Hexokinase bound to rat brain mitochondria uses externally added ATP more efficiently than internally generated ATP

Conformational changes in the mitochondrial channel protein, VDAC, and their functional implications

The voltage-dependent, anion-selective channel (VDAC) is generally considered the main pathway for metabolite diffusion across the mitochondrial outer membrane. It also interacts with several mitochondrial and cytosolic proteins, including kinases and cytochrome c. Sequence analysis and circular dichroism suggest that the channel is a bacterial porin-like beta-barrel. However, unlike bacterial porins, VDAC does not form tight trimeric complexes and is easily gated (reversibly closed) by membrane potential and low pH. Circular dichroism indicates that the protein undergoes a major conformational change at pH < 5, involving decreased beta-sheet and increased alpha-helical content. Electron microscopy of two-dimensional crystals of fungal VDAC provides direct information about the size and shape of its lumen and suggests that the N-terminal domain forms a mobile alpha-helix. It is proposed that the N-terminal domain normally resides in a groove in the lumen wall and that gating stimuli favor its displacement, destabilizing the putative beta-barrel. Partial closure would result from subsequent larger-scale structural rearrangements in the protein, possibly corresponding to the conformational change observed at pH < 5.

Source of ATP for hexokinase-catalyzed glucose phosphorylation in tumor cells: dependence on the rate of oxidative phosphorylation relative to that of extramitochondrial ATP generation

We isolated highly intact and tightly coupled mitochondria from the rat ascites hepatoma cell line AH130 by disruption of the cell membrane by nitrogen cavitation. These isolated mitochondria were found to have essentially the same functional properties as rat liver mitochondria, but unlike the latter, hexokinase (HK) was bound to their membrane. Using the tumor mitochondrial preparation, we examined the source of ATP for phosphorylation of glucose by HK under conditions in which intra- and extramitochondrial ATP-generation systems operated separately or together. Results showed that the membrane-bound HK utilized ATP derived from the most efficiently operating ATP generation system, i.e., oxidative phosphorylation. However, when the rate of extramitochondrial ATP generation was much greater than that of oxidative phosphorylation, HK used ATP from the extramitochondrial ATP-generation system.

Is there VDAC in cell compartments other than the mitochondria?

Higher eukaryotes, including mammals and plants, express a family of VDAC proteins each encoded by a distinct gene. Two human genes encoding VDAC isoforms (HVDAC1 and HVDAC2) have been characterized in greatest detail. These genes generate three proteins that differ primarily by the addition of distinct N terminal extensions in HVDAC2 and HVDAC2', a splice variant of HVDAC2, relative to HVDAC1. Since N terminal sequences have been demonstrated to target many proteins to appropriate subcellular compartments, this observation raises the possibility that the N terminal differences found in HVDAC isoforms may lead to targeting of each protein to different cellular locations. Consistent with this hypothesis, a large number of reports have provided evidence consistent with the notion that HVDAC1 and its homolog in related mammalian species may specifically be present in the plasma membrane or other nonmitochondrial cellular compartments. Here, we review this information and conclude that if VDAC molecules are present at nonmitochondrial locations in mammalian cells, these are unlikely to be the known products of the HVDAC1 or HVDAC2 genes.

Different properties of the mitochondrial and cytosolic hexokinases in maize roots

After tissue homogenization, 43% of the total hexokinase activity found in maize radicles was recovered in the mitochondrial fraction and 35% was soluble, in the cytosol. The maize submitochondrial particles obtained after mitochondrial sonication retained a high hexokinase activity. The mitochondrial respiration (state 4 rate) was activated by glucose. This activation was blocked by carboxyatractyloside (0.5 mM) and by oligomycin (2 micrograms/ml). The affinities for ATP and glucose of both soluble and membrane-bound maize hexokinases are similar to those of yeast hexokinase. The Km for ATP of these different forms of hexokinase varied between 0.15 and 0.37 mM, and the Km for glucose between 0.05 and 0.13 mM. A major difference between the two maize hexokinase forms is that only the mitochondrial enzyme was strongly inhibited by ADP (Ki 0.04 mM). The soluble forms of hexokinase found both in the cytosol of maize radicles and in yeast are not inhibited by ADP. In a previous report [de Meis, Grieco and Galina (1992) FEBS Lett. 308, 197-201] it was shown that the mitochondrial F1-F0-ATPase can use glucose 6-phosphate and yeast hexokinase as an ATP regenerating system. We now show that the membrane-bound hexokinase and glucose 6-phosphate can also serve as an ATP regenerating system for the mitochondria of maize radicles provided that the ADP concentration is kept below 0.05 mM. Higher ADP concentrations inhibit the reverse reaction of the mitochondrial hexokinase.

