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PubMed1993

Bicarbonate transport by rabbit duodenum in vitro: effect of vasoactive intestinal polypeptide, prostaglandin E2, and cyclic adenosine monophosphate.

BACKGROUND: Duodenal surface cells secrete bicarbonate that provides a barrier against injury. The current experiments were performed to identify duodenal bicarbonate regulatory and transport pathways. METHODS: Rabbit proximal duodenal mucosa were mounted in chambers under short-circuited conditions. Bicarbonate transport, short-circuit current (Isc), and potential difference (PD) were quantitated in response to prostaglandin E2 (PGE2), vasoactive intestinal polypeptide (VIP), and dibutyryl cyclic adenosine monophosphate (db-cAMP). Anoxia (N2), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and Cl(-)-free solutions, ouabain, and Na-free solutions were also studied, as was the effect of VIP and PGE2 on duodenocyte cAMP. RESULTS: PGE2, VIP, db-cAMP, and theophylline significantly increased bicarbonate secretion, Isc, and PD. Ouabain, Na(+)-free bathing solutions, and anoxia (N2) inhibited the responses. DIDS and Cl(-)-free solutions abolished the PGE2-induced response, reduced the response to VIP by about 50%, and had no effect on the response to db-cAMP. After PGE2 and VIP, cAMP concentration increased, yet was likely independent of bicarbonate secretion. CONCLUSIONS: Mammalian duodenal HCO3- transport requires Na+, Na+/K(+)-adenosine triphosphatase and O2-dependent metabolic pathways and is stimulated by PGE2, VIP, and cAMP, acting by distinct pathways.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1984

Secretin, VIP, and PHI stimulate rat proximal duodenal surface epithelial bicarbonate secretion in vivo.

The surface epithelial cells of the stomach and duodenum secrete bicarbonate at rest and in response to a number of agonists including the gastrointestinal hormones, glucagon, and GIP. Since those hormones with structural homology may have similar effects, the purpose of the present study was to examine the effect of graded doses (6, 24, and 96 nmol/kg) of pure porcine secretin, VIP, and PHI on bicarbonate secretion by the proximal duodenum containing Brunner's glands. Experiments were performed in vivo on unanesthetized Sprague-Dawley rats with chronic Thiry-Vella type loops of the proximal 2 cm of duodenum. The order of testing was random and only one hormone was tested on a single day. Compared to the saline control, each dose of VIP produced a significant increase in duodenal bicarbonate secretion in a dose-response manner. The two higher doses of secretin and only the 96 nmol/kg dose of PHI significantly increased bicarbonate output. The responses to 96 nmol/kg dose of secretin and VIP were similar, and each was significantly greater than observed with PHI. It is concluded that secretin and VIP stimulate proximal duodenal bicarbonate secretion and are more potent than PHI.

Animals↗
PubMed1984

A comparative study of bicarbonate secretion by ileum and colon of Oryctolagus cuniculus. Influence of caecal pH.

The influence of caecal pH on movements of bicarbonate ion in rabbit colon and ileum has been studied. A net secretion of bicarbonate is noted in both intestinal segments (at higher rates in colon than in ileum) in our experimental conditions. The bicarbonate secretion rate is lowered when bicarbonate is added to caecum. The introduction of acetic acid at caecal level increased bicarbonate secretion rate both in ileum as in colon.

Acetates↗
PubMed1987

Effects of bicarbonate on glial cell membrane potential in Necturus optic nerve.

Intracellular electrodes were used to continuously monitor the membrane potential of glial cells in the isolated Necturus optic nerve. Addition of up to 10 mM extracellular bicarbonate (with CO2), at constant pH, produced a hyperpolarization of up to 10 mV (with a time course almost as fast as that of a K+ depolarization) that returned toward baseline during the following 2-15 min. Upon bicarbonate withdrawal, the potential transiently became more positive. The bicarbonate effects were magnified when the K+ conductance was decreased and the cell depolarized by the addition of barium. Similar bicarbonate effects were observed in Cl- free solutions. These results suggest to us that: glial cells have a bicarbonate permeability of the same order as that to K+ and glial cells buffer transient changes in acid base balance in the neuronal microenvironment at the expense of their internal pH.

