Characterization of the mitochondrial porin expressed in malignant tumor cells.

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In tumor cells, a large amount of hexokinase is known to attach their mitochondria. The interaction of hexokinase and mitochondria is reversible ; hexokinase can be detached from mitochondria and rebinding can be observed when this hexokinase was added to the hexokinase-depleted tumor mitochondria. However, when mitochondria-bindable hexokinase was added to liver mitochondria, only a small portion can be attached to the mitochondria. This fact indicates the possible differences in the hexokinase binding sites between liver mitochondria and tumor mitochondria. Binding site of hexokinase on mitochondria is established as porin protein expressed in the outer mitochondrial membrane. Thus, in this study, we characterized porin protein expressed in malignant tumor cells. Following results were obtained.1. At least three porin isoforms were expressed in mammals. To examine possible structural differences of these three porin isoforms between normal tissues and tumor cells, we isolated and characterized the cDNA clones encoding three porin isoforms from cDNA library of malignant tumor cells. The revealed structures of tumor porins showed several mismatches with reported porins. However, detailed analyses showed that these mismatches were not tumor specific differences.2. To examine the possible expression of fourth member of porin in tumor cells, we carried out degenerated primer based PCR. However, only the DNA bands correspond to three porin isoforms were observed. This result indicates that only three porin isoforms were expressed in tumor cells as well as normal tissues.3. To explore the possible differences of porin protein between normal liver and tumor cells, their transcript levels were compared. As a result, transcript levels of three porin isoforms in tumor cells were remarkably higher than those in normal liver.