Structure and biosynthesis of (Na,K)-ATPase in developing brine shrimp nauplii

Molecular domains in epithelial salt cellNaCl of crustacean salt gland (Artemia)

The salt secretory cell has two distinct patterns of plasma membrane development. First, the basolateral surface forms a tubular labyrinth. It contains the subunit alpha-2 of the Na(+)-K(+)-ATPase bound together with a beta subunit for structural attachment within the lipid bilayer. Second, the apical plasma membranes form a multiple array of extending tufts. These tufts contain the subunit alpha-1 of the Na(+)-K(+)-ATPase bound together with a beta subunit for structural integrity within the lipid bilayer. The presence of an active transporter for chloride remains as an open question. It has been taken as preliminary evidence from brine shrimp cystic fibrosis toxicity that a cystic fibrosis transmembrane conductance regulator chloride channel could be present in the apical region. The presence of cytoskeletal elements being involved in the construction of a hypo-osmoregulatory apparatus is supported by the homeobox gene products derived from APH-1 m RNA found in the salt gland.

The resting membrane potential of cells are measures of electrical work, not of ionic currents

Living cells create electric potential force, E, between their various phases by at least three distinct mechanisms. Charge separation, F = [equation: see text] (Eqn 1) creates the potential, E = [equation: see text] of -120 to -145 mV between cytoplasmic and mitochondrial phases by unbalanced proton expulsion powered by the redox energy of the respiratory chain. Electrically unbalanced flow of Na+ through voltage gated Na+ channels raises the potential of nerve from -85 to +30 mV. The so-called resting potential of cells, which varies from -85 mV in heart to -4.5 mV in red cell, does not appear to result from the unbalanced flow of ions between phases, but rather to be a measure of the work required to move ions between phases. Movement of an ion between phases entails three types of energy. Concentration work is that required to move an ion between phases containing different concentrations of ions: [equation: see text] Electrical work is that work required to move an ion from phases with differing electric potentials: [equation: see text] The Nernst potential of an ion existing at different concentrations in two phases is: [equation: see text] The osmotic work term is small and can generally be ignored. In heart the measured resting potential between extra- and intracellular phases, EN is approximately -85 mV. The calculated Nernst potential of K+, E [K+]out/in, is -85 mV (Eqn 4). This means that in heart, K+ distributes itself between the two phases as if it moved through an open ion channel. Its concentration work (Eqn 2) is equal in magnitude but opposite in sign to its electrical work (Eqn 3). This makes net K+ current flow, I, equal 0, indicating that this potential cannot be a diffusion potential. In liver the resting potential ranges from -28 to -40 mV, and is equivalent to the E[Cl-]out/in, while in red cell the resting potential is about -4.5 mV, which is equivalent to the potential of all nine major inorganic ion species except Na+, K+ and Ca2+. Therefore the resting potential between extra- and intracellular phases of cells should be thought of, not as a diffusion potential but rather as a measure of the electrical work: [equation: see text] required to transport the most permeant ions in a Gibbs-Donnan near-equilibrium system, either K+ or Cl- or both, between the phases of an aqueous system during the flow of current required to measure potentials with intracellular KCl electrodes or during ion movements brought about during normal cellular activity. The resting electrical potential results from the existence of a mono-ionic Gibbs-Donnan near-equilibrium system between the extra- and intracellular phases of cell wherein the activity of free H2O within all phases of the system is equal and the energy of the gradients of the nine major inorganic ions, delta G[ionz]out/in, are in near-equilibrium with one another, with the potential between the phases, EN, and with the energy of ATP hydrolysis. delta GATP Hydrolysis. ranges from a low of -55 to slightly over -60 kJ/mole in all cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cloning of the beta-subunit of the Na,K-ATPase in the brine shrimp, Artemia. The cDNA-derived amino acid sequence shows low homology with the beta-subunits of vertebrates except in the single transmembrane and the carboxy-terminal domains

A cDNA encoding the beta-subunit of the Na,K-ATPase of brine shrimp (Artemia) has been cloned. Its nucleotide sequence and predicted amino acid sequence have been determined. The amino acid sequence shows considerable divergence from that of chicken, dog, human, pig, rat, sheep, Torpedo, and Xenopus. This is not entirely unexpected since brine shrimp is a 'fast clock' organism which diverged from the precursor of the vertebrates 0.5-1.0 billion years ago. However, a highly hydrophobic putative transmembrane domain and the carboxy-terminal domain show considerable conservation. The relatively small degree of conservation in the beta-subunit of Artemia should provide information about the functional significance of this protein.