Animals↗
PubMed1989

Bicarbonate dependence of glutamate receptor activation by beta-N-methylamino-L-alanine: channel recording and study with related compounds.

beta-N-methylamino-L-alanine (BMAA) is a neurotoxic glutamate agonist possibly responsible for the neuronal degeneration found in the Guam amyotrophic lateral sclerosis-Parkinsonism-dementia complex. The basis for glutamate receptor activation by BMAA has been unclear, as BMAA lacks the omega electronegative moiety characteristic of other excitatory amino acids. We recently reported that the neuroexcitatory and neurotoxic effects of BMAA depend strongly on the presence of bicarbonate ions and proposed that an interaction between bicarbonate and the beta amino group of BMAA produces a molecular configuration appropriate for activating glutamate receptors. We now report that bicarbonate potentiates the ability of BMAA to open NMDA receptor-activated channels in isolated membrane patches. Furthermore, the neurotoxic and neuroexcitatory effects of two structural analogs of BMAA, DL-2,4-diaminobutyrate and DL-2,3-diaminopropionate, were also potentiated by bicarbonate. These findings support the bicarbonate cofactor hypothesis for BMAA action and provide direct evidence that it may be generalizable to certain other compounds.

Amino Acids, Diamino↗
PubMed2005

Bicarbonate-induced membrane processing in sperm capacitation.

During capacitation, major changes take place in the sperm plasma membrane so as to render it fusogenic and responsive to zona pellucida glycoproteins. However, the mechanisms involved have not been defined. As bicarbonate is known to be the key component that induces capacitation, we have investigated the bicarbonate-dependent changes in the boar sperm's plasma membrane architecture. We have discovered that bicarbonate induces a rapid collapse of phospholipid transverse asymmetry, exposing phosphatidylethanolamine and phosphatidylserine at the outer surface of the lipid bilayer. The collapse, which is reversible, is brought about as a result of activation of the phospholipid scramblase that exchanges phospholipids in a non-specific fashion between the two leaflets of the lipid bilayer. The activation takes place via a cyclic AMP-protein kinase A-dependent pathway and is initiated via stimulation of the so-called 'soluble' adenylyl cyclase in the sperm cell by bicarbonate. As a result of the collapse and the concurrent increase in phospholipid exchange, removal of cholesterol by albumin is facilitated (perhaps due to increased lipid packing disorder). This finding is in conflict with earlier surmises that cholesterol loss precedes activation of the cyclic AMP-protein kinase A axis. We have noted that not all cells in a given sperm population show rapid changes in response to bicarbonate stimulation; samples from individual boars also differ in their response. Maturation differences between cells have been found to play an important role in such functional heterogeneity.

Animals↗
PubMed2003

The effect of chewing bicarbonate-containing gum on salivary flow rate and pH in humans.

OBJECTIVE: Gum chewing increases salivary flow rate and pH. The aim of this study was to compare the effects of chewing standard sugar-free gum with those of a gum containing sodium bicarbonate. DESIGN: Whole mouth saliva was collected from 20 volunteers who met inclusion criteria and gave informed consent. After unstimulated saliva was collected, stimulated saliva was collected at intervals during 30 min of chewing either a standard, mint-flavoured gum or bicarbonate-containing, mint-flavoured gum. The salivary flow and pH were measured for each sample. RESULTS: With the standard gum, the mean peak salivary flow rate was 3.1+/-1.27 ml/min and the peak salivary pH was 7.39+/-0.14. With the bicarbonate gum, the peak flow rate was 2.79+/-1.38 ml/min and the peak salivary pH was 8.06+/-0.18. The salivary flow rates with the two gums were not significantly different; however, the increase in salivary pH was significantly greater for the bicarbonate gum. CONCLUSION: The increased salivary pH with bicarbonate gum may have implications for oral health and prevention of dental caries.

Adolescent↗
PubMed1998

Bicarbonate binding to the water-oxidizing complex in the photosystem II. A Fourier transform infrared spectroscopy study.

The light-induced Fourier transform infrared difference (FT-IR) spectrum originating from the donor side of O2-evolving photosystem (PS) II was obtained in non-depleted and CO2-depleted PSII membrane preparations. The observed spectrum free of contributions from the acceptor side signals was achieved by employing 2 mM/18 mM ferri-/ferrocyanide as a redox couple. This spectrum showed main positive bands at 1589 and 1365 cm(-1) and negative bands at 1560, 1541, 1522 and 1507 cm(-1). CO-depleted PSII preparations showed a quite different spectrum. The main positive and negative bands disappeared after depletion of bicarbonate. The addition of bicarbonate partially restored those bands again. Comparison between difference FT-IR spectra of untreated and bicarbonate-depleted PSII membranes indicated that the positive bands at 1589 and 1365 cm(-1) can be assigned to COO- stretching modes from bicarbonate. The higher frequency corresponds to u[as] (COO-) and the lower frequency to u[s] (COO-). 13C-Labeling FT-IR measurements confirmed these findings and also suggested that the negative band at 1560 cm(-1) can be ascribed to u[as] (COO-). The data are discussed in the framework of the suggestion that bicarbonate can be a ligand to the Mn-containing water-oxidizing complex of PSII.

