Energy-dependent calcium sequestration activity in rat liver microsomes

Studies of gastric Ca2+-stimulated adenosine triphosphatase. I. characterization and general properties

Gastric microsomes do not contain any significant Ca2+-stimulated ATPase activity. Trypsinization of pig gastric microsomes in presence of ATP results in significant (2-3 fold) increase in the basal (with Mg2+ as the only cation) ATPase activity, with virtual elimination of the K+-stimulated component. Such treatment causes unmasking of latent Mg2+-dependent Ca2+-stimulation ATPase. Other divalent cations such as Sr2+, Ba2+, Zn2+, and Mn2+ were found ineffective as a substitute for Ca2+. Moreover, those divalent cations acted as inhibitors of the Ca2+-stimulated ATPase activity. The pH optimum of the enzyme is around 6.8. The enzyme has a Km of 70 microM for ATP and the Ka values for Mg2+ and Ca2+ are about 4 x 10(-4) and 10(-7) M, respectively. Studies with inhibitors suggest the involvement of sulfhydryl and primary amino groups in the operation of the enzyme. Possible roles of the enzyme in gastric H+ transport have been discussed.

ATP-dependent calcium transport by a Golgi-enriched membrane fraction from mouse mammary gland

Crude particulate preparations from the mammary glands of lactating mice were shown to transport calcium against a concentration gradient in the presence of ATP and mitochondrial inhibitors. Density gradient centrifugation with both sucrose and Percoll gradients indicated the presence of ATP-dependent transport in more than one membrane fraction. A Golgi-enriched membrane fraction possessed the highest specific activity of calcium transport. Digitonin, which increases the permeability of plasma membranes to calcium, did not affect this process. The Golgi fraction contained a 100,000 Dalton protein whose phosphorylation by gamma-[32P]-ATP was enhanced by a micromolar concentrations of free calcium. The phosphorylation was acid-stable and hydroxylamine-sensitive. These properties suggest that Golgi membranes in an activity secreting mammary epithelium possess a calcium transport system which resembles the calcium ATPase present in the sarcoplasmic reticulum of skeletal muscle.

Energy-dependent calcium sequestration activity in a Golgi apparatus fraction derived from lactating rat mammary glands

A fraction isolated from lactating rat mammary glands was shown by marker enzyme assays to be rich in Golgi apparatus vesicles. This Golgi apparatus-rich fraction was shown to accumulate calcium in the presence of ATP but not in its absence. Other nucleoside triphosphates were only partially effective in promoting calcium transport. Mg2+ was required for the uptake which was also temperature and pH dependent. The uptake was sustained by the use of oxalate and phosphate as intravesicular trapping agents. In the presence of 10 mM oxalate the apparent Km for calcium uptake was 0.24 muM ionized calcium. The V was 4.45 nmol calcium/min per mg protein. Preloaded calcium could be rapidly released by the addition of the ionophore A23187 indicating an intravesicular location for the sequestered ion. Addition of ethylene glycol bis(beta-aminoethylether)-N,N'-tetraacetic acid resulted in a slower release of preaccumulated calcium, indicating the existence of one or more efflux routes by which calcium leaves the vesicles in the presence of MgATP. Ruthenium red partially inhibited the uptake but lanthanum and particularly the sulphydryl inhibitor p-hydroxy-mercuribenzoate were much more effective. The properties of the calcium-sequestering system in the Golgi apparatus-rich fraction were similar to those reported for other non-muscular tissues and lend support to the hypothesis that calcium is secreted into milk via the Golgi apparatus of the mammary gland secretory gland.

Failure of ethanol or isopropanol pretreatment to affect carbon tetrachloride-induced inhibition of hepatic microsomal calcium pump activity

The calcium pump of liver microsomes is sensitive to CCl4 metabolism in vitro and in vivo. Treatment of rats with alcohol might involve an enhancement of the action of low doses of CCl4 on rat liver microsomal calcium pump. It was found that treatment of fasted rats with isopropanol or ethanol potentiated the increase of serum glutamic oxaloacetic transaminase elicited by CCl4. Alcohol pretreatment alone had no effect on calcium pump activity. After CCl4 alone, the calcium pump was 50% inhibited at 20 min, and 85% inhibited at 1 hr. Pretreatment with either alcohol had no effect on either rate of decline or extent of CCl4-dependent depression of the liver microsomal calcium pump. The mechanism of alcohol potentiation of CCl4-induced hepatotoxicity probably resides in alteration of processes developing after the initial events of CCl4 metabolism.