Subcellular localization of human voltage-dependent anion channel isoforms

The voltage-dependent anion channel of the outer mitochondrial membrane, VDAC (also known as mitochondrial porin), is a small abundant protein which forms a voltage-gated pore when incorporated into planar lipid bilayers. This protein forms the primary pathway for movement of major metabolites through the outer membrane. Recently, it has been demonstrated that two human VDAC genes, HVDAC1 and HVDAC2, produce three proteins that differ most significantly at their amino termini. These results suggest that the distinct amino termini lead to the targeting of individual VDAC isoforms to different cellular compartments. Consistent with this hypothesis, recent reports suggest that HIV-DAC1 is found in the plasma membrane of mammalian cells. To define the subcellular location of HVDAC isoforms, HVDAC genes were modified so that the encoded proteins contain COOH-terminal epitopes recognized by either of two monoclonal antibodies. Introduction of these epitope tags had no effect on the function of modified VDAC proteins. Epitope-tagged proteins were then individually expressed in COS7 cells or rat astrocytes and the intracellular location of each isoform subsequently identified by subcellular fractionation, light level immunofluorescence, and immunoelectron microscopy. Our results demonstrate that each HVDAC protein is exclusively located in fractions or subcellular regions containing mitochondrial marker proteins. In addition, immunofluorescence and immunoelectron microscopy show that an individual mitochondrion can contain both HVDAC1 and HVDAC2. Our results call into question previous reports demonstrating VDAC molecules in the plasma membrane and suggest that functional differences between individual VDAC isoforms may result in distinct regulatory processes within a single mitochondrion.

The importance of the outer mitochondrial compartment in regulation of energy metabolism

Substitution of physiologically present macromolecules during isolation of mitochondria and investigation of their functions led to a significant change in regulation of oxidative phosphorylation. The differences compared to conventionally isolated mitochondria were that stimulation of oxidative phosphorylation appeared to rather depend on the activity of peripheral kinases than on the addition of free ADP. The localisation of peripheral kinases such as hexokinase and mitochondrial creatine kinase are described as well as the effects of macromolecules on the regulation of bound hexokinase and of oxidative phosphorylation via this enzyme.

Effect of macromolecules on the structure of the mitochondrial inter-membrane space and the regulation of hexokinase

Macromolecules as components of the physiological mitochondrial environment were substituted by addition of 10% dextran 70. This led to a significant reduction of the space between the two envelope and the crista membranes and to an increase of contact sites as observed by freeze-fracture analysis. The preferential binding of hexokinase in these sites was employed to further analyze the dextran effect: (i) desorption of the enzyme by digitonin treatment was found to be significantly reduced in the presence of dextran although liberation of adenylate kinase and monoamine oxidase were not affected, (ii) the affinity of isolated hexokinase isozyme I to liver mitochondria was increased by dextran. Generally the binding of hexokinase to intact mitochondria (also control mitochondria) followed a co-operative mechanism and led to an activation. Cooperativity and activation were not observed when the contact formation was suppressed by dinitrophenol or glycerol. The binding of hexokinase to the isolated outer membrane resembled that of mitochondria in the absence of contacts (i.e., no cooperativity and activation). Conversely to the observation in intact mitochondria, dextran rather reduced the affinity of hexokinase to the isolated outer membrane. Kinetic analyses of the dextran effect served to explain the function of contact site specific hexokinase binding. We observed that dextran improved the hexokinase dependent stimulation of the oxidative phosphorylation (state 3 respiration), while the activity of the enzyme with internal or external ATP remained unaffected. The results suggest three things: (i) that contact sites are probably more frequent in the intact cell than in vitro in the absence of macromolecules, (ii) that the contact preference of hexokinase serves rather the ADP supply of the translocator than the ATP transfer to the enzyme and (iii) that the total cellular hexokinase activity may be regulated by specific binding of the enzyme to the contact sites, either because of a different pore structure or because of additional components exclusively exposed in these sites.

Respiratory control induced by ATP in human term placental mitochondria

Oxygen uptake in human placental mitochondria was stimulated by ATP addition. ATP-induced respiration was supported by malate, alpha-keto glutarate, and succinate, and inhibited by oligomycin and carboxytractyloside. This phenomenon was not caused by contamination with unspecific phosphatases or alkaline phosphatase, since NaF, L-phenyl alanine, or P1, P5-di-(adenosine-5') pentaphosphate failed to inhibit oxygen uptake induced by ATP. The stimulation of respiration was caused by an ATPase activity tightly bound to mitochondria, which yields ADP that is responsible for the oxygen uptake. The stimulation was not an uncoupling effect because ATP addition produced a transition between state 3 and 4 of respiration, indicating that ATP was not released from mitochondria.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.