EFFECT OF MONOVALENT CATIONS ON THE ADENOSINETRIPHOSPHATASE OF SONICATED ERYTHROCYTE MEMBRANE

The involvement of phosphatidylserine in adenosine triphosphatase activity of the sodium pump

1. A study has been made to see whether the ATPase of the Na pump is activated by phospholipids.2. Solubilization of a membrane-containing (microsomal) fraction from ox brain cortex with deoxycholate removed most or all of the ouabain-inhibited ATPase and p-nitrophenylphosphatase (pNPPase) activities. Addition of sonicated dispersions of crude commercial samples of phosphatidic acid, phosphatidylinositol or phosphatidylserine stimulated both enzymic activities, but neither phosphatidylcholine nor phosphatidylethanolamine affected them. After partial purification by chromatography, however, definite activation was demonstrated only with that component of crude samples which migrated like phosphatidylserine.3. The ATPase activity dependent on phosphatidylserine was eliminated by removal of Na and K or by addition of ouabain and was substantially inhibited by oligomycin. The corresponding component of pNPPase activity was partially inhibited by ouabain or by removal of most of the K, but was little affected by oligomycin. The phosphatidylserine-dependent enzyme(s) therefore exhibited properties characteristic of the ATPase of the Na pump.4. Phosphatidylserine dispersions similarly activated both the ATPase and the pNPPase of the particulate enzyme preparations, untreated with deoxycholate.5. The results suggest that the system for active transport of Na and K involves a complex unit of phosphatidylserine and the protein of the ATPase.

The localization of Mg-Na-K-activated adenosine triphosphatase on red cell ghost membranes

The lead salt method introduced by Wachstein and Meisel (12) for the cytochemical demonstration of ATPase activity was modified and used to determine sites of activity on red cell ghost membranes. Preliminary studies showed that aldehyde fixation and standard concentrations of the capture reagent Pb(NO(3))(2) resulted in marked inhibition of the ATPase activity of these membranes. By lowering the concentration of Pb(2+) and incubating unfixed red cell ghosts, over 50% of the total ATPase activity, which included an ouabain-sensitive, Na-K-activated component, could be demonstrated by quantitative biochemical assay. Cytochemical tests, carried out under the same conditions, gave a reaction product localized exclusively along the inner surfaces of the ghost membranes for both Mg-ATPase and Na-K-ATPase. These findings indicate that the ATPase activity of red cell ghosts results in the release of P(i) on the inside of the ghost membrane at sites scattered over its inner aspect. There were no deposits of reaction product on the outer surface of the ghost membrane, hence no indication that upon ATP hydrolysis P(i) is released outside the ghosts. Nor was there any clear difference in the localization of reaction product of Mg-ATPase as opposed to that of Na-K-ATPase.

The effects of magnesium on nucleoside phosphatase activity in frog skin

Histochemical tests, employing the Wachstein-Meisel medium, indicate that nucleoside triphosphatase activity is found predominantly in two areas of the frog skin epidermis: (1) in mitochondria, where activity is enhanced by dinitrophenol, Mg(2+) dependent, but inhibited by fixation; and (2) apparently associated with cell membranes of the middle and outer portions of the epidermis, where activity is inhibited by Mg(2+), unaffected by dinitrophenol, and only slightly reduced by fixation. Spectrophotometric analysis shows that Mg(2+) in the medium does not increase spontaneous hydrolysis of ATP, thus obviating the possible explanation that changes in substrate concentrations in the medium lead to alterations in the "staining" distributions. It is postulated that perhaps the two enzymes differ in their requirements for substrate-one requiring the polyphosphate to be in complexed form with Mg(2+), the other uncomplexed. Concentrations of Mg(2+) required to inhibit cell membrane nucleoside triphosphatase activity also inhibit the electrical potential difference and short-circuit current of the frog skin. Although these observations might be taken as presumptive evidence of the cell membrane enzyme as a component of the ion pump system, because of certain dissimilarities with respect to the biochemists' "transport ATPase" and for other reasons discussed in the paper, any definite conclusions in this regard are premature.

Cytochemistry of phosphatases of the sarcoplasmic reticulum. I. Biochemical studies

A microsomal fraction was isolated from rabbit psoas muscle by a modification of Muscatello's method. The fraction contained a Mg-dependent ATPase which had a pH optimum of 7.5. Activity was further stimulated by addition of Na or K or other monovalent cations to the reaction mixture, but synergistic activation by Na and K, and ouabain inhibition, could not be demonstrated. The enzyme hydrolyzed only ATP (adenosine triphosphate) and ITP (inosine triphosphate) at appreciable rates, but Na or K stimulated activity only when ATP was used as substrate. Activity was inhibited by Ca and by low concentrations of Na deoxycholate, and was sensitive to inhibition by thiol group reagents. The enzyme could be distinguished from another enzyme, also present in the fraction, which was Ca-activated, and which exhibited a wider substrate specificity, different pH activation characteristics, lower specific activity, lack of stimulation by Na or K, and less sensitivity to inhibition by deoxycholate and by thiol group reagents. These findings formed the basis for demonstration of the Mg-dependent ATPase in situ.

Adenosine triphosphatase localization in amphibian epidermis

The localization of ATPase(1) activity has been studied by light and electron microscopy in the epidermis of Rana pipiens, Rana catesbiana, and Bufo marinus. The reaction was carried out on skin (glutaraldehyde-fixed or fresh) sectioned with or without freezing. Best results were obtained with nonfrozen sections of fixed tissue. The incubation mixture was either a Wachstein-Meisel medium, or a modification which approximates assay systems used in biochemical studies of transport ATPases. The reaction product was found localized in contact with the outer leaflet of all cell membranes facing the labyrinth of intercellular spaces of the epidermis. It was absent from: (a) membrane areas involved in cell junctions (desmosomes, zonulae and maculae occludentes); (b) cell membranes facing the external medium (i.e., those on the distal aspect of the ultimate cell layer in s. corneum); (c) cell membranes facing the dermis (those on the proximal aspect of cells in s. germinativum). In the presence of (Na(+) + K(+)) the localization did not change, but the reaction was not appreciably activated. A similar though less intense reaction was obtained with ITP, but not with ADP, AMP, and GP as substrates. The results are discussed in relation to available data on transport ATPases in general, and on the morphology and physiology of amphibian skin in particular. Assuming that the ATPase studied is related to transport ATPase, the findings suggest a series of modifications to the frog skin model proposed by Koefoed-Johnsen and Ussing. The salient feature of this modified model is the localization of the Na(+) pump along all cell membranes facing the intercellular spaces of the epidermis.

Active transport and enzymes of the erythrocyte membrane under protein deprivation

1. Starvation for 3 days causes membrane damage of the rat erythrocyte manifested by several alterations. The adenosine-triphosphatase activity is decreased but that of acetylcholinesterase is not affected. 2. The ouabain-sensitive adenosine-triphosphatase activity increases at the expense of the non-sensitive enzyme moiety. 3. The Rb(+) uptake is not altered but the galactose transport is accelerated by the stated experimental conditions. 4. The modifications induced by starvation do not recover on re-feeding.