The localization and properties of membrane adenosine triphosphatases in the guinea-pig placenta

Summary The distribution and properties of cytochemically demonstrable phosphatases in the near-term guinea-pig placenta were examined using a strontium capture technique for sodium- and potassium-dependent adenosine triphosphatase (Na⁺, K⁺-ATPase) and a lead capture technique for magnesium-dependent adenosine triphosphatase (Mg²⁺-ATPase).Localizations with the strontium technique in the presence of an alkaline phosphatase inhibitor were mainly on the syncytiotrophoblast plasma membranes; the reaction was potassium-dependent and ouabain-sensitive. Reaction product using the lead capture method was found on both trophoblast and endothelial cell plasma membranes and was independent of magnesium and insensitive to p-hydroxymercuribenzoate (POHMB), an inhibitor of membrane ATPases. However, a very large proportion of this reaction could be blocked by an alkaline phosphatase inhibitor.It is concluded that the strontium capture technique gave a reliable localization for Na⁺, K⁺-ATPase. However, the lead capture method mainly demonstrated alkaline phosphatase, and does not offer a useful approach to specific ATPase studies in this particular system.     

The localization and properties of membrane adenosine triphosphatases in the guinea-pig placenta.

The distribution and properties of cytochemically demonstrable phosphatases in the near-term guinea-pig placenta were examined using a strontium capture technique for sodium- and potassium-dependent adenosine triphosphatase (Na+, K+-ATPase) and a lead capture technique for magnesium-dependent adenosine triphosphatase (Mg2+-ATPase). Localizations with the strontium technique in the presence of an alkaline phosphatase inhibitor were mainly on the syncytiotrophoblast plasma membranes; the reaction was potassium-dependent and ouabain-sensitive. Reaction product using the lead capture method was found on both trophoblast and endothelial cell plasma membranes and was independent of magnesium and insensitive to p-hydroxymercuribenzoate (POHMB), an inhibitor of membrane ATPases. However, a very large proportion of this reaction could be blocked by an alkaline phosphatase inhibitor. It is concluded that the strontium capture technique gave a reliable localization for Na+, K+-ATPase. However, the lead capture method mainly demonstrated alkaline phosphatase, and does not offer a useful approach to specific ATPase studies in this particular system.     

Localization of recently characterized membrane transport adenosine triphosphatases

Summary Na⁺, K⁺-ATPase is the best-known member of a family of membrane transport ATPases sharing a number of structural features. Methods recently developed for Na⁺, K⁺-ATPase localization may, therefore, be modifiable for use in studies of the distributions of related enzymes. Our experiences in the use of monophosphate and triphosphate substrate methods for localization of the gastric proton pump ATPase (H⁺, K⁺-ATPase) are described and the prospects for localizing other related enzymes by similar techniques are discussed.     

The distribution of free amino acids in subcellular fractions of guinea-pig brain

1. The free amino acids of homogenates of guinea-pig brain in 0.32m-sucrose and of subcellular fractions derived therefrom have been estimated by the method of Moore & Stein. 2. Seven amino acids together accounted for over 80% of the free amino compounds; these are, in decreasing order of abundance: glutamate, aspartate, gamma-aminobutyrate, glycine, serine, alanine and threonine. In addition, there are appreciable quantities of amide (presumably glutamine). 3. Control experiments showed that the pattern of free amino acid occurrence in sucrose homogenates was similar to that of brains of animals killed by freezing in liquid nitrogen and extracted immediately without thawing. 4. The subcellular distribution of the amino acids resembled that of soluble cytoplasmic markers; there was no specific localization in a fraction rich in isolated presynaptic nerve terminals of amino acids capable of exciting or depressing central neurones. 5. The significance of the results is discussed in relation to the possible role of centrally active amino acids as transmitters.     

Studies on intestinal adenosine triphosphatases. II. Stabilitiies in different rat subcellular fractions.

