The effect of different families of detergents on the solubilization and purification of the pore-forming protein (porin) of the mitochondrial outer membrane of bovine heart was investigated in detail. With Tritons, dimethylamine oxides and zwittergents, porin solubilization with respect to total mitochondrial membrane protein was more efficient with the more hydrophobic members of each series. With most detergents the protein eluted as protein-detergent micelles in the void volume of hydroxyapatite/cehte columns. In contrast, the protein was bound to the column material and was eluted after the addition of salt to the elution buffer when the detergents octylglucoside, zwittergent 2-314 and lauryl (dimethyl)-amine oxide were used. The protein purified in the presence of the latter detergent had a higher pore-forming activity in lipid bilayer membranes compared to porin isolated in the presence of Triton X-100. The binding of porin to the hydroxyapatite/celite column was used to study the lipid content of the active pore-forming complex. The analysis revealed that the complex contained no phospholipid but rather five molecules of cholesterol/polypeptide chain.The mitochondria1 outer membrane contains general diffusion pores which are responsible for the free permeability of this membrane towards small hydrophilic solutes [l]. The active component of this molecular sieve is a protein, called mitochondrial porin or voltage-dependent anion channel [2 -41. Mitochondria1 porins were isolated from a variety of eukaryotic cells and their pore-forming properties were studied in reconstitution experiments with planar lipid bilayers and liposomes [2 -51. According to these investigations the mitochondrial pore has a diameter of about 2 nm in the open state and is slightly anion-selective at low transmembrane potentials. Voltages larger than 20 mV result in a shift of the pore into closed states, which have a reduced permeability towards hydrophilic solutes and a completely different selectivity from the open state.The role of the mitochondrial outer membrane in the physiology and metabolism of mitochondria was underestimated in the past. It has been described as a simple barrier that prevented the destruction of the mitochondrial inner membrane during osmotic swelling. More recent papers gave some insight into the function of the mitochondrial outer Abbreviations. (cHxN)zC, dicyclohexylcarbodiimide; CaE3, octyl trioxyethylene; CsE4, octyl tetraoxyethylene; CsE5, octyl pentaoxyethylene; C8(HE)S0, octyl (hydroxyethyl) sulfoxide; Cs(DHP)SO, octyl (dihydroxypropane) sulfoxide; DDAO, decyl (dimethyl)-amine oxide; LDAO, lauryl (dimethy1)-amine oxide; IysoPtdCho, lysophosphatidylcholine; MEGA 6, exanoyl-N-methylglucamide; MEGA 7, heptanoyl-N-methylglucamide; MEGA 8, octanoyl-N-methylglucamide; MEGA 9, nonaoyl-N-methylglucamide; MEGA 10, decanoyl-N-methylglucamide; MEGA 1 1 , undecanoyl-N-methylglucamide; MEGA 12, dodecanoyl-N-methylglucamide; NDAO, nonaoyl (dimethyl)-amine oxide; octyl-POE, octyl-polydisperse-oligo(oxyethy1-ene);...