Isoforms of Na,K-ATPase in Artemia salina: II. Tissue distribution and kinetic characterization

To characterize the molecular properties conveyed by the isoforms of the alpha subunit of Na,K-ATPase, the two major transepithelial transporting organs in the brine shrimp (Artemia salina), the salt glands and intestines, were isolated in pure form. The alpha isoforms were quantified by ATP-sensitive fluorescein isothiocyanate (FITC) labeling. The salt gland enzyme exhibits only the alpha 1 isoform, whereas the intestinal enzyme exhibits both the alpha 1 and the alpha 2 isoforms. After 32 hours of development, Na,K-ATPase activity [in mumol Pi/mg protein/hr (1 mu)] in whole homogenates was 32 +/- 6 in the salt glands and 12 +/- 3 in the intestinal preparations (mean +/- SEM). The apparent half-maximal activation constants (K1/2) of the salt gland enzyme as compared to the intestinal enzyme were 3.7 +/- 0.6 mM vs. 23.5 +/- 4 mM (P less than 0.01) for Na+, 16.6 +/- 2.2 mM vs. 8.29 +/- 1.5 mM for K+ (P less than 0.01), and 0.87 +/- 0.8 mM vs. 0.79 +/- 1.1 mM for ATP (NS). The apparent Ki's for ouabain inhibition were 1.1 x 10(-4) M vs. 2 x 10(-5) M, respectively. Treatment of whole homogenates with deoxycholic acid (DOC) produced a maximal Na,K-ATPase activation of 46% in the salt gland as compared to 23% in the intestinal enzyme. Similar differences were found with sodium dodecyl sulfate (SDS). The two distinct forms of Na,K-ATPase isolated from the brine shrimp differed markedly in three kinetic parameters as well as in detergent sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Isoforms of Na,K-ATPase in Artemia saline: I. Detection by FITC binding and time course

Partially purified Na,K-ATPase from whole nauplii at various stages of development, analyzed by SDS-PAGE, reveals a polydisperse beta and two alpha subunits (denoted alpha 1 and alpha 2). In the absence of Ca2+, ATP-inhibitable fluorescein isothiocyanate (FITC) labeling is restricted to the alpha subunit of this enzyme, even in crude naupliar homogenates. The intensity of the alpha-specific fluorescent signal (i.e., the sum of the yield from both alpha isoforms) is proportional to Na,K-ATPase activity during development. FITC-labeled subunits were detected at 8 hr of development prior to the detection of measurable Na,K-ATPase activity. The alpha 2/alpha 1 ratio changed from an initial value of 1.25 to a peak of 1.75 at 32 hr of development, then reverted to a ratio of 1.25 by 42 hr, and remained constant thereafter. Pulse chase studies with 35S-methionine indicated that the developmental increase in enzyme activity is coincident with amino acid incorporation into the alpha subunits, implying that enzyme synthesis is active during enzyme accumulation.

Development and regional distribution of two molecular forms of the catalytic subunit of the Na,K-ATPase in rat brain

Two molecular forms of the catalytic subunit of the Na,K-ATPase can be isolated from brain (1). Only the alpha form was detected in rat embryo brain at 13 days of gestation (E13). The alpha (+) form, which is characteristic of myelinated axons, appeared at E15 before myelination begins. Hence its expression is not dependent on prior myelination. Axonal transport of the alpha (+) form was demonstrated in 4 day-old rats. The ratio of alpha (+): alpha was 1:1 in adult retina, cortex and cerebellum and 10:1 in brain stem. Although alpha (+) is characteristic of myelinated axons, this regional difference was present not only in enzyme extracted from crude microsomes, that contain myelinated axon fragments, but also in enzyme from isolated synaptosomes. Hence, the alpha (+): alpha ratio is an inherent characteristic of the neuron and does not depend on regional differences in myelination.

Binding sites for ouabain in human and rat erythrocytes and in rat heart cells

The presence and properties of two classes of binding sites for digitalis in erythrocytes and in heart cells are reviewed. Methods to distinguish between these two binding sites are summarized for intact cells and for their membranes. Our biochemical data do not suggest a physiological role for each class of sites. The membrane sites were modified by varying (a) the content of cholesterol or (b) the distribution of fatty acids leading to changes in the microviscosity thus affecting the degree of binding of ouabain to the two classes of sites. Oxidation of ouabain with periodate forms a di-aldehyde which could bind via Schiff base to the digitalis sites but could also attach covalently to these sites. The role of the sugar moiety in the process of ouabain binding becomes of increasing importance. A mild, controlled periodate oxidation of ouabain, especially in presence of phosphate cleaves only the bond between C-2 and C-3 of the rhamnose without affecting the steroid moiety. The periodate oxidation provided additional information for assigning a distorted chair conformation or a transient boat conformation for rhamnose in ouabain. It was also established by 1H NMR spectroscopy that this chair conformation is a 1C4 pyranose ring.