INHIBITION BY DIISOPROPYLFLUOROPHOSPHATE OF A KIDNEY TRANSPORT ADENOSINE TRIPHOSPHATASE BY PHOSPHORYLATION OF A SERINE RESIDUE

Inhibition of calcium-dependent ATPase from sarcoplasmic reticulum by a new class of indolizidine alkaloids, pumiliotoxins A, B, and 251D

Pumiliotoxins (PTX) A, B, and 251D, members of a new class of indolizidine alkaloids isolated from the skin of poison frogs of the family Dendrobatidae, inhibit Ca2+-ATPase activity in sarcoplasmic reticulum vesicles from frog and rat hind-limb muscles. PTX-B and PTX-A appear to be relatively specific inhibitors of Ca2+-ATPase; PTX-A is much less potent than PTX-B. PTX-251D is a potent inhibitor of Ca2+-ATPase, and was also found to inhibit Na+, K+, and Mg2+-ATPases in rat brain synaptosomes. Caffeine and verapamil, two drugs known to affect calcium translocation, are very weak inhibitors of the Ca2+-ATPase. The Ki values for inhibition of the Ca2+-ATPase of rat and frog sarcoplasmic reticulum by PTX-B were comparable and ranged between 22 and 36 microM. Inhibition of calcium-dependent ATPase in sarcoplasmic reticulum by pumiliotoxin-B is noncompetitive with calcium and is not readily reversible. Based on structure-activity profiles, it is concluded that inhibition of Ca2+-ATPase by the indolizidine alkaloids is responsible for the alkaloid-elicited prolongation of twitch in intact muscle.

Synaptosomal adenosine triphosphatase (ATPase) inhibition by organophosphates

Chicken spinal cord adenosine triphosphatases (both Na+, K+ stimulated and ouabain insensitive) were inhibited by tri-o-tolyl phosphate (TOTP, a neurotoxic organophosphate which is not a cholinesterase inhibitor) and mevinphos (a non-neurotoxic compound but inhibitor of cholinesterases). The inhibition was concentration and time dependent, with an initial rapid drop in activity followed by a gradual exponential decline.

The (Na++K+) activated enzyme system and its relationship to transport of sodium and potassium

It seems to be the membrane bound (Na++K+)-activated enzyme system which transforms the energy from a hydrolysis of ATP into a vectorial movement of sodium out and potassium into the cell against electrochemical gradients, i.e. this systems seems to be the transport system for sodium and potassium (see, for example, review by Skou, 1972; Hokin & Dahl, 1972).

Separation at neutral pH value of 32P-labelled proteins in a membrane preparation from ox brain. Partial characterization of component of the sodium-plus-potassium ion-activited adenosine triphosphatase

1. Ox brain microsomal fractions were labelled with [(32)P]ATP in the presence of Na(+) and the reaction was stopped with sodium dodecyl sulphate. The Na(+)-dependent bound phosphate was isolated on Sephadex G-25 and by acetone precipitation. The bound phosphate isolated under these neutral conditions was labile to hydroxylamine and gave the same pH profile of hydrolysis as that isolated by precipitation with strong acids. 2. When membrane protein was labelled with [(32)P]ATP, solubilized with sodium dodecyl sulphate and fractionated on Sepharose 6B, the Na(+)-dependent label emerged in a peak corresponding to protein of molecular weight 570000-580000. On fractionation of this protein peak on polyacrylamide gels containing detergent and urea, the Na(+)-dependent label occurred in a single band corresponding to a protein of molecular weight 102000. 3. Fractionation on Sepharose 6B of protein labelled with [(32)P]ATP in the absence of Na(+) revealed three labelled peaks, one of which corresponded in position to the Na(+)-dependent label. Electrophoresis of this peak material on polyacrylamide gels showed that most of the label occurred in two fast-running bands. Cyclic AMP stimulated the labelling in these two bands, but had no effect on the labelling of the band corresponding in position to the Na(+)-dependent label. 4. Di-isopropyl [(32)P]phosphorofluoridate also labelled the band corresponding to the Na(+)-dependent label on gel electrophoresis. The labelling of this band by the reagent was inhibited by 50-60% by 3mm-ATP, but there was no evidence to suggest that the group labelled is normally phosphorylated by ATP.

Phospholipid of the rat glomerular basement membrane in experimental nephrosis

Phospholipids were found to be a constant component of rat glomerular basement-membrane preparations. The concentration fell during preparation of basement membrane by sonication of whole glomeruli, but then remained constant despite continued sonication. The proportions of the individual phospholipids were different from those of whole renal tissue or of isolated glomeruli. The basement-membrane preparations had no (Na(+)+K(+))-activated adenosine triphosphatase activity, an enzyme that is bound to plasma membranes. The concentration of lipid P was decreased on exposure in vivo or in vitro to antiserum against basement membrane; 7 days after injection of antiserum there was a change in the phospholipid composition, with a relative increase in phosphatidylcholine and a decrease in sphingomyelin content. The metabolic turnover rate of the lipid P remaining in the membrane was normal, as determined by (32)P incorporation. The loss of phospholipid was associated with decreases in the relative concentrations of hydroxyproline, hydroxylysine and glycine, and relative increases in proline, lysine, serine, threonine and valine. Administration of aminonucleoside and daunomycin produced proteinuria but did not cause a decrease in lipid P. Anticollagen and anti-lymphocyte sera that attached to the basement membrane but failed to produce proteinuria, also failed to affect the phospholipid content.

