Photodynamic action of bilirubin on the inner mitochondrial membrane. Implications for the organization of the mitochondrial ATPase

Bilirubin in the presence of O₂ and light catalyzes the photodynamic modification of the proteins of the inner mitochondrial membrane as monitored by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Numerous polypeptide bands become streaked towards higher apparent molecular weight and decrease in staining intensity while other bands remain largely unchanged. The loss in staining intensity which occurs is at least partially due to apparent cross-linking of the polypeptides to produce aggregates which cannot penetrate into the gel. The α and β bands of the mitochondrial ATPase differ markedly in their susceptibility to modification. The β subunit is rapidly modified while the α subunit is largely inert. This differential susceptibility is a consequence of the binding of the soluble F₁ ATPase to the membrane. When submitochondrial particles with their normal complement of bound F₁ are mixed with free F₁ and are photolyzed together in the presence of bilirubin and O₂, it is found that the β subunit of the membrane-bound F₁, but not the α subunit, has been modified while neither subunit of the free F₁ has been modified. This increased susceptibility of the β subunit in the membrane state may represent cross-linking to membrane components and is consistent with the β subunit making more extensive contacts with membrane components than does the α subunit.