Nucleotide-induced alteration of rat liver microsome calcium pump activity

Rat liver microsomes sequester calcium by an energy dependent process that may be a nonmuscle cell analog of the sarcoplasmic reticulum Ca2+ pump of skeletal muscle (Moore, L., Chen, T.S., Knapp, H.R., Jr. and Landon, E.J. (1975) J. Biol. Chem. 250, 4562-4568). Homogenization of rat liver in the presence of ATP (5 mM) results in a 2-fold increase of the specific activity of the microsome Ca2+ pump. The effect of ATP is concentration dependent and is detected at ATP levels as low as 0.1 mM. ATP will produce this effect if added before homogenization, after homogenization or after any of the centrifugation steps of microsome isolation. Homogenization of rat liver in the presence of ADP and AMP also increases specific activity of the microsome Ca2+ pump, but to a lesser extent than ATP. Other nucleoside triphosphates have been tested and are in general less effective than ATP in increasing microsome Ca2+ pump activity. The phosphate group of nucleotides appears to be improtant to his effect in that adenosine does not affect Ca2+ pump activity, while sodium pyrophosphate will increase pump activity but to a smaller extent than ATP. The presence of nucleotides or pyrophosphate during microsome isolation results in the release of a small amount of protein material from microsomes. These proteins can be detected in 105 000 Xg supernatant by SDS-polyacrylamide gel electrophoresis. Three bands of molecular weight 46 000, 42 000 and 36 000 comprise the majority of protein material released from microsomes. The ability of nucleotides to release one of the these proteins, the 42 000 molecular weight band, from microsomes correlates with the ability of the nucleotide to increase microsome Ca2+ pump activity. Preliminary evidence indicates that the protein released from ATP-treated microsomes is able to suppress the stimulated calcium uptake measured in ATP-treated microsomes. It is possible that this protein functions to regulate Ca2+ pump activity in the endoplasmic reticulum of liver.

Characterization of the hormone-sensitive Ca2+ uptake activity of the hepatic endoplasmic reticulum

The characteristics and kinetics of calcium uptake activity were studied in isolated hepatic microsomes. The sustained accumulation of calcium was ATP- and oxalate-dependent. Glucagon increased microsomal Ca2+ uptake upon either in vivo injection, or in vitro perfusion of the hormone in the liver. In contrast, the effect of insulin depended on the route of administration. Calcium accumulation by subsequently isolated hepatic microsomes increased when insulin was injected intraperitoneally whereas it decreased when the hormone was perfused directly into the liver. These effects of glucagon and insulin were dose dependent. When insulin was added to the perfusate prior to the addition of glucagon, insulin blocked the glucagon-stimulated increase in microsomal Ca2+ uptake. Cyclic AMP mimicked the effect of glucagon on microsomal Ca2+ accumulation when the cyclic nucleotide was perfused into the liver. The effects of glucagon and insulin on the kinetics of hepatic microsomal Ca2+ uptake were investigated. In microsomes isolated from perfused rat livers treated with glucagon the V of the uptake was significantly increased over the control values (12.2 vs. 8.6 nmol Ca2+ per min per mg protein, P less than 0.02). In contrast, the addition of insulin to the perfusate significantly decreased the V of Ca2+ uptake by subsequently isolated microsomes (6.8 vs. 8.3 nmol Ca2+ per min per mg protein, P less than 0.05). However, neither hormone had an effect on the apparent Km for Ca2+ (4.1 +/- 0.5 microM) of the reaction. The effect of these hormones on the activity of Ca2+-stimulated ATPase was also studied. No significant changes in either V or Km for Ca2+ of the enzymatic reaction were detected.

