Study of the mitochondrial phosphate carrier in the course of calcium phosphate accumulation: a requirement for Mg2+ and ADP of its sensitivity to thiol reagents

The pathway of inorganic-phosphate efflux from isolated liver mitochondria during adenosine triphosphate hydrolysis

1. The distribution of P(i) between mitochondria and suspending medium during uncoupler-stimulated hydrolysis of ATP by rat liver mitochondria [Tyler (1969) Biochem. J.111, 665-678] has been reinvestigated, by using either mersalyl or N-ethylmaleimide as inhibitors of P(i) transport and either buffered sucrose/EDTA or LiCl/EGTA solutions as suspending medium. More than 75% of the total P(i) liberated was retained in mitochondria treated with either inhibitor at all ATP concentrations tested (0.2-2.5mm). With low ATP concentrations and mersalyl-treated mitochondria incubated in sucrose/EDTA, virtually all the P(i) liberated was retained in the mitochondria. 2. Larger amounts of P(i) appeared in the suspending medium during ATPase activity, despite the presence of N-ethylmaleimide, when LiCl/EGTA was used as suspending medium compared with sucrose/EDTA. Two sources of this P(i) were identified: (a) a slow efflux of P(i) from mitochondria to suspending medium despite the presence of N-ethylmaleimide; (b) a slow ATPase activity insensitive to carboxyatractyloside, which was stimulated by added Mg(2+), partially inhibited by oligomycin or efrapeptin and strongly inhibited by EDTA. 3. It is concluded that liver mitochondria preparations contain two distinct forms of ATPase activity. The major activity is associated with coupled mitochondria of controlled permeability to adenine nucleotides and P(i) and is stimulated strongly by uncoupling agents. The minor activity is associated with mitochondria freely permeable to adenine nucleotides and P(i), is unaffected by uncoupling agents and is activated by endogenous or added Mg(2+). 4. When mitochondria treated with mersalyl were incubated in buffered sucrose solution, almost all the P(i) liberated was recovered in the suspending medium, unless inhibitors of P(i)-induced large-amplitude swelling such as EDTA, EGTA, antimycin, rotenone, nupercaine or Mg(2+) were added. Thus the loss of the specific permeability properties of the mitochondrial inner membrane associated with large-amplitude swelling also influences the extent of P(i) retention during ATPase activity. 5. The results confirm the previous conclusion (Tyler, 1969) that the P(i) transporter provides the sole pathway for P(i) efflux during uncoupler-stimulated ATP hydrolysis by mitochondria. It is concluded that more recent hypotheses concerning the influence of Mg(2+) on mersalyl inhibition of the P(i) transporter [Siliprandi, Toninello, Zoccaroto & Bindoli (1975) FEBS Lett. 51, 15-17] and a postulated role of the adenine nucleotide exchange carrier in P(i) efflux [Reynafarje & Lehninger (1978) Proc. Natl. Acad. Sci. U.S.A.75, 4788-4792] are erroneous and should be discarded.

Molecular organization in bacterial cell membranes. Sulfhydryl groups and disulfide bridges in Streptomyces albus and Escherichia coli K 12 cytoplasmic membranes

Plasma membranes from Streptomyces albus had 5.2 mol of sulfhydryl groups and 6 mol of disulfide bridges/50 kg proteins whereas Escherichia coli membranes had 3.4 mol sulfhydryl groups and 4 mol disulfide bridges/50 kg protein. About 66% of the sulfhydryl groups of S. albus membranes and 22% of those of E. coli membranes were readily accessible to titration with 5,5'-dithiobis(2-dinitrobenzoic acid). o-[3 Hydroxymercuri-2-methoxypropyl)-carbamyl]-phenoxyacetic acid (mersalyc acid) and p-chloromercuribenzoate were effective in solubilizing membrane proteins from the two bacteria. Other sulfhydryl group reagents, such as N-ethylmaleimide, iodoacetamide and iodoacetic acid, were less effective. Dithiothreitol affected the dodecylsulphate gel electrophoresis patterns of S. albus membranes and soluble fractions. This effect resulted from the reduction of pre-existing disulfide intramolecular bridges and some interchain disulfide formed during solubilization and/or storage. Dithiothreitol also affected the dodecylsulphate gel electrophoresis patterns of E. coli membranes and their soluble fractions. These results suggest that sulfhydryl groups and disulfide bridges play a role in the structural organization of these prokaryotic membranes.

