STUDIES ON LOCALIZATION OF RAT LIVER MITOCHONDRIAL 5'-ENDONUCLEASE

Isolated rat liver mitochondria exposed to an hypotonic medium hydrolyzed poly A at a faster rate than did those incubated under isotonic conditions, indicating that the former medium causes unmasking of a substantial part of the 5′-endonuclease activity. Most of the unmasked activity is released in a soluble form in the incubation medium. The digitonin technique has been used to separate rat liver mitochondria into three fractions: inner membrane plus matrix, outer membrane, and a soluble fraction containing proteins localized between the membranes. The distribution of the 5′-endonuclease has been compared with those of four other enzymes: monoamine oxidase, cytochrome oxidase, malate dehydrogenase and adenylate kinase. The endonuclease was found in both the outer membrane and the soluble fraction: this suggests a dual localization. Attempts to measure the penetration of poly A into mitochondria were unsuccessful because this polymer was adsorbed on the mitochondrial membranes, resulting in a distribution space much larger than that of water. © 1969.