Relationship between calcium ion transport and (Ca2+ + Mg2+)-atpase activity in adipocyte endoplasmic reticulum

Calcium uptake by adipocyte endoplasmic reticulum was studied in a rapidly obtained microsomal fraction. The kinetics and ionic requirements of Ca2+ transport in this preparation were characterized and compared to those of (Ca2+ + Mg2+)-ATPase activity. The time course of Ca2+ uptake in the presence of 5 mM oxalate was nonlinear, approaching a steady-state level of 10.8--11.5 nmol Ca2+/mg protein after 3--4 min of incubation. The rate of Ca2+ transport was iM oxalate. The calculated initial rate of calcium uptake was 18.5 nmol Ca2+/mg protein per min. The double reciprocal plot of ATP concentration against transport rate was nonlinear, with apparent Km values of 100 muM and 7 muM for ATP concentration ranges above and below 50 muM, respectively. The apparent Km values for Mg2+ and Ca2+ were 132 muM and 0.36--0.67 muM, respectively. The energy of activation was 23.4 kcal/mol. These kinetic properties were strikingly similar to those of the microsomal (Ca2+ + Mg2+)-ATPase. The presence of potassium was required for maximum Ca2+ transport activity. The order of effectiveness of monovalent cations in stimulating both Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity was K+ greater than Na+ = NH4+ greater than Li+. Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity were both inhibited 10--20% by 6 mM procaine and less than 10% by 10 mM sodium azide. Both processes were completely inhibited by 3 mM dibucaine or 50 muM p-chloromercuribenzene sulfonate. The results indicate that Ca2+ transport in adipocyte endoplasmic reticulum is mediated by a (Ca2+ + Mg2+)-ATPase and suggest an important role for endoplasmic reticulum in control of intracellular Ca2+ distribution.

Effect of cell density on energy-dependent calcium uptake by Balb/c 3T3 membranes is independent of protein synthesis and attachment to substratum

Membranes isolated from subconfluent cultures of Balb/c 3T3 cells have low energy-dependent calcium uptake activity. Replating confluent cells at low density results in a prompt fall of energy-dependent calcium uptake by membrane fractions. The level to which uptake activity falls is a function of the density at which the cells are plated (Moore and Pastan, '77b). To determine if regulation of energy-dependent uptake of calcium by membrane fractions is dependent upon attachment to a substrate and to further characterize conditions that regulate the process, we examined calcium uptake activity of membranes isolated from cells in suspension. With cells in suspension energy-dependent calcium uptake activity of isolated membranes falls promptly if cells are diluted to a low density (less than 10(5) cells/ml) and is a function of cell density. When cells in suspension at low cell densities are concentrated to high cell densities (greater than 2 x 10(6) cells/ml), calcium uptake activity of the isolated membrane fraction is increased as a function of cell density. These changes of membrane calcium uptake activity occur promptly and do not require protein synthesis.

Further characterization of calcium-accumulating vesicles from human blood platelets

Human blood platelets are capable of removing Ca2+ from the cytoplasm by means of an active, ATP-dependent and cyclic AMP-stimulated transport system. Calcium-accumulating vesicles are obtained by sonicating platelets. On density gradient centrifugation, this activity is found in the heavier of two membrane fractions. Concentrated in this fraction are also the Ca2+-stimulated Mg2+-ATPase and glucose-6-phosphatase, believed to be a marker for internal membrane systems. When the isolated vesicles are loaded with Ca2+, a third band separates from the two vesicular fractions in the density gradient. This band C contains virtually all the Ca2+-accumulating activity. Evidence that this activity is due to an active uptake and not to surface binding or adsorption is presented. Whereas electron microscopy does not reveal striking differences between active and inactive fractions, differences in protein composition are revealed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Furthermore, this band contains an enzyme system which converts arachidonic acid to malondialdehyde and therefore this fraction must be the site of prostaglandin synthesis. Membranes prepared by loading platelets with glycerol, followed by osmotic lysis are unable to accumulate calcium. In sodium dodecyl sulphate-polyacrylamide gel electrophoresis such membranes show significant differences in their protein pattern as compared to the actively Ca2+-accumulating vesicular membranes of band C. All preparations with Ca2+-accumulating activity also contain markers for plasma membranes and the question whether this activity is due exclusively to an intracellular structural element equivalent to the sarcoplasmic reticulum of muscle or whether an "extrusion pump" expelling Ca2+ to the outside of the cell is also involved, cannot yet be ;nswered.