Complexes between hexokinase, mitochondrial porin and adenylate translocator in brain: regulation of hexokinase, oxidative phosphorylation and permeability transition pore.

Introduction Hexokinase I binds to the mitochondrial surface of brain [l], muscle [2], fat [3], kidney [4] and liver cells [S]. Responsibility for this association lies with a specific binding protein [6], which has been identified as mitochondrial porin in the outer membrane [7,8] and is also known as voltage-dependent anion channel W A C ) [9]. The isolated outer membrane pore, when reconstituted in liposomes, specifically binds hexokinase [6,7]. However, the distribution of hexokinase and porin at the mitochondrial surface of intact liver and brain mitochondria do not coincide. The localization of hexokinase was analysed by electron-microscopic techniques and by removing unattached outer membrane with digitonin. By the use of these techniques, the enzyme was found to be concentrated in contact sites between the mitochondrial envelope membranes in liver [ 101 and brain mitochondria [ 11,121. In contrast, porin appeared to be randomly distributed in the outer membrane [13,14]. This discrepancy was explained later by analysis of isolated mitochondrial contact sites. In contact-site fractions enriched from osmotically disrupted liver, brain and kidney mitochondria, hexokinase was concentrated [ 1 1,14,15] and had a significantly higher affinity for this membrane fraction than for isolated outer membrane [14,16]. By freeze-fracture analysis it was observed that contact sites were dynamic structures that were dependent not only on the functional state of the mitochondria [17,18] but also on the preservation of the physiological structure of the outer compartment during isolation [19]. In freeze-fractured mitochondria, fracture planes ran through the contact sites, pointing to a hydrophobic interaction between components of the two boundary membranes [ 171. Because of the higher affinity of hexokinase for contact sites, the binding of the enzyme to mitochondria was dependent on the frequency of these contact sites. Thus hexokinase binding could be used to indicate contact-site frequency.