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PubMed1989

Free sphingosine formation from endogenous substrates by a liver plasma membrane system with a divalent cation dependence and a neutral pH optimum.

Long-chain (sphingoid) bases may serve as another category of "lipid second messenger" because they inhibit protein kinase C and affect multiple cellular functions. Free sphingosine has been found in rat liver (Merrill, A. H., Jr., Wang, E., Mullins, R. E., Jamison, W. C. L., Nimkar, S., and Liotta, D. C. (1988) Anal. Biochem. 171, 373-381); hence, this study determined if liver plasma membranes contain free long-chain bases and have the ability to form them from endogenous enzymes and substrates. Isolated plasma membranes contained 0.45 nmol of sphingosine/mg of protein which, based on the recovery of the membranes, was equivalent to 3.5 +/- 1.2 nmol/g of liver and at least half of the total free sphingosine in liver. When the membranes were incubated at 37 degrees C, the amount increased at an initial rate of 5-25 pmol/min/mg, resulting in a 2-3-fold increase over an hour. Sphingosine formation required divalent cations, was optimal at neutral to alkaline pH, and was temperature-dependent. Activities with these characteristics were not identified in microsomes or lysosomes (lysosomal activities with acidic pH optima were detected, however); hence, they appear to reflect a separate plasma membrane system. Sphingosine formation was stimulated by ceramides either added exogenously or formed endogenously by treating the membranes with sphingomyelinase (but not endoglycoceramidase). Sphingomyelin hydrolysis to ceramide was also observed during incubation of the plasma membranes alone. Some of the properties of this system resembled the neutral sphingomyelinase and ceramidase activities of liver. While the physiological significance of this endogenous sphingosine is not known, this system has the appropriate subcellular location to provide sphingosine as a participant in signal transduction.

Animals↗
PubMed1990

Seizures induced by fluoroacetic acid and fluorocitric acid may involve chelation of divalent cations in the spinal cord.

Fluoroacetic and fluorocitric acid toxicity is often characterized by seizures, however the mechanism of this activity is unknown. Intrathecal (i.t.) injection of fluorocitrate in mice resulted in seizures after an average latency of 15 s, while intracerebroventricular (i.c.v.) injection produced seizures after 36.5 min, and required higher doses to achieve this effect. This indicates the probable site of fluoroacetate and fluorocitrate neurotoxicity is the spinal cord. To mimic citrate accumulation, characteristic of fluoroacetate and fluorocitrate poisoning, citric acid was injected i.t. and also found to produce seizures. The structurally unrelated compounds EDTA, EGTA, glutamic acid and lactic acid also produced seizures identical to fluorocitrate. The ability of these compounds to chelate Ca2+ correlates well with their ability to cause seizures when administered i.t. and coadministration of calcium greatly attenuated the neurotoxicity of these compounds as well as fluoroacetate and fluorocitrate. In contrast, Ca2+ was unable to inhibit seizures elicited by strychnine, suggesting calcium's ability to inhibit chelators of divalent cations is not due to a general anticonvulsant effect. These results suggest that changes in Ca2+ concentration in the spinal cord may be responsible for some forms of seizure activity.

Animals↗
PubMed1984

Toad egg-jelly as a source of divalent cations essential for fertilization.

Dejellied uterine eggs of the toad Bufo bufo japonicus are not fertilizable in 1/20 De Boer's solution (1/20 DB), but are fertilized when inseminated in a uv-solubilized jelly (UVJ) or the dialyzate of UVJ (UVJD). The present study was carried out to define this fertilization-supporting activity of egg-jelly. Dejellied eggs were fertilized in a high frequency when inseminated in a medium containing the ashes obtained by heating UVJD at 600 degrees C for 16 hr. Similarly, a reconstituted salt solution (RSS), which mimics the ionic composition of UVJD, supported a high rate of fertilization. To be effective in fertilization, however, RSS had to be present at the time of insemination. Analyses of individual salts revealed that dejellied eggs are successfully fertilized in CaCl2 and/or MgCl2 at 1-5 mM, only slightly in KCl at 10 mM, but not at all in NaCl at any of the concentrations tested. The activity of UVJD was lost reversibly when divalent cations were chelated by EDTA. The fertilization of dejellied eggs is therefore possible in a medium without any organic components of egg-jelly, provided that 2-5 mM Ca2+ or Mg2+ is present. Sperm were motile in media containing cations below 20-25 mM, regardless of the ionic composition. The egg-jelly possessed cations in a concentration of about 130 mM, but most ions were lost from intact jelly on immersion of eggs in water for 2-3 min, accompanied by the acquisition of fertilizability by sperm. Examination of the behavior of salts on dialysis or gel-filtration of jelly molecules revealed that the jelly retains Ca2+ and Mg2+, and possibly K+ as well, but not Na+ and Cl-. We propose that toad egg-jelly plays a function in fertilization by retaining Ca2+ and/or Mg2+ around each egg at the level necessary for successful sperm entrance into the egg.

