The intracellular location of pyruvate carboxylase in rat liver

Specificity and selectivity in post-translational biotin addition

Biotin, which serves as a carboxyl group carrier in reactions catalyzed by biotin-dependent carboxylases, is essential for life in most organisms. To function in carboxylate transfer, the vitamin must be post-translationally linked to a specific lysine residue on the biotin carboxyl carrier (BCC) of a carboxylase in a reaction catalyzed by biotin protein ligases. Although biotin addition is highly selective for any single carboxylase substrate, observations of interspecies biotinylation suggested little discrimination among the BCCs derived from the carboxylases of a broad range of organisms. Application of single turnover kinetic techniques to measurements of post-translational biotin addition reveals previously unappreciated selectivity that may be of physiological significance.

Electron microscopic localization of pyruvate carboxylase in rat liver and Saccharomyces cerevisiae by immunogold procedures

The intracellular location of pyruvate carboxylase (EC 6.4.1.1) in rat liver and Saccharomyces cerevisiae was investigated using the antibody-gold and protein A-gold techniques carried out as a postembedding immunoelectron microscopic procedure. The vast majority of gold particles (greater than 98%), indicative of the presence of antigenic sites of pyruvate carboxylase, were found in the mitochondria of rat liver. No other cellular compartment was labeled except the cytosol which did not account for more than 2% of the total labeling of a rat hepatocyte. Furthermore, 60% of labeled pyruvate carboxylase molecules within a mitochondrion were found adjacent to the matrix side of the inner mitochondrial membrane. In contrast, in S. cerevisiae, pyruvate carboxylase was found exclusively in the cytosol.

Analysis of the membrane potential of rat- and mouse-liver mitochondria by flow cytometry and possible applications

Washed and purified rat- or mouse-liver mitochondria exhibiting high membrane integrity and metabolic activity were studied by flow cytometry. The electrophoretic accumulation/redistribution of cationic lipophilic probes, rhodamine 123, safranine O and a cyanine derivative, 3,3'-dihexyloxadicarbocyanine iodide, during the energization process was studied and was consistent with the generation of a negative internal membrane potential. An exception to this was nonylacridine orange which spontaneously bound to the mitochondrial membrane by hydrophobic interactions via its hydrocarbon chain. Energized purified mitochondria stained with potentiometric dyes exhibited both higher fluorescence and population homogeneity than the non-energized or deenergized (nigericin plus valinomycin) mitochondria. By contrast, under non-energized or deenergized conditions, the mitochondrial population exhibited fluorescence intensity heterogeneity related to the residual membrane potential; two subpopulations were evident, one of low fluorescence which may be related to the autofluorescence of the mitochondria (plus non-specific dye binding) and a second population which exhibited high fluorescence. Flow cytometry of the unpurified, simply washed, rat-liver mitochondria stained with rhodamine 123, a classically used dye, provided evidence of their heterogeneity in terms of light-scattering properties and membrane-potential-related fluorescence. One third of the washed mitochondria were found to be non-functional by such assays. The fluorescence of purified rat-liver mitochondria due to the membrane potential built up by endogenous substrates indicates heterogeneity of the mitochondrial population with respect to levels of endogenous substrates. The low-angle light scattering increases upon energization and provides some original information about the shape and modification of the inner mitochondrial conformation accompanying the energization. The heterogeneity of the rat liver mitochondrial population, from a structural, metabolic (existence of endogenous substrates) and functional (active and non-active mitochondrial population dispersion) point of view could thus be demonstrated by flow-cytometry analysis. Two animal models were examined with regard to the alteration of the mitochondrial membrane potential under the effects of drugs (rat-liver mitochondria), and the effects of ammonium toxicity (mouse-liver mitochondria). These results are promising and open new perspectives in the study of mitochondriopathies.

Rapid, quantitative isolation of mitochondria from rat liver using Ficoll gradients in vertical rotors

A method for the rapid, quantitative isolation of rat liver mitochondria has been developed. The method consists of placing portions of whole liver homogenate on linear Ficoll gradients followed by reorienting rate-zonal centrifugation in a vertical rotor. Fractions collected from the gradient were analyzed for protein, cytochrome oxidase, and a number of marker enzymes to determine mitochondrial location and the extent of their contamination with other cellular organelles. In the fraction designated as the mitochondrial peak, 85% of the total cytochrome oxidase activity was recovered. Contamination with other cellular organelles ranged from 7.07 to 14.05%. The respiratory control ratio of the isolated mitochondria was 8.10, indicating they were well coupled.

