Lipid environment of gastric potassium ion-stimulated adenosine triphosphatase

General and specific interactions of the phospholipid bilayer with P-type ATPases

Protein structure and function are modulated via interactions with their environment, representing both the surrounding aqueous media and lipid membranes that have an active role in shaping the structural topology of membrane proteins. Compared to a decade ago, there is now an abundance of crystal structural data on membrane proteins, which together with their functional studies have enhanced our understanding of the salient features of lipid-protein interactions. It is now important to recognize that membrane proteins are regulated by both (1) general lipid-protein interactions, where the general physicochemical properties of the lipid environment affect the conformational flexibility of a membrane protein, and (2) by specific lipid-protein interactions, where lipid molecules directly interact via chemical interactions with specific lipid-binding sites located on the protein. However, due to local differences in membrane composition, thickness, and lipid packing, local membrane physical properties and hence the associated lipid-protein interactions also differ due to membrane location, even for the same protein. Such a phenomenon has been shown to be true for one family of integral membrane ion pumps, the P2-type adenosine triphosphatases (ATPases). Despite being highly homologous, individual members of this family have distinct structural and functional activity and are an excellent candidate to highlight how the local membrane physical properties and specific lipid-protein interactions play a vital role in facilitating the structural rearrangements of these proteins necessary for their activity. Hence in this review, we focus on both the general and specific lipid-protein interactions and will mostly discuss the structure-function relationships of the following P2-type ATPases, Na⁺,K⁺-ATPase (NKA), gastric H⁺,K⁺-ATPase (HKA), and sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), in concurrence with their lipid environment.

Purification and functional characterization of a low-molecular-mass Ca2+,Mg2+- and Ca2+-ATPase modulator protein from rat brain cytosol

A low-molecular-mass modulator protein having a molecular mass of about 12 kDa has been purified from rat brain cytosol following gel filtration and FPLC/Mono Q anion-exchange chromatographic separation. A number of protein fractions were obtained from an FPLC column when eluted with a 0.1 M NaCl hold gradient. One fraction (peak no. 5) was found to stimulate Ca2+,Mg2+-ATPase but inhibit Ca2+-ATPase isolated from goat spermatozoa. The S50 (concentration producing 50% stimulation) and I50 were found to be in the nanomolar range. The modulator seems to bind to Ca2+, Mg2+- or Ca2+-ATPase at a site distal from the ATP binding site. The binding to both the ATPases is reversible and non-competitive in nature. The inhibitory activity is found to depend significantly on -SH or -NH2 group(s) of the modulator, whereas no appreciable dependency of the stimulatory effect was apparent. The study indicates that the modulator is not a glycoprotein. CD analysis suggests that the protein exists as an unordered secondary structure. An immuno-cross-reactivity study with specific antibody and inhibition by thapsigargin suggests that the Ca2+,Mg2+- and Ca2+-ATPases from goat testes microsomal membranes are two isoforms of the sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA) family. The modulator does not contain any Trp molecules, as evident from Trp fluorescence analysis. Amino acid analysis shows that glycine, serine, derivatives of tyrosine and phenylalanine are the predominant amino acids. The data suggest that the modulator is a negatively charged protein and is a good tool for distinguishing the regulation of Ca2+,Mg2+- and Ca2+-ATPase activities.

An Na+/K(+)-ATPase inhibitor protein from rat brain cytosol

A protein isolated from rat brain cytosol is found to inhibit Na+/K(+)-ATPase in rat brain and kidney and H+/K(+)-ATPase from toad gastric mucosa, but has no effect on Ca2+,Mg(2+)-ATPase and Ca(2+)-ATPase isolated either from rat testis or goat spermatozoa. The inhibitor has been partially purified by ammonium sulphate precipitation followed by gel-filtration through Sephadex G-100. The inhibitor seems to bind at or close to the ATP binding site of Na+/K(+)-ATPase, such that the binding of the inhibitor to ATPase is reversible and competitive in nature with respect to the substrate. Optimum inhibition is observed at around the phase transition temperature of brain Na+/K(+)-ATPase and the inhibitory activity is only partially dependent on -SH or -NH2 group(s) of the inhibitor protein.

Brain NaK-ATPases in mice differentially sensitive to alcohols

The isozymes of NaK-ATPase were studied in the particulate fraction of homogenates prepared from cortex of LS and SS mice, two lines of mice that have been selectively bred for differential response to ethanol. In addition to a ouabain-insensitive Mg-ATPase, ouabain dose-response curves have suggested the presence, in brain, of two NaK-ATPase activities with different sensitivities to ouabain. These are designated low Ki (4 X 10(-7) M) and high Ki (2 X 10(-4) M). Ethanol differentially inhibited the ATPases: The ouabain-insensitive activity was less sensitive to ethanol inhibition than were the two ouabain-inhibitable activities. The low Ki (brain specific) NaK-ATPase was more sensitive than was the high Ki activity. However, ethanol inhibited all three components of the ATPase activity in an identical fashion in the two mouse lines. The low Ki activity was also more labile to thermal and p-hydroxymercurobenzoate denaturation than was the high Ki activity. These measures did not differ between the LS and SS lines. SDS-polyacrylamide electrophoresis of particulate fraction of cortical homogenates obtained from LS and SS mice revealed the presence of two protein bands with similar 32P labeling in both lines. Given that electrophoretic pattern and heat or p-hydroxymercurobenzoate inhibition were identical, it seems unlikely that differences in ATPase activity, or inhibition by ethanol, are responsible for the different response of LS and SS mice to ethanol.

