The submitochondrial localization of monoamine oxidase. An enzymatic marker for the outer membrane of rat liver mitochondria

Controlled osmotic lysis (water-washing) of rat liver mitochondria results in a mixed population of small vesicles derived mainly from the outer mitochondrial membrane and of larger bodies containing a few cristae derived from the inner membrane. These elements have been separated on Ficoll and sucrose gradients. The small vesicles were rich in monoamine oxidase, and the large bodies were rich in cytochrome oxidase. Separation of the inner and outer membranes has also been accomplished by treating mitochondria with digitonin in an isotonic medium and fractionating the treated mitochondria by differential centrifugation. Treatment with low digitonin concentrations released monoamine oxidase activity from low speed mitochondrial pellets, and this release of enzymatic activity was correlated with the loss of the outer membrane as seen in the electron microscope. The low speed mitochondrial pellet contained most of the cytochrome oxidase and malate dehydrogenase activities of the intact mitochondria, while the monoamine oxidase activity could be recovered in the form of small vesicles by high speed centrifugation of the low speed supernatant. The results indicate that monoamine oxidase is found only in the outer mitochondrial membrane and that cytochrome oxidase is found only in the inner membrane. Digitonin treatment released more monoamine oxidase than cytochrome oxidase from sonic particles, thus indicating that digitonin preferentially degrades the outer mitochondrial membrane.

Lipogenesis in adipose tissue of meal-fed rats. A possible regulatory role of alpha-glycerophosphate formation

The incorporation of acetate-1-14C into fatty acids by isolated epididymal adipose tissue of fed and fasted rats adapted to a single daily 2-hour meal (meal eaters) or fed ad libitum (nibblers) was investigated. Fasting (22 hours) markedly depressed lipogenesis whereas fatty acid synthesis increased linearly with time of refeeding in meal-fed but not in nibbling rats. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and NADP-malic dehydrogenase in adipose tissue of meal-fed or nibbling rats were not altered as a consequence of a 22-hour fast or of subsequent feeding for 2 hours. The incorporation of acetate-1-l4C into fatty acids by adipose tissue of fasted meal-eating or nibbling animals was markedly enhanced by the addition of unlabeled pyruvate or oxaloacetate to the incubation medium. This stimulatory effect was not observed with adipose tissue front fed meal-eating rats. The addition of unlabeled glucose and insulin to the incubation medium markedly enhanced acetate-1-14C incorporation into fatty acids by isolated adipose tissue and completely overcame any effect of fasting. Adipose tissue converted pyruvate-1-14C, -2-14C, or -3-14C to fatty acids and glyceride-glycerol. The results obtained are consistent with the functioning of a pathway in adipose tissue involving mitochondrial carboxylation of pyruvate to oxaloacetate, and equilibration of the newly formed oxaloacetate with malate and fumarate, followed by cytoplasmic conversion of oxaloacetate to phosphoenol pyruvate. The data are interpreted to support a control mechanism in which fatty acid synthesis is inhibited by tissue fatty acids and fatty acyl-CoA derivatives. The inhibition could in turn be reduced by the availability of α-glycerophosphate, for the esterification of fatty acids. This control mechanism is proposed as the explanation for the refeeding response observed in adipose tissue of meal-fed rats.

Biochemical and ultrastructural properties of osmotically lysed rat-liver mitochondria