Leukocidin, tetraethylammonium ions, and the membrane acyl phosphatases in relation to the leukocyte potassium pump

The response of the leukocyte to leukocidin and its relevance to excitable and secreting tissues are described. New data are presented on the leukocyte membrane phosphatases and the action of tetraethylammonium ion (TEA) on the leukocyte. The leukocyte surface membrane lacks a cation-sensitive ATPase but possesses a potassium- and ouabain-sensitive p-nitrophenyl phosphatase. The p-nitrophenyl phosphatase shows peak activity at three pH values and the pH dependence and potassium sensitivity depend on the state of the membranes. In the presence of magnesium, potassium can stimulate over the range pH 6 to 8. The relation of the leukocyte p-nitrophenyl phosphatase to electrolyte control in the leukocyte and to the properties of cation-sensitive phosphatases in other cells suggests that the leukocyte enzyme is a component of an electrogenic potassium pump. Leukocidin stimulates the leukocyte p-nitrophenyl phosphatase under all the conditions studied. The effect is specific and occurs under conditions that induce cytotoxic effects in the cell. It is concluded that the potassium pump is the site of action of leukocidin. TEA prevents the effects of leukocidin by inhibiting the action of leukocidin and not the responses of the cell to injury. TEA does not inhibit the p-nitrophenyl phosphatase nor prevent its stimulation by leukocidin. The enhancement of leukocidin by diisopropylphosphofluoridate (DFP) is briefly described. It is concluded that TEA acts in the opposite way to DFP and blocks the ion pathway activated by leukocidin in the leukocyte potassium pump.

Identification of components of (Na+ plus K+)-adenosine triphosphatase by double isotopic labeling and electrophoresis

A microsomal adenosine triphosphatase (ATPase) that requires both sodium and potassium ions is thought to be identical with, or an integral part of, the active cation transport system located in cell membranes. Attempts to isolate and purify (Na(+) + K(+))-ATPase have met with limited success because solubilization of microsomal protein causes partial, if not complete, loss of enzymatic activity. We now report the isolation from rat kidney microsomes of proteins which, though enzymatically inactive, could still be identified as components of the (Na(+) + K(+))-ATPase system. Phosphoproteins known to be intermediates in the hydrolysis of ATP by (Na(+) + K(+))-ATPase were prepared by incubating rat kidney microsomes with gamma-labeled ATP(33) in the presence of sodium or with P(32)-orthophosphate in the presence of ouabain. After the P(32)- and P(33)-labeled microsomes had been dissolved in phenol-acetic acid-urea, the resultant solutions were mixed and subjected to polyacrylamide gel electrophoresis. The radioactivity from both phosphorus isotopes was found almost exclusively in one of the resultant 21 protein bands. In contrast, the radioactive protein from DFP(32)-labeled microsomes moved slightly faster than the radioactive protein from microsomes labeled with P(33)-orthophosphate in the presence of ouabain. DFP inhibits (Na(+) + K(+))-ATPase by reacting with a nucleophilic site at or near the active site. These results suggest that while a single protein component of (Na(+) + K(+))-ATPase accepts the terminal phosphate from ATP, the final splitting of this phosphoprotein intermediate may be catalyzed by nucleophilic sites on a second protein.

Nucleoside phosphatase and cholinesterase activities in dorsal root ganglia and peripheral nerve

IN DORSAL ROOT GANGLIA AND PERIPHERAL NERVE OF THE RAT AND OTHER SPECIES, NUCLEOSIDE PHOSPHATASE AND UNSPECIFIC CHOLINESTERASE REACTION PRODUCTS ARE FOUND IN THE PLASMA MEMBRANES AND SPACES BETWEEN THEM AT TWO SITES: (1) Schwann cell-axon interfaces and mesaxons of unmyelinated fibers, and (2) sheath cell-perikaryon interfaces and interfaces between adjacent sheath cells. Acetylcholinesterase reaction product is found in the perikaryon (within the endoplasmic reticulum) and the axon (axoplasmic surface). Nucleoside phosphatase reaction product is also found in the numerous vacuoles at the surface of perineurium cells, ganglion sheath cells, and cells surrounding some ganglion blood vessels. Nucleoside phosphatase activities in the sections fail to respond, in the manner described for "transport ATPase," to diisopropylphosphofluoridate, sodium and potassium ions, and ouabain. Nucleoside diphosphates are hydrolyzed more slowly than triphosphates in unmyelinated fibers, and are not hydrolyzed at the perikaryon surface. Nucleoside monophosphates are either not hydrolyzed or hydrolyzed very slowly. In contrast to these localizations, which are believed to demonstrate sites of enzyme activity, it is considered likely that diffusion artifacts account for the nucleoside phosphatase reaction product frequently found along the outer surfaces of myelinated fibers and within vacuoles at the Schwann cell surfaces of these fibers. The diffuse reaction product seen in basement membranes of ganglion and nerve may also be artifact.

Evidence that a phosphorylated intermediate in a brain transport adenosine triphosphatase is an acyl phosphate

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