Animals↗
PubMed1983

Inability of divalent cation complexes of A23187 to induce platelet secretion.

Extracellular calcium produces a dose-dependent inhibition of the early time course of A23187-induced 14C-5HT secretion in human platelets which is independent of the occurrence of aggregation. Pre-incubation of A23187 with calcium to form the A23187-Ca complex results in markedly prolonged lag periods prior to the onset of secretion at Ca concentrations of 0.25 - 1.0 mM and total abolition of the secretion response at 2.0 mM Ca, indicating that these effects of extracellular Ca are due to specific interactions of Ca and A23187. Under conditions which favor dissociation, the complexes of A23187 with Sr, Ca, and Mg, but not Mn, all induce platelet secretion but produce an inhibition of the early secretion response which generally parallels their relative complex stabilities, i.e., Sr less than Ca approximately equal to Mg less than Mn. Under conditions which prevent dissociation, none of these complexes induce secretion. These results indicate that the complexes of A23187 with divalent cations, including Ca, are unable to induce platelet secretion, most likely because of an inability to penetrate the platelet plasma membrane.

Animals↗
PubMed1986

Role of divalent cations on DNA polymorphism under low ionic strength conditions.

We have examined the conformational properties of poly(dG-m5dC) under a variety of low salt conditions and sample preparations. Extensive dialysis against 0.5 mM Na-cacodylate resulted in a left-handed polynucleotide conformation as determined by circular dichroism, in agreement with recently reported results. Similarly, extensive dialysis against Tris-EGTA also led to a left-handed conformation. Dilution of these samples led to a transition to the right-handed conformation. More stringent treatments such as dialysis followed by passage over an ion exchange column also resulted in a right-handed conformation. When these various solutions were examined using atomic absorption spectroscopy, significant levels of Mg+2 were observed (greater than or equal to 190 per 1000 nucleotides) in all samples showing a left-handed form, while much lower levels (less than or equal to 45 per 1000 nucleotides) were found in the low salt samples displaying a right-handed conformation. Addition of MgCl2 to samples in which divalent cations had been almost completely removed led to the reformation of the left-handed form. These results indicate that the left-handed form seen under certain low salt conditions is due to the presence of Mg+2 ions that remain bound to the polynucleotide, even in the presence of EDTA.

Cations, Divalent↗
PubMed1989

Angiotensin II vascular receptors in fowl aorta: binding specificity and modulation by divalent cations and guanine nucleotides.

In the domestic fowl, angiotensin (ANG) II causes a unique vasodepressor response in vivo and vascular relaxation of aortic rings in vitro that appear to be mediated by ANG II receptors. In initial studies using radioligand binding techniques, we identified specific vascular ANG II receptors in the fowl aorta. In the present study, we have characterized fowl vascular ANG II receptors in terms of binding specificity and their modulation by divalent cations and guanine nucleotide, to understand how the fowl receptor might differ from mammalian vascular ANG II receptors that mediate vasoconstriction. Competitive displacement of [125I] ANG II binding by ANG agonist and antagonist analogs revealed a unique pattern of receptor specificity, with the potency rank order: [Asn1, Val5]ANG II greater than [Asp1, Ile5]ANG II greater than [Asp1, Val5, Ser9] ANG I = [Asp1, Val5]ANG II much greater than [Val5]ANG III greater than [sarcosine(Sar)1, Ile5]ANG II greater than [Sar1, Ile8]ANG II much greater than [Sar1, Thr8]ANG II. Divalent cations (Ca++, Mg++ and Mn++) inhibited equilibrium radioligand binding by as much as 50% at 100 mM, with the potency order: Ca++ greater than Mn++ greater than Mg++. Mg++ and Mn++ stimulated binding very slightly (110%) at low doses (1-10 mM). The stable guanine nucleotide analog 5'-guanylyl imidodiphosphate inhibited equilibrium radioligand binding moderately (15% at 100 microM) in the presence of 10 mM MgCl2, but failed to alter the dissociation rate of receptor-bound ligand (half-time = 10.92 min). These results suggest that fowl vascular ANG II receptors exhibit specificity and regulatory properties fundamentally different from those of mammalian vascular ANG II receptors.