Permeability measurements on mitochondria from wild-type and poky strains of Neurospora crassa

The permeability properties of isolated Neurospora mitochondria were determined by measuring the rate at which the mitochondria swell in isotonic solutions of various organic and inorganic molecules. Like mammalian mitochondria, wild-type Neurospora mitochondria were impermeable to sucrose and only slightly more permeable to most inorganic ions (K, Na, Cl). Their permeability to K was greatly increased by valinomycin and by monensin. In addition, the mitochondria contain specific systems mediating PO4 uptake and PO4- malate, fumarate, and succinate exchange. Mitochondria from the maternally inherited poky strain of Neurospora, previously demonstrated to possess defective ribosomes and a grossly cytochrome chain, showed a slight but significant increase in permeability to inorganic ions. They contained, however, the specific uptake and exchange systems for phosphate and dicarboxylate anions, a result suggesting that these systems do not depend upon mitochondrially synthesized polypeptides.

Thiols related to mitochondrial ATPase and transports: unmasking upon conformational changes supported by the comparative effects of ethacrynate and dihydroethacrynate

Comparison between the effects on various rat liver mitochondrial functions of ethacrynate, a thiol reagent inhibitor of oxidative phosphorylations [3, 4] and those of dihydroethacrynate its saturated derivative which is not a thiol reagent, has been performed. Both, ethacrynate and dihydroethacrynate increase oxygen consumption by mitochondria in state 4 (succinate as substrate) in a concentration dependent way (from 1 to 5 X 10(-4) M EA or DHEA). This activation is followed, only with ethacrynate, by an inhibition appearing sooner with higher concentrations. After preincubation or mitochondria with ethacrynate (1 to 5 X 10(-4) M), the stimulation of respiration by (ADP + Pi) is completely inhibited whereas it is only weakly affected by dihydroethacrynate at the same concentrations. Ethacrynate and dihydroethacrynate provoke variations of intramitochondrial Mg2+ and K+ levels which need energy from the respiratory chain. These are affected by Pi or (Pi + ADP) in a different way with ethacrynate and with dihydroethacrynate. After preincubation with mitochondria, ethacrynate and to a smaller extent dihydroethacrynate, inhibit partially ADP translocation; ADP increases the inhibitory effect of EA on translocation and not that of dihydroethacrynate. Ethacrynate increases the oligomycin sensitive ATPase activity and dihydroethacrynate still more. After a ten minutes preincubation with mitochondria, ethacrynate and dihydroethacrynate hardly affect the 2.4 DNP stimulated ATPase activity. Preincubation with succinate or ADP strongly increases the ethacrynate inhibition whereas it decreases dihydroethacrynate inhibition. Ethacrynate and dihydroethacrynate do not affect the efflux of Pi produced by ATP hydrolysis but ethacrynate enforces the inhibitory effect of mersalyl (Mg2+ containing medium). After ten minutes of preincubation with mitochondria, ethacrynate binds 25 nmoles of -SH/mg protein (DTNB titration) and dihydroethacrynate has no effect. These results show an effect of ethacrynate on two types of thiols linked with energy conservation mechanisms and ADP translocation. These thiols could be unmasked or made accessible by conformational modifications of the inner membrane upon energization or addition of ADP.

[Permeability of yeast mitochondrial internal membrane: structure-activity relationship]

In order to investigate the possible relations between the anionic permeability and the functions (or the structure ) of the inner mitochondrial membrane, three types of organelles isolated from S. cerevisiae were tested: mitochondria (aerobic culture), promitochondria (anaerobic culture) and CAP-mitochondria (aerobic culture with chloramphenicol added). By using the technique of swelling in isoosmotic potassium salts, after a derermination of the isotonic conditions, it was possible to discriminate between an electrogenic (valinomycin induced) or an electroneutral (both valinomycin and uncoupler induced) translocation. 1) Mitochondria: The permeability properties of mitochondria are energy dependent: a) Respiring mitochondria are permeable to Cl-; Mg2+, however, inhibits this translocation. Phosphate transport seems to be exclusively electrogenic and mersalyl sensitive, but swelling inhibition by that thiol reagent is restored by Mg2+. b) Non respiring mitochondria are impermeable to Cl-, but ATP addition restores the permeability. Thiocyanate permeates as the anionic form and acetate as the undissociated form. The phosphate transport, sensitive to mersalyl, seems to be partially electrogenic. 2) Promitochondria: Deficient of respiratory enzymes but containing an oligomycin sensitive ATPase, they are impermeable to Cl- only when Mg2+ is added. In these conditions, an electrogenic phosphate transport, sensitive to mersalyl, is observed. 3) CAP-mitochondria: Although CAP-mitochondria are cytochrome deficient and contain an oligomycin insensitive ATPase, they are also impermeable to Cl- in presence of Mg2+. As in fully differenciated mitochondria, an electroneutral phosphate entry is observed; Mg2+ is required for mersalyl sensitivity.

