Isolation of ionophores from ion-transport systems

Possible role of Ca2+ in heavy metal cytotoxicity

1. Organic xenobiotic metabolism often results in oxidative stress, involving GSH depletion, alteration of thiol/disulphide balance and peroxidation of membrane lipids. These events can lead to the disruption of Ca2+ homeostasis, through impairment of the Ca2+ translocases present in cellular membranes. Inhibition of the activity of Ca,Mg-ATPases due to oxidation of their SH groups would lead to uncontrolled rises in cytosolic Ca2+ levels resulting in loss of cell viability. 2. These observations seem to be of interest when interpreting the biochemical mechanisms of heavy metal cytotoxicity. Since these cations (such as Hg2+, Cu2+, Cd2+ and Zn2+) have an extremely high affinity for SH groups, they may affect the function of SH containing proteins, such as the Ca,Mg-ATPases, as in the case of oxidative stress. 3. Results are reported indicating that Hg2+ may stimulate Ca2+ influx through voltage-dependent channels in different experimental systems. Moreover, evidence is presented that heavy metals can inhibit Ca,Mg-ATPase activity and affect mitochondrial functions in the cells of different organisms. 4. The possibility that heavy metal cytotoxicity is mediated through disruption of Ca2+ homeostasis is discussed.

Presence of the plasma membrane proteolipid (plasmolipin) in myelin

Plasma membrane proteolipid (plasmolipin), which was originally isolated from kidney membranes, has also been shown to be present in brain. In this study, we examined the distribution of plasmolipin in brain regions, myelin, and oligodendroglial membranes. Immunoblot analysis of different brain regions revealed that plasmolipin levels were higher in regions rich in white matter. Plasmolipin was also detected in myelin, myelin subfractions, and oligodendroglial membranes. Immunocytochemical analysis of the cerebellum revealed that plasmolipin was localized in the myelinated tracts. Plasmolipin levels in myelin were enriched during five successive cycles of myelin purification, similar to the enrichment of myelin proteolipid apoprotein (PLP) and myelin basic protein (MBP). In contrast, levels of Na+,K(+)-ATPase and a 70-kDa protein were decreased. When myelin or white matter was extracted with chloroform/methanol, it contained, in addition to PLP, a significant amount of plasmolipin. Quantitative immunoblot analysis suggested that plasmolipin constitutes in the range of 2.2-4.8% of total myelin protein. Plasmolipin, purified from kidney membranes, was detected by silver stain on gels at 18 kDa and did not show immunological cross-reactivity with either PLP or MBP. Thus, it is concluded that plasmolipin is present in myelin, possibly as a component of the oligodendroglial plasma membrane, but is structurally and immunologically different from the previously characterized myelin proteolipids.

Expression of a plasma membrane proteolipid during differentiation of neuronal and glial cells in primary culture

Plasma membrane proteolipid protein (PM-PLP) synthesis was examined in embryonic rat neurons and neonatal rat glial cells during differentiation in culture. Glial cultures were treated with 1 mM N6, O2, dibutyryl cyclic adenosine monophosphate (dbcAMP) following confluency to induce differentiation, which resulted in the elaboration of long cellular processes. However, no changes in the biosynthetic level of PM-PLP was observed during the differentiation of these cells. Neurons differentiated spontaneously in culture, forming cellular aggregates immediately following plating and elaborating a network of neurites over 7 days. The differentiation of neurons was accompanied by a seven-fold increase in PM-PLP synthesis with increases in biosynthetic increase in PM-PLP synthesis with increases in biosynthetic rate observed between days 1 and 3 and between days 3 and 7 in culture. Ultrastructural examination of neurons indicated that the Golgi apparatus was also developing during this period of time, with an increase in both the number of lamellae and generation of vesicles. The transport of PM-PLP to the plasma membrane was therefore examined in neurons at day 7 in culture by pulse labeling experiments with monensin and colchicine. Monensin (1 microM) was found to inhibit the appearance of radiolabeled PM-PLP in the plasma membrane by 63%, indicating that a functional Golgi apparatus is required for transport of PM-PLP to its target membrane. Colchicine (125 microM) also inhibited the appearance of newly synthesized PM-PLP in the plasma membrane by greater than 40%, suggesting that microtubules may also be required for PM-PLP transport to the plasma membrane.