Angiotensin I↗
PubMed1980

On the role of inorganic phosphate in divalent-cation sequestration by mitochondria.

Rat liver mitochondria which have actively accumulated Mn2+ ions exhibit an electron para-magnetic resonance (EPR) spectrum dominated by a component E in which the usual hyperfine sextet has been collapsed to a single line through the process of spin exchange narrowing. When uptake occurs in the absence of added Pi, the line width of E shows considerable variation. By comparing experimentally observed spectra with computer-generated simulations it is demonstrated however that, under these conditions, E can be as narrow as approximately equal to 190 Oe, considerably narrower than the spectrum from a manganese phosphate precipitate. Furthermore parallel quantification on a single mitochondrial sample shows the level of Pi to be several times lower than the amount of Mn2+ observable in E. It is concluded therefore that the spectral component E, obtained under these conditions, cannot be accounted for in terms of manganese phosphate precipitate. These findings further indicate that a substantial number of sites (approximately equal to 100 nmol/mg protein), other than Pi, must exist inside the mitochondria which are capable of complexing divalent cations in regions of high local concentration.

Animals↗
PubMed1981

An investigation into the effects of manganese and other divalent cations on tyrosine hydroxylase activity.

The activity of tyrosine hydroxylase (TH) was measured in homogenates of hypothalamic tissue and corpus striatum obtained from 24 day old rats. The addition of manganese to the preparations had no direct effect on TH activity in either brain region. Similarly, the addition of calcium, magnesium and lead at concentrations of 1.0 mM did not affect TH activity. Cadmium and zinc (1.0 mM) significantly reduced TH activity in both the hypothalamic area and corpus striatum. Iron (1.0 mM) significantly stimulated TH activity in the corpus striatum but did not affect TH activity in the hypothalamic area. At conentrations of 1.0 mM, neither disodium ethylenediamine tetra-acetic acid (EDTA) nor ethylene glycol bis (beta-amino-ethyl ether)-N,N'-tetraacetic acid (ECTA) affected directly TH activity in either brain region. Cyclic adenosine 3',5'-monophosphate (cAMP) at a concentration of 1.0 mM did not affect TH activity in either the hypothalamic area or corpus striatum. Results from these in vitro studies show clearly that manganese does not inhibit directly TH activity and that manganese does not compete with other physiologically important divalent cations to cause the reduction in TH activity seen in vivo after chronic exposure.

Animals↗
PubMed1979

Intracellular divalent cations and neuronal excitability.

Itracellular injections of Mg into cat spinal motoneurones have a depolarizing action, associated with a fall in input conductance, and depression of the postspike hyperpolarizing after-potential (a.h.p.) as well as its underlying conductance increase. There is also an increase in excitability, sometimes leading to outright discharge, and a change in the current-firing relation: the normal primary range is largely abolished and the firing appears to have the characteristics of the normal secondary range. Intracellular effects of Mg are thus mainly opposite to those of Ca, possibly owing to competition at sites where Ca activates K channels. Intracellular injections of Mn also tend to depress the a.h.p. but have relatively little effect on resting potential and conductance, or action potentials. Co also depresses the a.h.p. but has a more pronounced depolarizing action, and produces particularly strong depression of action potentials. By contrast intracellular Sr tends to raise the membrane conductance and has a mild hyperpolarizing effect. During the injection of Sr, a.h.p's are depressed but this is followed by a rebound of increased a.h.p. amplitude and conductance. Unlike the other divalent cations tested, Sr strongly depressed excitatory postsynaptic potentials. In most respects Sr appears to behave like Ca.

Action Potentials↗
PubMed1978

Intracellular divalent cation release in pancreatic acinar cells during stimulus-secretion coupling. I. Use of chlorotetracycline as fluorescent probe.