Bicarbonates↗
PubMed1985

Gastric bicarbonate secretion in patients with duodenal ulcer.

The gastric mucosa secretes both acid and bicarbonate. In patients with duodenal ulcers, gastric acid hypersecretion is well-established. However, gastric bicarbonate secretion rates in these patients have not been reported. Accordingly, we calculated gastric acid and bicarbonate secretion rates in 13 patients with duodenal ulcer and 13 healthy subjects (controls) using a recently validated method that uses measurements of gastric juice volume, acidity (hydrogen-ion concentration), and osmolality. Non-parietal and parietal gastric volume secretion were also calculated. Although ulcer patients secreted significantly more acid than controls under basal conditions and during a submaximal intravenous pentagastrin infusion, they secreted similar amounts of bicarbonate. Ulcer patients secreted significantly more parietal and nonparietal fluid than controls. Increased nonparietal secretions in duodenal ulcer patients diluted excessive, acidic parietal secretions, preventing significant differences in gastric acidity in ulcer patients and control subjects (50.1 +/- 8.7 vs. 44.0 +/- 9.7 mmol/L basally; 111.0 +/- 3.1 vs. 102.4 +/- 7.5 mmol/L during pentagastrin infusion, respectively). These studies indicate that gastric bicarbonate secretion is close to normal in patients with duodenal ulcer, whereas acid secretion from parietal cells and nonparietal volume secretion are greater than normal.

Adult↗
PubMed1999

Bicarbonate binding to hemoglobin links oxygen and carbon dioxide transport in hagfish.

Hagfish are unusual among vertebrates in having red cells that almost completely lack the anion-exchanger membrane protein band III and hemoglobins that are highly sensitive to CO2 but only weakly affected by protons. This suggests a different linkage between oxygen and CO2 transport from that of the majority of vertebrates, which is characterized by oxygen-linked proton binding to hemoglobin and chloride-bicarbonate exchange across band III protein. We here report that the hemoglobin of the hagfish Myxtne glutinosa shows oxygen-linked binding of bicarbonate, which decreases the oxygen affinity and thereby enhances oxygen unloading to the tissues. Bicarbonate binding to the hemoglobin moreover facilitates the hydration of CO2 and its transport as intraerythrocytic bicarbonate, and may compensate at least in part for the virtual absence of band III protein. This represents a unique linkage between oxygen and CO2 transport among vertebrates, where the physiological role of protons in the Bohr and Haldane effects is played by bicarbonate ions.

Acetazolamide↗
PubMed1996

Intracerebroventricular secretin enhances pancreatic volume and bicarbonate response in rats.

BACKGROUND: Although secretin has been found within the brain, its central role in pancreatic exocrine function has not been previously addressed. The hypothesis that intracerebroventricular secretin enhances pancreatic volume and bicarbonate output at doses that have no effect when given intravenously was tested. METHODS: Sprague-Dawley rats had a cannula stereotactically placed into the left lateral cerebral ventricle 24 hours before study. At laparotomy the bile and pancreatic ducts were separately cannulated and excluded for tared collections and bicarbonate assay. RESULTS: Increasing doses of intracerebroventricular secretin (0.005, 0.05, and 0.5 microgram/1.0 microliter) induced a significant dose-related increase in bicarbonate output (2.95, 3.32, and 4.02 microEq/30 min, respectively) above basal (2.62 microEq/30 min) compared with control or intracerebroventricular saline treated animals. Pancreatic volume increased to 59.7 microliters at the lowest intracerebroventricular dose and increased (p < 0.025) to 65.8 microliters at the 0.05 intracerebroventricular secretin dose when compared with basal (59.4 microliters). To show that this was not a systemic effect of secretin, intravenous infusion of secretin at 0.005 and 0.05 microgram/kg/hr failed to stimulate either volume or bicarbonate output compared with that observed with intracerebroventricular secretin over the same dose range. CONCLUSIONS: These observations indicate that intracerebroventricular secretin stimulates pancreatic volume and bicarbonate output and suggest that central secretin may play a role in the regulation of exocrine pancreatic secretion.