Characterization and biosynthesis of the plasma membrane proteolipid protein in neural tissue

In this study we have characterized, in brain, the expression of a plasma membrane proteolipid protein (PM-PLP) complex that can form cation-selective channels in lipid bilayers. We isolated PLP fractions from synaptic plasma membrane and glial microsomes and found a high degree of similarity in both size and amino acid composition to the complex we had previously isolated from kidney. Antibodies specific to the kidney PM-PLP were prepared, and, on the basis of immunoblot and immunoprecipitation studies, the PM-PLP complex isolated from neural membranes was shown to be immunologically related to the kidney PM-PLP. These proteolipid proteins exhibited a molecular weight of approximately 14K and contained a high percentage of hydrophobic amino acids with an apparent absence of cysteine. The biogenesis of PM-PLP in brain was studied by in vitro translation of free and bound polysomes and total RNA in a rabbit reticulocyte lysate followed by immunoprecipitation of the translation products. From these studies it is concluded that the PM-PLP complex is synthesized on the rough endoplasmic reticulum. On the basis of the identical electrophoretic mobility of material isolated from plasma membranes and material immunoprecipitated after translation of bound polysomes and isolated RNA, it appears that the PM-PLP does not undergo detectable posttranslational processing between its site of synthesis and its incorporation into the plasma membrane.

Protein interactions with lipid bilayers: the channels of kidney plasma membrane proteolipids

Proteolipids extracted from bovine kidney plasma membrane induce irreversible changes in the electrical properties of lipid bilayers formed from diphytanoyl phosphatidylcholine. The interaction with the proteolipid produces channels which are cation selective. At low protein concentrations (i.e., less than 0.6 microgram/ml), the single-channel conductance is approximately 10 pS in 100 mM KCl and 3 pS in 100 mM NaCl. In the presence of protein concentrations above 1 microgram/ml, another population of channels appears. These channels have a conductance of about 100 pS in 100 mM KCl and 30 pS in 100 mM NaCl. Further, these channels are voltage dependent in KCl, closing when the voltage is clamped at values greater than or equal to 30 mV. The steady-state membrane conductance, measured at low voltages, was found to increase proportional to a high power (2-3) of the proteolipid concentration present in one of the aqueous phases. In 100 mM NaCl, the conductance increases at protein concentrations above 5 microgram/ml, whereas in 100 mM KCl it increases at protein concentrations above 0.6 microgram/ml. These measurements indicate that the higher steady-state conductance observed in KCl at a given proteolipid concentration in a multi-channel membrane presumably results because more channel incorporate in the presence of KCl than in the presence of NaCl. The two major fractions which comprise the proteolipid complex were also tested on bilayers. It was found that both fractions are required to produce the effects described.

Phospholipids as the molecular instruments of ion and solute transport in biological membranes

Partition studies have established that phospholipids generally have the capabilities to mediate the transmembrane transport of the full range of ions and solutes that physiologically cross biological membranes. The list of transportable species includes cations, anions, amino acids, citric acid cycle intermediates, nucleotides, and sugars. Phospholipid-mediated transport can be readily modulated by altering the phospholipid mixture or by addition of detergents, nucleotides, divalent metals, proteins, peptides, or ring compounds. Containment of phospholipid within channels in protein appears to be the precondition for the formation of the micellar structure requisite for solute transport. Phospholipid-mediated transport is postulated to be a central feature of energy coupling, membrane-spanning systems, and membrane-bound, phospholipid-requiring enzymes.

