BIOCHEMICAL CONSTITUTION OF MAMMALIAN MITOCHONDRIA

Abstract

Almost 20 years have passed since the isolation of mitochondria from guinea pig liver was announced by Bensley and Hoerr (6) in 1934. This important discovery remained largely unexploited until the researches of Claude on the particulate components of cytoplasm were begun some 5-6 years later. Although Claude at first mistook the mitochondria for secretion granules (17), his methods for the isolation of these cell organelles, which were the first to be described in the literature (17, 19, 20), laid the foundations for the large amount of research on mitochondria that has been carried out in the last few years. The recognition of the key role played by the mitochondrion in the metabolism of the cell is largely the result of the localisation of a number of important enzymatic functions in isolated mitochondria (cf. 97). These enzymatic activities have not merely been detected in isolated mitochondria but, in many cases, have been shown to account for 50 per cent or more of the total cellular activity. The ability to recover large fractions of the total enzyme activity in isolated mitochondria and to demonstrate concentration of this activity in mitochondria indicated to us that the findings were accurate from the cytochemical standpoint. On the other hand, we have hesitated to attach cytochemical significance to the presence of small percentages of the total tissue enzyme activity in mitochondna or in any other subcellular structure, since such results could very readily be the result of adsorption, contamination, or other factors which could not be satisfactorily proved or disproved. Even in those cases in which it has been possible to demonstrate almost exclusive localisation of a cellular function in mitochondria, the possibility of artifacts has always been considered and we have sought and welcomed the appearance of independent methods that would either support or deny the results obtained with isolated mitochondria. Although some such evidence has been reported in support of the localisation of certain enzymes in mitochondria (3, 11), confirmation of the results of the cell fractionation technic has not kept pace with new results obtained with this method. It is hoped that this situation will be corrected in the future.