Stimulus-secretion coupling in pancreatic exocrine cells was studied using dissociated acini, prepared from mouse pancreas, and chlorotetracycline (CTC), a fluorescent probe which forms highly fluorescent complexes with Ca2+ and Mg2+ ions bound to membranes. Acini, preloaded by incubation with CTC (100 microM), displayed a fluorescence having spectral properties like that of CTC complexed to calcium (excitation and emission maxima at 398 and 527 nm, respectively). Stimulation with either bethanechol or caerulein resulted in a rapid loss of fluorescence intensity and an increase in outflux of CTC from the acini. After 5 min of stimulation, acini fluorescence had been reduced by 40% and appeared to be that of CTC complexed to Mg2+ (excitation and emission maxima at 393 and 521 nm, respectively). The fluorescence loss induced by bethanechol was blocked by atropine and was seen at all agonist concentrations that elicited amylase release. Maximal fluorescence loss, however, required a bethanechol concentration three times greater than that needed for maximal amylase release. In contrast, acini preloaded with ANS or oxytetracycline, probes that are relatively insensitive to membrane-bound divalent cations, displayed no secretagogue-induced fluorescence changes. These results are consistent with the hypothesis that CTC is able to probe some set of intracellular membranes which release calcium during secretory stimulation and that this release results in dissociation of Ca(2+)-complexed CTC.

Animals↗
PubMed1985

Properties of a tyrosine protein kinase from calf thymus. Response to ionic strength and divalent cations.

A tyrosine protein kinase activity has been partially purified from calf thymus using the phosphorylation of the tyrosine-containing peptide angiotensin I as an assay. Detergent extracts of calf thymus possessed only low levels of specific peptide phosphorylating activity when assayed at low ionic strength. The inclusion of NaCl at a concentration of 2 M stimulated endogenous tyrosine protein kinase activity, while the activity of other endogenous kinases was inhibited. This sensitivity to NaCl was retained following partial purification of the enzyme. The phosphorylation of other substrates such as casein or the R-R-SRC peptide (Arg-Arg-Leu-Ile-Glu-Asp-Ala-Glu-Tyr-Ala-Ala-Arg-Gly) by the tyrosine protein kinase was less sensitive to NaCl. Phosphorylation of the PK-1 peptide (Leu-Arg-Arg-Ala-Ser-Leu-Gly) by the purified catalytic subunit of cAMP-dependent protein kinase was inhibited by NaCl. The effect of NaCl on angiotensin I phosphorylation could be mimicked by KCl or sodium acetate. The principal effect of NaCl was to increase the Vmax of the enzyme for the phosphorylation of angiotensin I. At low ionic strength, Mn2+ and Co2+ were the preferred required divalent cations. At elevated NaCl concentrations Mg2+ was preferred, with half-maximal activation occurring at 35 mM Mg2+. By conducting peptide phosphorylation assays in the presence of elevated levels of Mg2+ and NaCl, tyrosine protein kinase activity can readily be detected in extracts from cell lines that express low levels of the enzyme.

Angiotensin I↗
PubMed1987

Interaction of polylysine with bovine factor Xa: effect of divalent cations.

Poly-L-lysine has been demonstrated to partially replace biological cofactors in the activation of prothrombin by factor Xa. The present study was initiated to determine if poly-L-lysine has an effect on the enzymatic activity of factor Xa in the absence of prothrombin. At low ionic strength (50 mM Tris-Cl, pH 8.0, ambient temperature), poly-L-lysine inhibits amidase activity (S-2222) of bovine factor Xa with high affinity (Ki = 7 nM). The inhibition was readily reversed by 100 mM NaCl. The inhibition was also markedly reduced by the addition of 1.0 mM CaCl2 but not by MnCl2 or MgCl2. All three metal ions enhance amidase activity in the absence of poly-L-lysine. Poly-L-lysine also inhibits the amidase activity of factor Xa from which the gamma-carboxyglutamic acid domain has been removed by limited proteolysis with chymotrypsin (factor Xa-GD) but with somewhat lower avidity (Ki = 35 nM). As with native factor Xa, calcium ions reduce the observed inhibition while either manganese or magnesium ions are much less effective. The amidase activity of factor Xa-GD is enhanced with any one of the three divalent cations. These results provide additional support for the existence of a functionally significant binding site for calcium ions outside of the gamma-carboxyglutamic domain of factor Xa.

Amidohydrolases↗
PubMed2004

Divalent cations modulate the activity of metabotropic glutamate receptors.