Ionophorous properties of the 20,000-dalton fragment of (Ca2+ + Mg2+)-ATPase in phosphatidylcholine: cholesterol membranes

The purified 20,000-dalton fragment of sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase has been shown by us (A.E. Shamoo, T.E. Ryan, P.S. Stewart, D.H. MacLennan, 1976. J. Biol. Chem. 251:4147) to have Ca2+-selective ionophoric activity. The Ca2+-ionophoric fragment has been purified by either SDS-column chromatography or SDS-preparative gel electrophoresis. The Ca2+-ionophoric fragment has been subjected to prolonged dialysis to insure the removal of bound SDS from the fragment. The selectivity sequence of this fragment in black lipid membranes (BLM) formed from either oxidized cholesterol or phosphatidylcholine/cholesterol is the same, PBa greater than PCa greater than PSr greater than PMg greater than PMn. This selectivity sequence is the same as that for the intact (Ca2+ + Mg2+)-ATPase. Treatment of the fragment with cholate to absolutely insure the removal of bound SDS resulted in the fragment having a selectivity sequence as above except that PMn greater than PMg. This and other data indicate that the 20,000-dalton fragment is the site containing the Ca2+-ionophoric activity of the (Ca2+ + Mg2+)-ATPase.

Active calcium treatment transport via coupling between the enzymatic and the ionophoric sites of Ca2+ + Mg2+-ATPase

The 20K dalton fragment of Ca2+ + Mg2+-ATPase obtained from th tryptically digested sarcoplasmic reticulum has been further purified using Bio-Gel P-100. This removed low-molecular-weight UV-absorbing and positive Lowry-reacting contaminants. The ionophoric activity of the 20K fragment in both oxidized cholesterol and phosphatidylcholine:cholesterol membranes is unaltered by this further purification. The 20K selectivity sequence in phosphatidylcholine:cholesterol membrane is Ba2+ greater than Ca2+ greater than Sr2+ greater than Mn2+ Mg2+. Digestion of intact sarcoplasmic reticulum vesicles with trypsin, which results in the dissection of the hydrolytic site (30K) from the ionophoric site (20K), is shown to disrupt energy transduction between ATP hydrolysis and calcium transport. This further implicates the 20K dalton fragment as a calcium transport site. These data and previous evidence are discussed in terms of a proposed model for the ATPase molecular structure and the mechanisms of cation transport in sarcoplasmic reticulum.

Separate effects of mercurial compounds on the ionophoric and hydrolytic functions of the (Ca++ +Mg++)-ATPase of sarcoplasmic reticulum

We have shown that a Ca++-ionophore activity is present in the (Ca++ +Mg++)-ATPase of rabbit skeletal muscle sarcoplasmic reticulum (A. E. Shamoo & D. H. MacLennan, 1974. Proc. Nat. Acad. Sci. USA 71:3522). Methylmercuric chloride inhibited the (Ca++ +Mg++)-ATPase and Ca++ transport, but had no effect on the activity of the Ca++ ionophore. Mercuric chloride inhibited ATPase, transport and ionophore activity. The ATPase and transport functions were more sensitive to methylmercuric chloride than to mercuric chloride. The two functions were inhibited concomitantly by methylmercuric chloride but slightly lower concentrations of mercuric chloride were required to inhibit Ca++ transport than were required to inhibit ATPase. Methylmercuric chloride and mercuric chloride probably inhibited ATPase and Ca++ transport by blocking essential -SH groups. However, it appears that there are no essential -SH groups in the Ca++ ionophore and that mercuric chloride inhibited the Ca++ ionophore activity by competition with Ca++ for the ionophoric site. Blockage of Ca++ transport by mercuric chloride probably occurs both at sites of essential -SH groups and at sites of ionophoric activity. These data suggest the separate identity of the sites of ATP hydrolysis and of Ca++ ionophoric activity.