools of calcium bound to the Ca2+-ATPase.