Animals↗
PubMed1997

Effect of two-chambered bicarbonate lactate-buffered peritoneal dialysis fluids on peripheral blood mononuclear cell and polymorphonuclear cell function in vitro.

Low pH, high osmolality, increasing glucose concentration, and glucose degradation products (GDP) formed during heat sterilization of conventional peritoneal dialysis (PD) fluids have been shown to have a detrimental effect on cells involved in peritoneal host defense. The two-chambered PD fluid bag in which glucose at pH approximately 3 is separated from a bicarbonate (25 mmol/L)-lactate (15 mmol/L) buffer during heat sterilization permits PD fluids with lower GDP to be delivered to the patient at neutral pH. To establish the possible benefit of two-chambered bag PD fluids on peripheral blood mononuclear cell (PBMC) and polymorphonuclear (PMN) cell function, we compared conventional 1.5% Dianeal (1.5%D) with 1.5% two-chambered bag bicarbonate-lactate (1.5%D-B), and conventional 4.25% Dianeal (4.25%D) with 4.25% two-chambered bag bicarbonate-lactate (4.25%D-B). Furthermore, to study the effect of the sterilization process on PBMC and PMN function, we compared filter-sterilized 4.25%D (4.25%D-F) with 4.25%D and 4.25%D-B. PBMC were harvested by Ficoll-Hypaque separation, and 2.5 x 10(6) cells in RPMI were incubated with an equal volume of the test fluids for 4 hours, pelleted, and resuspended in RPMI containing 10 ng endotoxin for a further 20 hours. Tumor necrosis factor alpha (TNF-alpha) production by endotoxin-stimulated PBMC was not significantly different (P = 0.10) between 1.5%D-B and 1.5%D, but was significantly higher (P = 0.01) with 4.25%D-B compared with 4.25%D. PBMC exposed to filter-sterilized fluid (4.25%D-F) showed significantly higher endotoxin-stimulated TNF-alpha production compared with 4.25%D (P = 0.02), but was not significantly different from 4.25%D-B (P = 0.40). PMN were harvested by Ficoll-Hypaque separation and 10 x 10(6) cells incubated with test fluids for 30 minutes. After incubation, phagocytosis (phagocytosis index) was determined by the uptake of 14C-labeled Staphylococcus aureus, oxidative burst by reduction of ferricytochrome C to ferrocytochrome C on stimulation with PMA, and enzyme release by measurement of endotoxin-stimulated bactericidal/permeability increasing protein (BPI). Bicarbonate-lactate two-chambered fluids of similar osmolality and glucose concentration conferred a significant improvement in phagocytosis (P = 0.02 for 1.5%D-B and P < 0.001 for 4.25%D-B). Oxidative burst and BPI release were significantly higher in 4.25%D-B compared with 4.25%D (P < 0.001). Filter-sterilized 4.25%D-F conferred a significant improvement in phagocytosis and oxidative burst compared with 4.25%D (P < 0.001) or 4.25%D-B (P < 0.001). Furthermore, conventional 4.25%D was associated with significantly lower BPI release compared with 4.25%D-F (P = 0.01). GDP's acetaldehyde and 5-HMF were analyzed in 4.25%D-B, 4.25%D, and 4.25%D-F. Acetaldehyde was below the lower limit (0.79 ppm) of the standard curve in 4.25%D-B and 4.25%D-F fluids but was detected (3.76 to 5.12 ppm) in all of the 4.25%D fluids. Relative levels of 5-HMF in the 4.25%D-B (0.032 to 0.041 Abs @ 284 nm) and 4.25%D (0.031 to 0.036 Abs @ 284 nm) were similar. The lowest levels (0.001 Abs @ 284 nm) were observed in the filter-sterilized 4.25%D-F. The beneficial effects of two-chambered bicarbonate lactate-buffered PD fluids on PBMC and PMN function are probably related to reduction of GDP from heat sterilization of glucose in a separate chamber at a lower pH. This improvement in biocompatibility could have a beneficial affect on peritoneal defenses.

Bicarbonates↗
PubMed2001

Sodium bicarbonate improves the chance of resuscitation after 10 minutes of cardiac arrest in dogs.

