Measurement of microsomal ATPase activities: a comparison between the inorganic phosphate-release assay and the NADH-coupled enzyme assay

Measuring Ca2+ pump activity in overexpression systems and cardiac muscle preparations

Sarco-/endoplasmic reticulum (SR/ER) Ca(2+) pumps (SERCAs) build up vital Ca(2+) gradients across the intracellular SR/ER membrane, helping to control cell function, proliferation, growth, differentiation, and death. We describe two techniques to measure the SERCA activity either in mammalian culture cells overexpressing SERCAs or in muscle tissue containing high levels of endogenous SERCAs. As Ca(2+) transport is tightly coupled to ATP hydrolysis, it is possible to determine the rate of Ca(2+)-dependent ATP hydrolysis and use it as a measure for SERCA activity or, in a second approach, to quantify ATP-stimulated uptake of radioactive (45)Ca(2+). Here, we first provide an overview of the mechanism of Ca(2+)-transport ATPases and show how this can be taken advantage of in protocols for measuring Ca(2+) pump activity.

A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation

The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.

A generic assay for phosphate-consuming or -releasing enzymes coupled on-line to liquid chromatography for lead finding in natural products

A generic continuous-flow assay for phosphate-consuming or -releasing enzymes coupled on-line to liquid chromatography (LC) has been developed. Operating the LC-biochemical assay in combination with mass spectrometry allows the fast detection and identification of inhibitors of these enzymes in complex mixtures. The assay is based on the detection of phosphate, released by the on-line continuous-flow enzymatic reaction, using a fluorescent probe. The probe consists of fluorophore-labeled phosphate-binding protein, which shows a strong fluorescence enhancement upon binding to inorganic phosphate. To detect very small changes of the phosphate concentration in a postcolumn enzymatic reaction medium, the enzymatic removal of phosphate impurities from solvents, reagents, and samples was optimized for application in continuous flow. The potential of the phosphate probe is demonstrated by monitoring the enzymatic activity, i.e., the phosphate release, from alkaline phosphatase. The selectivity of the phosphate readout, necessary to distinguish between phosphate containing substrate or product and free inorganic phosphate released after enzymatic conversion, is shown. The applicability of LC coupled to the enzymatic assay using the phosphate readout was demonstrated by detection of tetramisole in a plant extract as inhibitor of alkaline phosphatase. Parallel mass spectrometry allowed the simultaneous confirmation of the identity of the inhibitor.

Effect of cytochalasin B on intestinal smooth muscle cells

The inhibitory effect of cytochalasin B on contraction of smooth muscle cells isolated from guinea-pig taenia coli was investigated. Cytochalasin B (10-70 microM) inhibited the high K+ (70 mM)-induced contraction in a dose-dependent manner, and the maximum and the half-maximum effects were obtained at 50 and 15 microM, respectively. Cytochalasin B (70 microM) decreased ATPase activity in skinned guinea-pig taenia coli. However, cytochalasin B (50 microM) had no significant effect on the voltage-dependent Ca2+ currents, the passive membrane properties or the membrane potential. Cytochalasin B also had no effect on the phosphorylation of 20 kDa myosin light chain induced by high K+ and cytosolic Ca2+ levels. These results suggest that the inhibition of contraction by cytochalasin B may be due to its effects on actin of microfilaments and contractile filaments of guinea-pig taenia coli smooth muscle cells.

Effects of myosin kinase inhibiting peptide on contractility and LC20 phosphorylation in skinned smooth muscle

A peptide inhibitor, myosin kinase inhibitor (MKI), of myosin light chain kinase (MLCK) was tested for its effects on contractility and myosin light chain phosphorylation in Triton X-100 skinned guinea pig taenia coli. MKI is based on the amino acid sequence of the myosin light chain (residues 11-19 LC20) and is a competitive inhibitor [inhibitory constant (Ki) congruent to 10 microM] of purified MLCK with respect to myosin light chain (LC20). MKI inhibited unloaded shortening velocity (V(us)) and the calcium-sensitive ATPase activity of the skinned fibers but had no significant effect on steady-state isometric force or myosin light chain phosphorylation, as measured by IEF-polyacrylamide gel electrophoresis analysis. MKI had no significant effect on V(us) of thiophosphorylated fibers in the absence of calcium. MKI inhibited MLCK activity in protein extracts from taenia coli, as measured by radioactive phosphate incorporation into LC20. Surprisingly, MKI also inhibited the phosphatase activity of these same extracts. This peptide slowed the rate and extent of relaxation of calcium-contracted fibers and elicited a contraction in relaxed fibers. These results are consistent with the hypothesis that MKI may be a phosphatase inhibitor as well as an inhibitor of MLCK. Our data further suggest that the rate of phosphorylation-dephosphorylation turnover may be important in regulating V(us) in smooth muscle.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.