Metabotropic glutamate receptors (mGluRs) and calcium receptors (CaR) are closely related G protein-coupled receptors (GPCRs). The similar structural and functional properties of mGluRs and CaRs include conserved amino acid residues involved in glutamate binding in mGluRs and Ca2+ binding in the CaR. Furthermore, recent findings have demonstrated that mGluRs can respond to high extracellular Ca2+ (Ca2+(o)) whereas CaR activity is potentiated by L-amino acids. We show that both mGluR1 and mGluR2 are activated by Ca2+(o) in the absence of glutamate in the extracellular media. This activation by Ca2+(o) is antagonized by Mg2+(o). Unlike the CaR, in which the intracellular carboxyl tail has been reported to be involved in Ca2+(o)-dependent activity, the carboxyl tail of mGluRs does not seem to play a role in mediating Ca2+(o) actions. On the other hand, we find that preservation of disulfide bonds in the N-terminal extracellular domain of mGluRs is essential for stimulation by Ca2+(o) as well as glutamate. Because the mGluR1 EC50 for Ca2+(o) is within the physiologic range of Ca2+ in the synaptic cleft, mGluR function is likely regulated by changes in divalent cations caused by synaptic activity under normal or pathologic conditions.

Calcium↗
PubMed1983

Inhibition of human red cell sodium and potassium transport by divalent cations.

The influx and efflux of Na and K across the human red cell membrane by the bumetanide-sensitive (Na-K co-transport) and residual (ouabain- and bumetanide-insensitive) routes were inhibited by increasing concentrations of external Mg. Ca, Sr, Ba, Mn and Co also inhibited bumetanide-sensitive and residual K transport. External Mg inhibited choline uptake and the Na-dependent fractions of L-alanine and L-serine uptakes. External Mg reduced the maximal rate (app. Vmax) but not the affinity (app. Km) of the bumetanide-sensitive K and Na influxes when they were measured as functions of external K and Na respectively. The inhibitory effect of Mg was not due to a small reduction in zeta potential since much larger reductions in zeta potential produced by neuraminidase did not affect transport. Internal Mg stimulated the ouabain-sensitive K influx but inhibited the co-transport and residual components of K influx. Bumetanide was a poor co-transport inhibitor in red cells pre-treated with A23187 and EDTA. It was concluded that the inhibitory effects of external Mg were probably not due to changes in the ionic composition of the diffuse double layer adjacent to the cell membrane. Mg and other divalent cations should not be used as 'inert' ionic substitutes in human red cell Na and K transport studies.

Amino Acids↗
PubMed1990

Effects of divalent cations on lipid flip-flop in the human erythrocyte membrane.

Treatment of human erythrocytes with ionophore A23187 (10 mumol.l-1) and Ca2+ (0.05-0.5 mmol.l-1) or Sr2+ (0.2-1 mmol.l-1) in results in a concentration-dependent acceleration of the transmembrane reorientation (flip) of the lipid probes lysophosphatidylcholine and palmitoylcarnitine to the inner membrane leaflet after their primary insertion into the outer leaflet. Mg2+, Mn2+, Zn2+ and La3+ do not accelerate flip. Ca2(+)-induced flip acceleration depends also on the ionophore concentration. It is reversed by removal of Ca2+ with EDTA. A causal role of Ca2(+)-induced membrane protein degradation and decrease of the polyphosphoinositide level in flip acceleration could be excluded. Likewise, calmodulin-dependent processes are probably not involved since the calmodulin antagonist calmidazolium (2-10 mumol.l-1) does not suppress but even enhances the Ca2(+)-induced flip acceleration. The same is true for the Ca2+ antagonist flunarizine. These drugs do not alter flip rate in the absence of Ca2+. At high Ca2+ (1-5 mmol.l-1) an initial flip acceleration is followed by flip normalization. High concentrations of Mn2+ and Mg2+ slow down flip rates. The selective acceleration of flip by Ca2+ and Sr2+ is discussed to be due to a local detachment of the membrane skeleton from the bilayer, whereas the unselective slow down of flip by divalent cations might be due to a stabilization of the membrane bilayer by the cations. After loading of cells with Ca2+ (but not with Mn2+) the inner membrane leaflet phospholipid phosphatidylserine becomes rapidly exposed to the outer membrane surface, as detectable by its accessibility to phospholipase A2 (5 min).(ABSTRACT TRUNCATED AT 250 WORDS)

Biological Transport↗
PubMedDate not supplied

Divalent cation and ATP dependent motility of Toxoplasma gondii tachyzoites after mild treatment with trypsin.