The likelihood of successful defibrillation and resuscitation decreases as the duration of cardiac arrest increases. Prolonged cardiac arrest is also associated with the development of acidosis. These experiments were designed to determine whether administration of sodium bicarbonate and/or adrenaline in combination with a brief period of cardiopulmonary resuscitation (CPR) prior to defibrillation would improve the outcome of prolonged cardiac arrest in dogs. Ventricular fibrillation (VF) was induced by a.c. shock in anaesthetised dogs. After 10 min of VF, animals received either immediate defibrillation (followed by treatment with bicarbonate or control) or immediate treatment with bicarbonate or saline (followed by defibrillation). Treatment with bicarbonate was associated with increased rates of restoration of spontaneous circulation. This was achieved with fewer shocks and in a shorter time. Coronary perfusion pressure was significantly higher in NaHCO3-treated animals than in control animals. There were smaller decreases in venous pH in NaHCO3-treated animals than in controls. The best outcome in this study was achieved when defibrillation was delayed for approximately 2 min, during which time NaHCO3 and adrenaline were administered with CPR. The results of the present study indicate that in prolonged arrests bicarbonate therapy and a period of perfusion prior to defibrillation may increase survival.

Animals↗
PubMed1998

Cytosolic pH regulation in perfused rat liver: role of intracellular bicarbonate production.

The contribution of metabolic bicarbonate to cytosolic pH (pHcyto) regulation was studied on isolated perfused rat liver using phosphorus-31 NMR spectroscopy. Removal of external HCO3- decreased proton efflux from 18.6+/-5.0 to 1.64+/-0.29 micromol/min per g liver wet weight (w.w.) and pHcyto from 7.17+/-0.06 to 6.87+/-0.06. In the nominal absence of bicarbonate, inhibition of carbonic anhydrase by acetazolamide induced a further decrease of proton efflux of 0.69+/-0.26 micromol/min per g liver w.w. reflecting a reduction in metabolic CO2 hydration, and hence a decrease of H+ and HCO3- supplies. Even though 27% of the proton efflux was amiloride-sensitive under bicarbonate-free conditions, amiloride did not change pHcyto, revealing the contribution of additional regulatory processes. Indeed, pH regulation was affected by the combined use of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and amiloride since pHcyto decreased by 0.16+/-0.05 and proton efflux by 0.60+/-0.14 micromol/min per g liver w.w. The data suggest that amiloride-sensitive or SITS-sensitive transport activities could achieve, by themselves, pHcyto regulation. The involvement of two mechanisms, most likely Na+/H+ antiport and Na+:HCO3 symport, was confirmed in the whole organ under intracellular and extracellular acidosis. The evidence of Na-dependent transport of HCO3- in the absence of exogenous bicarbonate implies that the amount of metabolic bicarbonate is sufficient to effectively participate to pHcyto regulation.

4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfo↗
PubMed2001

Sodium-hydrogen exchangers and sodium-bicarbonate co-transporters: ontogeny of protein expression in the rat brain.

We used western blotting to examine the developmental profiles (at embryonic day 16 and postnatal days 1, 13, 23, 33 and 105) of protein expression for three sodium-hydrogen exchanger isoforms (1, 2 and 4) and for a sodium-bicarbonate co-transporter in three CNS regions (cortex, cerebellum and brainstem-diencephalon). In microsomal preparations, sodium-hydrogen exchanger isoform 1 and sodium-bicarbonate co-transporter protein expression in the CNS increases gradually from embryonic day 16 (25-40% of the adult level) to postnatal day 105. In contrast, sodium-hydrogen exchanger isoform 2 and 4 expression reaches a maximum (three to 20 times the adult level) at around three to four weeks of age. There is significant regional heterogeneity in the expression of sodium-hydrogen exchanger and sodium-bicarbonate co-transporter proteins in the rat CNS. Sodium-hydrogen exchanger isoform 1 was highly expressed in the brainstem-diencephalon, whereas the sodium-bicarbonate co-transporter was robustly expressed in the cerebellum and brainstem-diencephalon. These data indicate that the expression of sodium-hydrogen exchanger and sodium-bicarbonate co-transporter proteins varies as a function of both development and specific brain region.

Acid-Base Equilibrium↗
PubMed1999

Diagnostic aids in the differentiation of pyloric stenosis from severe gastroesophageal reflux during early infancy: the utility of serum bicarbonate and serum chloride.