Isolated rat-liver mitochondria were osmotically lysed by suspension and washing 3 times in cold, distilled water. Pellets obtained by centrifugation at 105,000 g for 30 min were resuspended, fixed with glutaraldehyde and OsO(4), and embedded in Epon 812. Thin sections show the presence of two distinct membranous populations, each of which is relatively homogeneous in size and appearance. Swollen mitochondria ( approximately 1.5 micro in diameter), which have been stripped of their outer membranes, are largely devoid of matrix and normal matrix granules and are referred to as "ghosts." The smaller (0.2 to 0.4 micro in diameter), empty appearing, vesicular elements, derived primarily from the outer mitochondrial membrane, can be differentiated from the ghosts on the basis of their smaller size and complete absence of internal structures, especially cristae. Each membranous element is enclosed by a single, continuous membrane; the "double membrane" organization typical of intact mitochondria is not observed. These findings indicate that the outer membrane of rat-liver mitochondria is spatially dissociated from the inner mitochondrial membrane by osmotic lysis of the mitochondria in distilled water. Three parameters of structural and functional significance in freshly isolated rat-liver mitochondria have been correlated with the structural alterations observed: (a) chemical composition (total protein, lipid phosphate and total phosphate), (b) specific and total activities of marker enzymes for mitochondrial matrix and membranes (malate dehydrogenase (MDH), D-beta-hydroxybutyrate dehydrogenase (BDH) and cytochromes), and (c) integrated multienzyme functions (respiration, phosphorylation, and contraction). The data presented indicate that all mitochondrial membranes are completely conserved in the crude ghost preparation and that, in addition, about (1/3) of the matrix proteins (estimated by assays for MDH activity and protein) are retained. The study of integrated mitochondrial functions shows that a number of physiologically important multienzyme activities also are preserved in the water-washed preparation. The respiratory rate of ghosts per milligram of protein is 1.5 to 2.0 times that of intact mitochondria, which shows that the respiratory chain in the ghosts is functionally intact. The rate of phosphorylation is reduced, however, to about 25% of that measured in freshly isolated mitochondria and accounts for lowered P:O ratios using succinate as substrate (P:O ranges from 0.4 to 0.9). The phosphorylation of ADP to ATP is the only biochemical function, so far investigated, that is greatly affected by osmotic lysis. In addition, two lines of evidence suggest that the ghosts undergo an energy-dependent transformation resulting in contraction: (a) suspensions of the crude ghost preparation in 0.02 M Tris-0.125 M KCl medium show a marked increase in optical density upon the addition of ATP, and (b) ghost preparations incubated in ion-uptake medium in the absence of added calcium but in the presence of added ATP contain a large number of highly condensed ghosts (about 50% of the total profiles) when viewed as thin sections in the electron microscope. The correlated biochemical and morphological study presented here shows that the outer membrane of rat-liver mitochondria can be removed by controlled osmotic lysis without greatly impairing a number of integrated biochemical functions associated with the inner membrane.

Cofactor requirements of the L-malate dehydrogenase of Pseudomonas ovalis Chester

1. The l-malate dehydrogenase of Pseudomonas ovalis Chester, which is independent of nicotinamide nucleotides and which is structurally and functionally bound to the cell-wall membrane, has been prepared in a soluble form and partially purified. 2. The purified dehydrogenase exhibits a triple cofactor requirement for FAD, quinone and phospholipid, and in the presence of these cofactors can utilize 2,6-dichlorophenol-indophenol as hydrogen acceptor. 3. The formation of reduced forms of FAD was not detected, but in the presence of both FAD and phospholipid the enzyme catalysed the reduction of quinone by l-malate at rates equivalent to those obtained with 2,6-dichlorophenol-indophenol as terminal acceptor. The l-malate dehydrogenase of Ps. ovalis Chester is therefore an l-malate-quinone oxidoreductase. 4. The quinone and the phospholipids present in the fragments of the cell-wall membrane from which the soluble dehydrogenase was prepared have been extracted and purified. The quinone was identified as coenzyme Q(9). At least eight phospholipids were detected, and the major component is an unsaturated phosphatidylethanolamine. 5. The nature of the phospholipid required to activate the enzyme depends on the nature of the quinone used in the assay system. When 2-methyl-1,4-naphthaquinone is used, a wide variety of phospholipids, including all those isolated from the organism, will activate the enzyme, but when coenzyme Q(9) is used the phospholipid specificity of the enzyme is much more restricted, and the most effective activator is the unsaturated phosphatidylethanolamine isolated from the organism. 6. Evidence is presented to support the view that the restricted phospholipid specificity exhibited by the enzyme in the presence of coenzyme Q(9), as opposed to the broad specificity exhibited when 2-methyl-1,4-naphthaquinone is used, is due to the fact that coenzyme Q(9) has a large substituent on position 3.