Some actions of purified toxins from Bungarus fasciatus venom on isolated pig-heart mitochondria

I. In the presence of succinate as an oxidation substrate, neurotoxins alpha, beta and gamma induce the following. Firstly, an increasing stimulation of oxygen uptake, which in potentiated by 25 muM Ca2+, Mg2+ 1.3 mM completely inhibits the effect of toxin alpha but not of toxins beta and gamma. Secondly, a depletion of the Mg2+, Ca2+ and K+ content of the water-soluble and membrane-bound mitochondrial compartments, following complex kinetics, which suggest a multistep interaction mechanism of the toxins with the mitochondria. 25 muM Ca2+ also potentiates the effect of the toxins on these ionic flows. Thirdly, no decrease of turbidity with toxin alpha, and a limited decrease with toxins beta and gamma. 2. In the absence of respiration, the neurotoxins induce a cationic depletion, the kinetics of which are different than with succinate, suggesting an instantaneous maximal effect on the inner membrane. Toxins beta and gamma (but not alpha) induce, under these conditions, a turbidity decrease of large amplitude, which is proportional to the amount of toxin added and tends to reach a maximum. With gamma toxin this turbidity decrease is faster than the rate of water uptake (which never exceeds 18%) indicating that it is due rather to structural modifications than to swelling. The same is observed with beta toxin, provided the mitochondrial protein concentration to be lower than 0.7 mg/ml. For higher concentrations, a continuous decrease of turbidity with a considerable uptake of water probably reflects the onset of phospholipasic activities. 3. It is postulated that structural modifications of the mitochondrial membranes are initiated which lead to the loss of their selective impermeability. The simultaneous loss of respiratory control with succinate may be due to the direct (though Ca2+-potentiated) displacement of the fraction of the membrane-bound Mg2+ ions which controls its energy-transducing properties. 4. In addition, correlations between the effects of the toxins on mitochondria and their neurotoxicity in vivo are discussed.

Special features of Pi transport in pig heart and rat liver mitochondria as revealed by 6,6'-dithiodinicotinic acid (CPDS)

CPDS (6,6'-dithiodinicotinic acid), a non permeant thiol agent which affects several mitochondrial functions in a way different to that of mersalyl [18-19] revealed striking differences between the phosphate translocating systems of pig heart and rat liver mitochondria. Pi entry was measured either by swelling in 0.12 M ammonium phosphate or by rapid centrifugation in 32Pi medium. Pi efflux was measured after preloading of mitochondria with 32Pi, by exchange against Pi or malate; the "ATP-FCCP" system has been tested previously [19]. In pig heart mitochondria, Pi entry seems to proceed exclusively via the Pi/OH- carrier; CPDS completely inhibits this transport and the energy-linked functions. In contrast n-butyl-malonate does not affect the Pi-entry and the energy-linked functions. The Pi efflux is not affected either by CPDS or mersalyl, which do not produce a swelling in the "ATP-uncoupler system". In rat liver mitochondria, CPDS inhibits only the Pi/OH- carrier; both CPDS and n-butylmalonate are necessary to inhibit completely Pi entry. CPDS as well as mersalyl provokes a swelling in the presence of the "APT-uncoupler system". The results suggest two distinct functions of phosphate transport in both types of mitochondria.

Temperature-induced transitions in the conformation of intermediates in the hydrolytic cycle of myosin

The conformations of the transitory intermediates of the myosin ATPase occurring during the hydrolytic cycle, enzyme without ligand, enzyme-substrate complex and two different forms of enzyme-product complex, have been characterized in terms of numbers and classes of reactive thiol groups based on incorporation of radioactively labeled alkylation reagent. The techniques employed allowed this to be done under steady-state conditions in the presence of high ligand concentrations on intact myosin from rabbit fast skeletal muscles at low ionic strength where the protein is in the gel state as it is in muscle. The binding of a divalent cation (Mg2+ or Ca2+) nucleotide complex exposes thiol-1 as well as thiol-2 groups. The long-lived ATPase intermediate occurring at temperatures above 10 degrees C adopts the same conformation with Mg2+ and Ca2+ ions. This intermediate does not protect the thiol-1 and thiol-2 groups but exposes a number of thiol-3 groups which seem to be located distant from the active site. The conformation of the intermediate prevailing in the presence of ATP changes with lowering temperature below 10 degrees C and is identical with that found in the presence of ADP at 0 degree C indicating a change in the rate-limiting step of the hydrolytic cycle. In the absence of divalent cations no such temperature-dependent change in conformation was observed. Evaluation of the activation entropies shows that the structure of the long-lived intermediate occurring above 10 degrees C in the presence of Mg2+ ions goes through a transformation from low to high order at around 20 degrees C. In the case of the monovalent-cation-stimulated ATPase a constant activation energy of around 70 kJ/mol, typical of many enzyme reactions, was found over the entire temperature range from 0--35 degrees C.