Localization and cation dependence of a Ca2+- or Mg2+-ATPase from electrocytes of Electrophorus electricus, L

Electrocyte membranes of Electrophorus electricus exhibit high ATPase activity, as demonstrated by cytochemical and biochemical techniques. This activity is visualized as electron-dense deposits in electron micrographs, and appears to be localized only at the innervated face of the electrocyte. ATP hydrolysis can be detected cytochemically or biochemically only in the presence of calcium or magnesium. The effects of Ca or Mg on ATPase activity can be described by Michaelis-like functions with similar apparent Km values for Ca and Mg (0.41 mM and 0.23 mM, respectively). Vmax, however, is fivefold higher in the presence of Mg. The effects of the two cations are not additive, and pH dependence of ATP hydrolysis is identical in the presence of Ca or Mg (maximal at pH 8-9). Therefore, it can be concluded that Ca and Mg activate the same enzyme, the differences in Vmax being attributable to influences in kcat.     

Adsorption of 5′-adenosine monophosphate onto precipitated calcium phosphate: Effects of inorganic polyphosphates and carbamyl phosphate

In this paper it is shown that the adsorption of 5-adenosine monophosphate (5-AMP) onto precipitated calcium phosphate exhibits a sigmoidal profile as revealed by isotherms at 45 °C. This result indicates a cooperative behavior in the adsorption of 5-AMP. The relationship between adsorption capacity and surface area of the sedimented matrix may be interpreted as an indication that there is a monolayer of the adsorbed nucleotide on the solid surface. The pH dependence of adsorption suggests that the negatively charged phosphoryl group of 5-AMP interacts with a positively charged site (possibly Ca²⁺) on the matrix surface. The adsorption of the nucleotide is markedly decreased at pH values above 8.0. The Dixon-like plot of the effect of pH suggests an inhibitory role of hydroxyl ions in the adsorption of 5-AMP. At pH 7.5, other anions such as pyrophosphate, tripolyphosphate and carbamyl phosphate also inhibit the adsorption of the nucleotide, probably by interacting with its adsorption site. We suggest that these phosphorylated molecules could have played a role in chemical evolution by modulating the amount of nucleotides adsorbed onto mineral surfaces. The significance of these phenomena in chemical evolution is discussed.     

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. Received: 11 December 1995 / Accepted: 5 April 1996     

Reversible inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid of the plasma membrane (Ca(2+)+Mg2+)ATPase from kidney proximal tubules

Calcium accumulation by purified vesicles derived from basolateral membranes of kidney proximal tubules was reversibly inhibited by micromolar concentrations of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of anion transport. The inhibitory effect of this compound on Ca2+ uptake cannot be attributed solely to the inhibition of anion transport: (Ca(2+)+Mg2+)ATPase and ATP-dependent Ca2+ transport, respectively. The rate constant of EGTA-induced Ca2+ efflux from preloaded vesicles was not affected by DIDS, indicating that this compound does not increase the permeability of the membrane vesicles to Ca2+. In the presence of DIDS, the effects of the physiological ligands Ca2+, Mg2+, and ATP on (Ca(2+)+Mg2+)ATPase activity were modified. The Ca2+ concentration that inhibited (Ca(2+)+Mg2+)ATPase activity in the low-affinity range decreased from 91 to 40 microM, but DIDS had no effect on the Km for Ca2+ in the high-affinity, stimulatory range. Free Mg2+ activated (Ca(2+)+Mg2+)ATPase activity at a low Ca2+ concentration, and DIDS impaired this stimulation in a noncompetitive fashion. The inhibition by DIDS was eliminated when the free ATP concentration of the medium was raised from 0.3 to 8 mM, possibly due to an increase in the turnover of the enzyme caused by free ATP accelerating the E2----E1 transition, and leading to a decrease in the proportion of E2 forms under steady-state conditions. Alkaline pH totally abolished the inhibition of the (Ca(2+)+Mg2+)ATPase activity by DIDS, with a half-maximal effect at pH 8.3.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pyrite Suspended in Artificial Sea Water Catalyzes Hydrolysis of Adsorbed ATP: Enhancing Effect of Acetate

Minerals have been implicated in different catalytic processes during chemical evolution. It has been proposed that exergonic synthesis of pyrite (FeS₂) could have served to promote the endergonic synthesis of biomonomers in early stages of life formation on Earth. The present study was aimed to investigate whether pyrite can adsorb nucleotides and oxo acids in the potentially mild prebiotic conditions found away from the hot hydrothermal vents. It is shown that pyrite strongly adsorbs adenosine 5-triphosphate in an artificial medium that simulates primordial aqueous environments, and that adsorption is enhanced in the presence of acetate and in an oxygen-free atmosphere. Moreover, the mineral catalyzes the sequential hydrolysis of the and phosphoanhydride bonds of the nucleotide.     

Protein kinase C-mediated inhibition of renal Ca2+ ATPase by physiological concentrations of angiotensin II is reversed by AT1- and AT2-receptor antagonists

Angiotensin II (Ang II) increases the cytosolic Ca2+ concentration in different cell types. In this study, we investigate the effect of Ang II on the Ca2+ ATPase of purified basolateral membranes of kidney proximal tubules. This enzyme pumps Ca2+ out of the cytosol in a reaction coupled to ATP hydrolysis, and it is responsible for the fine-tuned regulation of cytosolic Ca2+ activity. Ca2+-ATPase activity is inhibited by picomolar concentrations of Ang II, with maximal inhibition being attained at approximately 50% of the control values. The presence of raising concentrations (10(-11) to 10(-7) M) of losartan (an AT1-receptor antagonist) or PD123319 (an AT2-receptor antagonist) gradually reverts inhibition by Ang II. Both the phospholipase C (PLC) inhibitor U-73122 (10(-6) M) and the inhibitor of protein kinase C (PKC) staurosporine (10(-7) M) prevent inhibition of the Ca2+ pump by Ang II. Incubation of the previously isolated membranes with a PKC activator-the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (10(-8) M)-mimics the inhibition found with Ang II, and the effects of the compounds are not additive. Taken as a whole, these results indicate the Ang II inhibits Ca2+-ATPase by activation of a PKC system present in primed state in these membranes after binding of the hormone to losartan- and PD123319-sensitive receptors coupled to a PLC. Therefore, inhibition of the basolateral membrane Ca2+-ATPase by kinase-mediated phosphorylation appears to be one of the pathways by which Ang II promotes an increase in the cytosolic Ca2+ concentration of proximal tubule cells.     

Macroinvertebrates as bioindicators of water quality in the Mkondoa River,Tanzania, in an agricultural area

The suitability of using macroinvertebrates as bioindicators of stream water quality was tested in the Mkondoa River in an agricultural area at Kilosa, using the rapid bioassessment protocol. The family biotic index (FBI) showed marked variation in water quality along the stream from values ranging from 4.1 to 5.0 in the upstream reaches, indicating good water quality, 5.3 to 5.5 in the mid-reaches and 6.0 to 6.5 in the lower reaches. The water quality index (WQI) indicated that water quality was fair (77 ± 0.98) in the upstream reach of the Mkondoa, marginal (55 ± 0.86) in the midstream reach and poor (33 ± 0.45) in the downstream reach. There were significant relationships between biological oxygen demand and dissolved oxygen and the occurrence of specific taxa, mainly Chironomus and Caenis. Significant changes in macroinvertebrate abundance were mostly related to changes in water quality. As in other parts of the world, macroinvertebrate communities proved to be good biological indicators of water quality and they should be used as bioindicators in long-term monitoring of this river.