Subcellular fraction (brush border, mitochondria, microsomes and plasma membranes) are isolated from the rat intestinal epithelial cells. A comparison was made between the effect of cold storage, freeze-thawing, heating and of some chemicals (DMSO, DTT, glycerol, sucrose) on the stability of Mg2+ and (Na+-K+) dependent ATPases in these fractions in order to determine possible difference linked to the localization in the enterocyte. Enzymatic activities were found more stable at -20 degrees C than at +4 degrees C. Microsomal (Na+-K+)-ATPase increased in activity until the 8th day, then declined. Brush border (Na+-K+)-ATPase was the least resistant of all fractions. For Mg2+-ATPase, that from mitochondria was that had lost much more activity (84%) in 15 days at +4 degrees C. With freeze-thawing there was a comparable decrease in all activities (20-35%). by heating between 35 and 60 degrees C, Mg2+-ATPase was shown to be more heat resistant than (Na+-K+)-ATPase. The addition of some stabilizing chemicals (DMSO, glycerol, sucrose) improved the heat stability of the two enzymes: better results were obtained with glycerol for Mg2+-ATPase and sucrose for (Na+-K+)-ATPase. These differences might be due to the compositon in membraine lipids or to the nature of the enzymes studied.     

The distribution of glutamate decarboxylase and aspartate transaminase in subcellular fractions of rat and guinea-pig brain

1. The subcellular distributions of glutamate decarboxylase and aspartate transaminase were studied in rat and guinea-pig brain. 2. Glutamate decarboxylase is localized in the synaptosome fraction. The mean density of the particles containing the enzyme is slightly greater than those derived from cholinergic neurones, though overlap is substantial. 3. The enzyme is readily released from synaptosomes by hypo-osmotic treatment, but in the presence of Ca(2+), Na(+) and K(+) it sediments with particulate material. 4. The release and binding of the enzyme to membrane fractions by Ca(2+) were investigated. 5. Aspartate transaminase is present in brain as two isoenzymes with different kinetic properties. One isoenzyme is associated with the cytoplasm and the other with mitochondria.     

The subcellular distribution of triphosphoinositide phosphomonoesterase in guinea-pig brain

1. Some properties of the triphosphoinositide phosphomonoesterase from the homogenates of guinea-pig brain were studied. The enzyme has an optimum pH range 6.7-7.3, is stimulated with KCl at a concentration of 0.1m, and under these conditions has K(m)1.43x10(-4)m. 2. A factor from the ;pH5 supernatant' of guinea-pig brain stimulates the enzyme activity over and above the stimulation produced by KCl. Subcellular fractions of guinea-pig brain varied in their response to the ;pH5 supernatant'. Maximum stimulation was observed with the P(1) fraction, containing myelin and nuclei. 3. An assay system for the enzyme was developed that contained optimum concentrations of both KCl and the ;pH5 supernatant'. Acid phosphatases were inhibited by NaF, but, in contrast with previous work, no EDTA was added to the assay system to inhibit the alkaline phosphatases. This reagent inhibited the triphosphoinositide phosphomonoesterase. It was estimated that the remaining fraction of non-specific phosphatases can account for only 14% of the observed triphosphoinositide phosphomonoesterase activity. 4. Subcellular fractions of guinea-pig brain were characterized by electron microscopy and subcellular markers. The triphosphoinositide phosphomonoesterase exhibited a distribution between the fractions similar to that of 5'-nucleotidase, but different from that of alkaline phosphatase.     

Lysophospholipase activity in rat brain subcellular fractions

Lysophospholipase activity in brain subcellular fractions was measured by the release of myristic acid from 1-myristoylglycerophosphocholine or through the formation of [³²P]glycerophosphocholine from [³²P]lysophosphatidylcholine. Although the lysophospholipase activity was highest in microsomes, considerable enzyme activity was also found in other subcellular membrane fractions. The pH optimum for the microsomal enzyme was around 7, whereas the synaptosomes and non-synaptic plasma membranes exhibited a pH maximum around 8. Although the enzyme did not require divalent cations for activity, divalent cations (1 mM) such as Hg²⁺, Cu²⁺, and Zn²⁺ inhibited potently the enzyme activity. Enzyme activity was also partially inhibited by both saturated and polyunsaturated fatty acids (25–200 M), and the inhibition seemed to be greater in the membrane than in the cytosolic fractions. Ionic detergents such as deoxycholate and taurocholate inhibited the lysophospholipase. On the other hand, the effect of Triton X-100 was biphasic, i.e., stimulation at concentrations below 100 g/mg protein and inhibition at higher concentrations. Addition of cholesterol (50–250 g/ml), but not cholesteryl esters, also potently inhibited enzyme activity. The presence of active lysophospholipase(s) in brain is probably an important mechanism for preventing unnecessary accumulation of lysophospholipids which may exert a deleterious effect on the membranes because, of their detergent properties.     