Large percentages of Toxoplasma gondii tachyzoites could be induced to display two types of movement associated with active invasive behavior by exposing them for 1 min to 0.002% trypsin in phosphate-buffered saline (PBS). The motile activity, consisting of clockwise rotation around the posterior end (about 20 revolutions per min) and twirling-gliding over a poly-L-lysine substrate (1.2 +/- 0.2 microns/s standard deviation), was observed and recorded by video-enhanced contrast microscopy. The number of active tachyzoites reached a maximum 1 min after trypsinization; the motile response of the population lasted for about 5 min. Activation was prevented by soybean trypsin-inhibitor, and could not be induced again in previously treated specimens. Electronmicroscopy of trypsinized tachyzoites fixed in the presence of ruthenium-red revealed discrete discontinuities of the plasma membrane, which sealed within 90 min after washing with PBS. Treated tachyzoites were able to invade cultured epithelial cells with a higher relative infectivity than that of untreated parasites. Perfusion of trypsinized tachyzoites with 1 mM of either CaCl2 or MgCl2 and 1 mM ATP increased the number of activated parasites to over 60%; on the other hand, all induced motility was inhibited or blocked by agents that chelate divalent cations. The present preparation, which provided the first serial illustrations of T. gondii movements induced by a defined chemical stimulus, may offer a useful experimental model for the study of motility in this parasite.

Adenosine Triphosphate↗
PubMed1983

Dimethonium, a divalent cation that exerts only a screening effect on the electrostatic potential adjacent to negatively charged phospholipid bilayer membranes.

Calcium and other alkaline earth cations change the electrostatic potential adjacent to negatively charged bilayer membranes both by accumulating in the aqueous diffuse double layer adjacent to the membrane and by adsorbing to the phospholipids. The effects of these cations on the electrostatic potential are described adequately by the Gouy-Chapman-Stern theory. We report the results of experiments with ethane-bis-trimethylammonium, a cation that has been termed "dimethonium" or "ethamethonium" in analogy with hexamethonium (hexane-1,6-bis-trimethylammonium) and decamethonium (decane-1,10-bis-trimethylammonium). We examined the effect of dimethonium on the zeta potential of multilamellar vesicles formed from the negative lipid phosphatidylserine (PS) and from 5:1 phosphatidylcholine/phosphatidylserine mixtures in solutions containing 0.1, 0.01 and 0.001 M sodium, cesium, or tetramethylammonium chloride. We also examined the effect of dimethonium on the conductance of planar PS bilayer membranes and the 31P NMR signal from sonicated PS vesicles formed in 0.1 M NaCl. We found no evidence that dimethonium adsorbs specifically to bilayer membranes. All the results, except for those obtained with vesicles of low charge density formed in a solution with a high salt concentration, are consistent with the predictions of the Gouy-Chapman theory. We conclude that dimethonium, which does not have the pharmacological effects of hexamethonium and decamethonium, is a useful divalent cation for physiologists interested in investigating electrostatic potentials adjacent to biological membranes.

Bis-Trimethylammonium Compounds↗
PubMed1996

Divalent cations modify adsorption of 5'-AMP onto precipitated calcium phosphate: a model for cation modulation of adsorptive processes in primitive aqueous environments.

The adsorption of 5'-AMP onto precipitated calcium phosphate (CaPi) requires the presence of soluble calcium and this dependence exhibits a Michaelian-like behavior. This result suggests that the formation of a complex between 5'-AMP and free Ca2+ (CaAMP) is a prelude to the adsorption of the nucleotide in the solid matrix. At concentrations one order of magnitude higher, Mn2+ and Mg2+ can substitute for soluble Ca2+ in the adsorption of 5'-AMP onto solid CaPi. However, when added simultaneously with 5'-AMP to a heterogeneous mixture that contains CaPi and soluble Ca2+, Mn2+ and Mg2+ inhibit the adsorption of 5'-AMP in a concentration-dependent manner. This suggests the formation of complexes that are much less effective for 5'-AMP adsorption than the CaAMP complex. On the other hand, Mn2+ and Mg2+ cannot promote desorption of the nucleotide attached to the precipitate in the presence of soluble Ca2+ if they are added after adsorption has attained equilibrium. Although desorption of 5'-AMP can be obtained by a sequential dilution of the soluble phase with buffer and no nucleotide in a process that obeys a Langmuir equation, the lack of effect of Mn2+ or Mg2+ when adsorption has attained its maximal value suggests strong interactions between the CaAMP complex and the solid matrix when adsorption equilibrium is reached. The divalent cations present in the matrix also participate with different selectivity in the attachment of the CaAMP complex, indicating that a cation-exchange mechanism could have acted in the modulation of adsorptive/desorptive processes involving biomonomers and phosphate surfaces in primitive aqueous environments.

Adenosine Monophosphate↗