Role of cholesterol in the structure and function of gastric microsomal vesicles

Digitonin was used as a tool to investigate the organization and function of cholesterol in gastric microsomes. Microsomal vesicles were treated with digitonin for different time at 0-4 degrees C under isotonic conditions. The effects of digitonin treatment of the vesicles on removal of cholesterol, ultrastructural changes, (H+ + K+)-ATPase activity, and gastric ATPase-dependent H+ uptake ability were investigated. Microsomal cholesterol was extracted in an exponential manner with a t1/2 of 32 min. There was no release of microsomal phospholipids by digitonin treatment during the same period. Digitonin treatment (30 min) produced visible "holes" in the vesicles; at the same time (H+ + K+)-ATPase-dependent H+ uptake was abolished. Under the same conditions the K+-stimulated ATPase activity, however, was moderately (about 35%) reduced, although the response of K+ stimulation to valinomycin was obliterated. Longer digitonin treatment resulted in gradual diffusion and eventual disappearance of the "holes" with the generation of distorted cup-shaped microsomes. The data strongly suggest that membrane lipids are freely mobile and that there is a certain degree of specialization in the organization of gastric microsomal cholesterol for the proper maintenance of the membrane structure and function.

Inhibition of gastric (H+ + K+)-ATPase by unsaturated long-chain fatty acids

Arachidonic acid and unsaturated C18 fatty acids at concentrations near 10(-5) M markedly inhibited (H+ + K+)-ATPase in hog or rat gastric membranes. Arachidonic acid was a more potent inhibitor than unsaturated C18 fatty acids, but the involvement of the metabolites of arachidonic acid cascade was ruled out. Linolenic acid inhibited the formation of phosphoenzyme and the K+ -dependent p-nitrophenylphosphatase activity of the hog ATPase. Treatment with fatty acid-free bovine serum albumin abolished only the inhibitory effect of the fatty acid on the phosphatase activity without restoring the overall ATPase action. These data suggest the existence of at least two groups of hydrophobic binding sites in the gastric ATPase for unsaturated long-chain fatty acids which affect differentially the catalytic reactions of the ATPase. (H+ + K+)-ATPase in rat gastric membranes was found more susceptible to the fatty acid inhibition and also more unstable than the ATPase in hog gastric membranes. The presence of a millimolar level of lanthanum chloride or ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid stabilized the rat ATPase probably via the inhibition of Ca2+ -dependent phospholipases in the gastric membranes.

Studies on (K+ + H+)-ATPase. IV. Effects of phospholipase C treatment

(1) The total phospholipid content of a gradient purified (K+ + H+)-ATPase preparation from pig gastric mucosa is 105 mumol per 100 mg protein, and consists of 29% sphingomyelin, 29% phosphatidylcholine, 28% phosphatidylethanolamine, 10% phosphatidylserine and 4% phosphatidylinositol. The cholesterol content corresponds to 50 mumol per 100 mg protein. (2) Treatment with phospholipase C (from Clostridium welchii and Bacillus cereus) results in an immediate decrease of the phosphate content. Up to 50% of the phospholipids are hydrolyzed by each phospholipase C preparation alone, without further hydrolysis by increased phospholipase concentration or prolonged incubation time. Combined treatment with the two phospholipase C preparations, sequentially or simultaneously, hydrolyzes up to 65% of the phospholipids. (3) The (K+ + H+)-ATPase and K+ stimulated p-nitrophenylphosphatase activities are decreased proportionally with the total phospholipid content, indicating that these enzyme activities are dependent on phospholipids. (4) Phospholipase C treatment does not change optimal pH, Km value for ATP and temperature dependence of the gastric (K+ + H+)-ATPase, but slightly decreases the Ka value for K+. (5) Phospholipase C treatment lowers the AdoPP[NH]P binding and phosphorylation capacities, suggesting that inactivation occurs primarily on the substrate binding level. (6) Most of the results can be understood by assuming that hydrolysis of the phospholipids by phospholipase C leads to aggregation of the membrane protein molecules and complete inactivation of the aggregated ATPase molecules.

Characterization of gastric-mucosal membranes. Distribution of lipid- and protein-associated amino groups across pig gastric microsomes

Two NH2-reactive probes (2,4,6-trinitrobenzesulphonic acid and 1-fluoro-2,4-dinitrobenzene) were used to study the vectorial orientation of the membrane-associated free NH2 groups across pig gastric microsomal vesicles. Unlike 1-fluoro-2,4-dinitrobenzene, 2,4,6-trinitrobenzenesulphonic acid is ordinarily an impermeant probe that becomes permeant in the presence of K+ and valinomycin. Although 2,4,6-trinitrobenzenesulphonic acid alone reacts with about 28% of the total microsomal phosphatidylethanolamine, 2,4,6-trinitrobenzenesulphonic acid in the presence of valinomycin plus K+ or 1-fluoro-2,4-dinitrobenzene alone reacted with 75% of the phosphatidyl- ethanolamine. Under similar conditions the free NH2 groups associated with the microsomal proteins also exhibited an asymmetric labeling pattern, the intra- and extravesicular orientation being 74 and 26% respectively.