The role of ATP and of a bound phosphoryl group acceptor on Ca binding and exchangeability in sarcoplasmic reticulum

Partition of lindane in synthetic and native membranes

Partition coefficients of the insecticide gamma-1,2,3,4,5,6-hexachlorocyclohexane (trivially, lindane) were determined in model and native membranes. Partition in egg phosphatidylcholine bilayers decreases linearly with temperature, over a range (10-40 degrees C) at which the lipid is in the liquid-crystalline state. Addition of 50 mol% cholesterol dramatically decreases partition (2100 falls to 100, at 10 degrees C) and abolishes the temperature dependence. First-order phase transitions of dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines (DMPC, DPPC and DSPC) are accompanied by a sharp increase in lindane partition. Apparently, the insecticide is easily accommodated in bilayers of short-aliphatic-chain lipids, since the partitions were 2450, 600 and 50 in DMPC, DPPC and DSPC, respectively, at temperatures 10 Cdeg below the midpoint of their transitions. The lindane partition sequence in native membranes is as follows: mitochondria, sarcoplasmic reticulum, myelin, brain microsomes and erythrocytes. This sequence correlates reasonably well with the relative content of cholesterol and is similar in liposomes of total extracted lipids, although the absolute partitions showed decreased values. Therefore, the presence of proteins in native membranes contributes to the insecticide partition, probably by favouring its interaction with lipids.

Partition of parathion in synthetic and native membranes

Partition coefficients of [14C]parathion were determined in several types of membranes. Model membranes of egg phosphatidylcholine, dimyristoyl- (DMPC), dipalmitoyl- (DPPC) and distearoyl- (DSPC) phosphatidylcholines, and native membranes of mitochondria, sarcoplasmic reticulum, brain microsomes, myelin and erythrocytes were investigated. Parathion partition is variable among the membranes under study and depends on temperature and cholesterol content. First-order transition of membrane lipids from the gel to the liquid crystalline state is accompanied by a sharp increase in the partition coefficient of parathion. The insecticide is easily accommodated in bilayers of short-chain lipids, since the partitions were 1950, 650 and 270 in DMPC, DPPC and DSPC, respectively, at temperatures 10 degrees C below the midpoint of their transitions. Preferential interaction with short-chain lipids promotes phase separation in heterogeneous bilayers, favouring segregation of lateral domains enriched in insecticide-phospholipid complexes. Cholesterol incorporation in membranes prevents the binding of the insecticide either through competition for similar interacting sites or as a consequence of changes in structural organization of phospholipids.

State of translocated Ca2+ by sarcoplasmic reticulum inferred from kinetic analysis of calcium oxalate precipitation

Uptake of Ca2+ by sarcoplasmic reticulum in the presence of oxalate displays biphasic kinetics. An initial phase of normal uptake is followed by a second phase coincident with precipitation of calcium oxalate inside the vesicles. The precipitation rate induced by actively transported Ca2+ is depressed by increasing the added Ca2+ concentration. This correlates linearly with the reciprocal of precipitation rate. Therefore, a maximal limit rate could be extrapolated at zero Ca2+ (V0). The rate of precipitation, also a function of added amount protein, gives a linear correlation in a double reciprocal plot. Thus, it was possible to estimate the maximal precipitation rate occurring at infinite protein concentration (V infinity). With the combined extrapolated values a maximal expected precipitation rate could be calculated (V infinity 0). Kinetics of calcium oxalate precipitation was studied in the absence of calcium uptake and empirical equations relating the rate of precipitation with the added Ca2+ were established. Entering V infinity 0 in the equations, an internal free Ca2+ concentration of approx. 2.5 mM was estimated. Additionally, it is shown that the ionophore X-537A does not suppress the Ca2+ uptake, if added during the oxalate-dependent phase, albeit the uptake proceeds at a slower rate after the release of approx. 70 nmol Ca2+/mg protein. This amount presumably equals the internal free Ca2+ not sequestered by oxalate, producing a maximal concentration approx. 14 mM. Taking into account low affinity binding of internal binding sites and the transmembrane Ca2+ gradients built up during the uptake of Ca2+, values of free Ca2+ ranging from 3 to 6 mM, approaching those estimated by the precipitation analysis, could be estimated.

Oxalate transfer across the membranes of sarcoplasmic reticulum during the uptake of Ca++

The kinetic profile of Ca++ uptake in the presence of oxalate is biphasic. An initial phase independent on oxalate is followed by an oxalate-dependent phase delayed in time. The ionophore X-537A only abolishes the net Ca++ uptake if added before the onset of the oxalate phase. However, during this phase, X-537A suddenly releases an amount of Ca++ similar in quantity to that released in the initial phase. The delay of the oxalate-dependent phase is a function of pH. At pH of about 5.5, the oxalate phase and simultaneous calcium oxalate precipitation would theoretically start at the beginning, with no delay. Ejection of protons during Ca++ uptake is strongly depressed by oxalate, but not by other organic anions which do not trap Ca++. It is suggested that oxalate is transferred to the inside of the vesicles as a monoprotonated species at expense of protons ejected by the Ca++-pump during the uptake of Ca++.

Ca2+ transport mediated by a synthetic neutral Ca2+ -ionophore in biological membranes

The effect of a synthetic neutral ligand on the Ca2+ permeability of several biological membranes has been investigated. The ligand had been previously shown to possess Ca2+ -ionophoric activities in artificial phospholipid membranes. The neutral ionophore is able to transport Ca2+ across the membranes of erythrocytes and sarcoplasmic reticulum, when lipophilic anions such as tetraphenylborate and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) are present, presumably to facilitate the diffusion of the charged Ca2+ -ionophore complex across the hydrophobic core of the membrane. In mitochondria, the neutral ionophore promotes the active transport of Ca2+ in response to the negative membrane potential generated by respiration, in the presence of the specific inhibitor of the natural carrier ruthenium red.

Subunits of the calcium ion-pump system of sarcoplasmic reticulum

Sarcoplasmic reticulum membranes with high content of Ca2+ -ATPase (80% of total protein) were dissolved in a non ionic medium and were submitted to isoelectric focusing in polyacrylamide gels. The membrane protein was resolved into six main bands whose isoelectric points range from 6 to 5. The mol. wt. of these peptides is about 100 000 as estimated by second dimension electrophoresis in sodium dodecyl sulfate-polyacrylamide system. The electrophoretic behaviour of the purified ATPase enzyme is similar to that of crude membranes.

Active calcium ion uptake by inside-out and right side-out vesicles of red blood cell membranes

The relationship between active extrusion of Ca(++) from red cell ghosts and active uptake of Ca(++) by isolated red cell membrane fragments was investigated by studying the Ca(++) uptake activities of inside-out and right side-out vesicles. Preparations A and B which had mainly inside-out and right side-out vesicles, respectively, were isolated from red cell membranes and were compared with respect to Ca(++) adenosine triphosphatase (ATPase) and ATP-dependent Ca(++) uptake activities. Preparation A had nearly eight times more inside-out vesicles and took up eight times more (45)Ca in the presence of ATP compared to preparation B. Separation of the (45)Ca-labeled membrane vesicles by density gradient centrifugation showed that the (45)Ca label was localized to the inside-out vesicle fraction. In addition, the (45)Ca taken up in the presence of ATP was lost during a subsequent incubation in the absence of ATP. The rate of (45)Ca loss was not influenced by the presence of EGTA, but was slowed in the presence of La(+8) (0.1 mM) in the efflux medium. The results presented here support the thesis that the active uptake of Ca(++) by red cell membrane fragments is due to the active transport of Ca(++) into inside-out vesicles.