This study evaluated whether serum bicarbonate levels, serum chloride levels, and other diagnostic criteria could be used to differentiate pyloric stenosis (PS) from severe gastroesophageal reflux (GER) during early infancy. The investigation was a retrospective, case-control study conducted in the emergency department of a large, academic children's hospital. Cases were 75 consecutive infants with PS confirmed in the operating room. Controls were 75 consecutive infants 12 weeks of age or younger with the diagnosis of GER whose serum electrolytes had been examined. Projectile vomiting was sensitive (0.93) but not specific (0.39) for PS. The mean serum bicarbonate level was 27.2 mmol/L for PS patients and 22.3 mmol/L for GER patients (P < .00001), and the mean serum chloride level was 95.7 mmol/L and 103.6 mmol/L for PS patients and GER patients, respectively (P < .00001). Serum bicarbonate levels of > or =29 mmol/L and serum chloride levels of < or =98 mmol/L had high positive predictive values (0.96 and 0.97, respectively) and were specific (0.99 for both) but not very sensitive (0.36 and 0.50, respectively) in identifying patients with PS. Only one patient would have been misclassified (false positive) as having PS using either of these cutoff values. These laboratory tests can also help discriminate between PS and GER when the history and physical examination fail to do so. For example, of the 20 patients with PS who did not have a pyloric mass palpated, 3 (15%) had serum bicarbonate levels of > or =29 mmol/L, and 6 (30%) had serum chloride levels of < or =98 mmol/L. In conclusion, the serum bicarbonate or serum chloride level offers a useful additional diagnostic tool in the evaluation of children presenting during early infancy with vomiting of uncertain etiology.

Bicarbonates↗
PubMed2000

A five way crossover human volunteer study to compare the pharmacokinetics of paracetamol following oral administration of two commercially available paracetamol tablets and three development tablets containing paracetamol in combination with sodium bicarbonate or calcium carbonate.

This report concerns a single dose randomized five way crossover study to compare the pharmacokinetics of paracetamol from two commercially available paracetamol (500 mg) tablets and three different development paracetamol (500 mg) tablet formulations containing either sodium bicarbonate (400 mg), sodium bicarbonate (630 mg) or calcium carbonate (375 mg). The results demonstrated that addition of sodium bicarbonate (630 mg) to paracetamol tablets, increased the rate of absorption of paracetamol relative to conventional paracetamol tablets and soluble paracetamol tablets. Addition of sodium bicarbonate (400 mg) to paracetamol tablets increased the absorption rate of paracetamol relative to conventional paracetamol tablets, but there was no difference in the rate of absorption compared to soluble paracetamol tablets. Inclusion of calcium carbonate (375 mg) to paracetamol tablets had no effect on absorption kinetics compared to the conventional paracetamol tablet. The faster absorption observed for the sodium bicarbonate formulations may be as a result of an increase in gastric emptying rate leading to faster transport of paracetamol to the small intestine where absorption takes place.

Acetaminophen↗
PubMed1975

Bicarbonate stabilization of ribulose 1,5-diphosphate carboxylase.

The carboxylase and oxygenase activities of purified soybean ribulose 1,5-di-P carboxylase (EC4.1.1.39) were unstable when reactions were initiated with enzyme. Time courses of carboxylase and oxygenase activities were curvilinear, approximating hyperbolas. Double reciprocal plots of amount of CO2 incorporated and P-glycolate produced vs. time were constructed to determine a constant representing the half-time of initial enzyme activity, K. K increased with increasing bicarbonate concentration but was independent of O2 tensions between 0.21 and 5 atm. When time courses of carboxylase and oxygenase activities were determined simultaneously, K was identical for both activities. Linear time courses were obtained py preincubation of the enzyme for 10 min in the absence of bicarbonate or by adding 46 mM MgCl2 to the reaction mixture. The observed bicarbonate-dependent decline in ribulose 1,5-di-P carboxylase activity with time is the probable cause for the anomalously high Km(CO2) values previously reported for this enzyme. In the experiments reported here, the apparent Km(CO2) at pH 8.5 increased from 6 muM CO2 at zero time to 78 muM CO2 at 10 min. The corresponding bicarbonate Km values ar 1;3 and 17 mM, respectively, The interaction between bicarbonate and enzyme may be important in the light activation of photosynthetic CO2 fixation in vivo.

Bicarbonates↗