The subcellular localization of di- and tri-peptide hydrolase activity in guinea-pig small intestine

1. Two different subcellular fractionation techniques were applied to guinea-pig intestinal mucosa and the composition of the brush borders prepared by the two methods were compared. 2. By using a kinetic assay system the subcellular distribution of activity against ten dipeptides and five tripeptides was studied. 3. Only small amounts (5–10%) of activity against dipeptides were found in the brush-border region, the enzymes being concentrated in the cytosol. 4. Significant amounts (10–60%) of activity against tripeptides were found in the brush border with the remainder largely present in the soluble fraction. 5. The relevance of these studies to the localization in vivo and the possible role of peptidases in protein digestion is discussed.     

Properties and subcellular localization of adenosine diphosphatase in rat heart

Some properties and subcellular localization of adenosine diphosphatase (ADPase) activity from rat heart have been investigated. The pH optimum was 7.4, maximal activity was found with 5 mM MgCl2, and the apparent Km was 20 microM. ADPase activity was strongly inhibited by NaF and AppNHp, and to a lesser extent by AMP and GppNHp. The enzyme was not inhibited by p-nitrophenylphosphate, beta-glycerophosphate, or pyridoxal phosphate. The distribution of ADPase activity in subcellular fractions obtained by differential centrifugation parallel ouabain-sensitive (Na+-K+)ATPase and 5'-nucleotidase activities, suggesting a plasma membrane-bound localization. The functional significance of ADPase in adenosine production and hemostasis is discussed.     

Structure of the vacuolar adenosine triphosphatases

Vacuolar adenosine triphosphatases (V-ATPases) represent an important class of proton pumps found in endomembrane systems of eucaryotic cells, where they are involved in pH regulation. Progress has been made in the structure determination of this large, membrane-bound multisubunit enzyme complex. Electron microscopy of the V-ATPase has revealed a ball-and-stalk-like structure similar to F1F0-type ATP synthase, to which the V-ATPase is evolutionary related. Aside from the overall structural similarity of the V-ATPase and F-ATP synthase, a number of distinct structural differences exist between the two related enzymes, giving clues to their different function and regulation in the organism.     

The mass distribution of the phosphatidylcholines in subcellular fractions of rat brain

Whole brains from 20-22-day-old rats were separated into the 15,000 g supernatant, myelin, nerve ending and mitochondrial fractions. Gas chromatography of the trimethylsilyl derivatives of 1,2-diglycerides obtained by hydrolysis with phospholipase C of the phosphatidylcholine from each fraction showed marked differences of carbon number distribution (i.e. the sum of the carbon atoms in the two fatty acids of the diglyceride) among the different membranous fractions. Further characterization of each diglyceride was obtained by preparative gas chromatography of the diglyceride-trimethylsilyl ethers and determination of the acyl moieties after collection, methanolysis and gas chromatography. The results indicate that at least three distinct populations of phosphatidylcholine exist in the brain. Nerve endings and the 15,000 g supernatant fraction exhibit a very similar diglyceride pattern with dipalmitoylglycerophosphorylcholine representing over 30 per cent of the species present. Myelin has a unique phosphatidylcholine composition with much less polyenoic species in the 36 and 38 carbon number peaks. Mitochondria contain phosphatidylcholines with relatively more long-chain polyunsaturated fatty acids. TLC of the phosphatidylcholines yielded partial separation into two spots, which differed in distribution of fatty acids. The faster migrating spot contained most of the polyenoic acids, whereas the slower migrating spot contained most of the palmitic, stearic and oleic acids.