Investigations on the turnover of adrenocortical mitochondria. XII. Studies on the mechanism of the ACTH-induced elongation of the half-life of rat zona fasciculata mitochondria

The half-life of rat adrenocortical mitochondria was determined by high resolution autoradiography and liquid scintillation spectroscopy as previously described (Mazzocchi et al., '76). The results obtained by the two approaches were in good agreement. In the normal rats the half-life of adrenocortical mitochondria averaged 11 days. ACTH significantly increased mitochondrial half-life to about 16 days, and chloramphenicol significantly decreased this parameter in both untreated and ACTH-administered rats to about seven days. It is suggested that the ACTH-induced maintenance and slowing down of the degradation rate of adrenocortical mitochondria requires continuous mitochondrial DNA-dependent protein synthesis.

Growth dynamics of mitochondria in synchronized Chinese hamster cells

The increase in cell volume (from electronic cell sizing) and the apportionment of this volume amongst the nuclear, cytoplasmic, and mitochondrial subcellular compartments (from electron microscopy) were studied throughout the cell division cycle in partially synchronized cultures of Chinese hamster V79-S171 cells. Average whole cell volume was found to increase smoothly, consistent with the doubling in one generation of individual cell volume. Nuclear size increased in like fashion. Mean total mitochondrial volume and number of mitochondria per cell both showed a different kind of variation, most notably a significant decrease in G(1) and G(2) as compared with mid S. These results are therefore counter to a model of simple doubling of mitochondria either synchronously with the cell division cycle or asynchronously. Absolute mean values per cell for log phase Chinese hamster cells were also determined, as follows: whole cell volume, 710 mu(3); nuclear volume, 190 mu(3); total mitochondrial volume, 37.5 mu(3); number of mitochondria per cell, 90.

Biochemical and structural changes in mitochondria and other cellular components of pea cotyledons during germination

Integrated studies comprising biochemical and electron microscopic analysis suggested that the increase in respiratory activity of pea cotyledon mitochondria during germination results from further development of the original mitochondria present in dormant seeds. Electron microscopy of isolated mitochondria as well as mitochondria in situ has revealed that membranes are scarce in the mitochondria present in dormant seeds. Mitochondrial cristae become well developed during the initial stages of germination. Crude mitochondrial preparations from pea cotyledons were fractionated by sucrose density gradient centrifugation and analyzed through electron microscopy. These studies showed that, at all stages of germination, "peroxisome"-like structures were present in the fractions of higher sucrose densities than that containing mitochondria. Biochemical studies revealed that the activities of catalase (H2O2:H2O2 oxidoreductase, EC 1.11.1.6) and peroxidase (guaicol:H2O2 oxidoreductase, EC 1.11.1.7) were associated mainly with these fractions and their activities increased during germination.

The influence of temperature on bone growth in the chick

Images

Synchronous mitochondrial division in Neurospora crassa

Samples of mycelium of Neurospora crassa of known age were harvested from agar plates and examined with the electron microscope. The relative volume of the mitochondria was determined for mycelium of different ages. The volume measurements indicated that the mitochondria were dividing synchronously in fronts 6, 13, and 22(1/2) hr behind the growing hyphal tips. The sequence of mitochondrial division is hypothesized to include mitochondrial cupping followed by division which results in closely associated daughter mitochondria. On the basis of percentages of mitochondrial cupping and association, mitochondrial division was postulated to be occurring at 6, 14, and 26 hr. Close agreement between the mycelial mass doubling time and the calculated mitochondrial mass doubling time indicates that synchronous mitochondrial division is sufficient to maintain growth. The possibility that mitochondrial division is due to intercellular regulation of a mitochondrial genetic system is advanced.

Distribution of cytochrome c peroxidase activity in wild-type and petite cells of bakers' yeast grown aerobically and anaerobically

Studies of mitochondrial biogenesis in yeast have been hampered by a lack of suitable membrane markers in anaerobically grown cells subsequently grown in air. Cytochrome c peroxidase activity and subcellular location was studied to determine whether it would be a useful marker for an analysis of mitochondrial formation. Cytochemical tests revealed enzyme reaction product on all mitochondrial membranes in aerobically grown wild-type cells. Anaerobically grown wild-type and all petite cultures contained cytochrome c peroxidase cytochemical reaction deposits on abundant cytoplasmic membranes and on the few mitochondrial profiles which also were seen in the electron photomicrographs. Biochemical studies corroborated the cytochemistry because mitochondrial fractions were greatly enriched in cytochrome c peroxidase activity for aerobically grown wild-type cultures, but petite and anaerobically grown wild-type cultures showed higher enzyme activities in supernatant fractions than was present in the corresponding particulate fractions after differential centrifugation. Evidence from low-temperature microspectroscopy, spectrophotometric assays of mitochondrial enzyme activities, and electron microscopy showed mitochondrial formation during the time required for preparation and lysis of spheroplasts from anaerobically grown cultures. The data were interpreted as indicating that cytochrome c peroxidase was an oxygen-inducible enzyme, and that there was a developmental relationship between enzyme-reactive membranes of mitochondria and cytoplasm during the period of respiratory adaptation.