Submitochondrial location of ruthenium red-sensitive calcium-ion transport and evidence for its enrichment in a specific population of rat liver mitochondria

1. Seven fractions sedimenting at between 3000 and 120000g-min were prepared from a rat liver homogenate by differential centrifugation in buffered iso-osmotic sucrose. The following measurements were carried out on each of these fractions: Ruthenium Red-sensitive Ca(2+) transport in the absence and in the presence of P(i) as well as in the presence of N-ethylmaleimide to prevent P(i) cycling, succinate-supported respiration in the absence and in the presence of ADP, the DeltaE and -59 DeltapH components of the protonmotive force, cytochrome oxidase, uncoupler-stimulated adenosine triphosphatase, alpha-glycerophosphate dehydrogenase, P(i) content and the effect on the ;resting' rate of respiration of repeated additions of a fixed Ca(2+) concentration. 2. Ca(2+) transport either in the presence or in the absence of added P(i) and in the presence of N-ethylmaleimide exhibits significantly higher rates in the fraction sedimenting at 8000g-min. By contrast, respiration in the presence or in the absence of added ADP and the values for DeltaE and -59 DeltapH were similar in those fractions sedimenting between 4000 and 20000g-min, indicating that the driving force for Ca(2+) transport was similar in each of these fractions. 3. Experiments designed to determine the capacity of the individual fractions for Ca(2+), as measured by the effect of repeated additions of Ca(2+) on the resting rate of respiration, showed that fraction 2, i.e. that sedimenting at 8000g-min, also exhibited the greatest tolerance towards the uncoupling action of the ion. 4. Of the three enzyme activity profiles, only that of alpha-glycerophosphate dehydrogenase was similar to that of Ca(2+) transport. Because previous workers have assigned this enzyme to loci in the inner peripheral membrane [Werner & Neupert (1972) Eur. J. Biochem.25, 379-396], it is concluded that the Ruthenium Red-sensitive Ca(2+)- transport system also is located in this domain of the inner membrane. The relation of these findings to the mechanisms of mitochondrial Ca(2+) transport and the biogenesis of mitochondria is discussed.

The intracellular location of pyruvate carboxylase, citrate synthase and 3-hydroxyacyl-CoA dehydrogenase in lactating rat mammary gland

The intracellular location of pyruvate carboxylase (EC 6.4.1.1), citrate synthase (EC 4.1.3.7) and 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) in rat mammary gland was investigated by using a fractional-extraction technique. The results indicate a mitochondrial location for all three enzymes.

Subcellular distribution of a factor inactivating tyrosine aminotransferase. Study of its mechanism and relationship to different forms of the enzyme

The subcellular distribution of a tyrosine aminotransferase inactivating factor in rat liver has been investigated. Most of its activity is associated with plasma membranes, with minor amounts in mitochondria and endoplasmatic reticulum. The factor is also found in kidney and inactivates the enzyme reversibly in presence of cysteine, most likely by modification of -SH groups. ATP counteracts this inactivation only, when crude enzyme extracts are inactivated by purified subcellular fractions or when the purified enzyme is inactivated in presence of liver or kidney cortex homogenates. The relationship of this inactivation to reported different forms of the enzyme has been investigated. Form I of three different forms, that can be obtained by hydroxyl-apatite chromatography, is readily inactivated, form III can be partly converted to form I by incubation in presence of purified plasma membranes. The relationship of these findings to a possible multistep mechanism in the turnover of the enzyme discussed.

Different types of mitochondria in parenchymal and non-parenchymal rat-liver cells

1. Intact and pure parenchymal and non-parenchymal cells were isolated from rat liver. The specific activities of several mitochondrial enzymes were determined in both parenchymal and non-parenchymal cell homogenates to characterize the mitochondria in these liver cell types. 2. In general the activities of mitochondrial enzymes were lower in non-parenchymal liver cells than in parenchymal cells. The specific activity of pyruvate carboxylase in non-parenchymal cells expressed as the percentage of that in parenchymal cells was onlu 2% for glutamate dehydrogenase 4.3% and for cytochrome c oxidase 79.4%. Monoamine oxidase, as an exception, has an equal specific activity in both cell types. 3. The activity ratio of pyruvate carboxylase at 10 mM pyruvate over 0.1 mM pyruvate is 3.35 for parenchymal cells and 1.50 for non-parenchymal cells. This indicates that non-parenchymal liver cells only contain the high affinity form of pyruvate carboxylase in contrast to parenchymal cells. 4. The ratio of glycerol-3-phosphate cytochrome c reductase over succinate cytochrome c reductase activity differs from parenchymal (0.01) and non-parenchymal cells (0.10). This might indicate that the glycerol-3-phosphate shuttle, which is important for the transport of reduction equivalents for cytosol to mitochondria is relatively more active in non-parenchymal cells than in parenchymal cells. 5. The activity pattern of mitochondrial enzymes in parenchymal and non-parenchymal cell homogenates indicates that these cell types contain different types of mitochondria. The presence of these different cell types in liver will therefore contribute to the heterogeneity of isolated rat liver mitochondria in which the mitochondria from non-parenchymal cells might be considered as "non-gluconeogenic".

Compartmentation of acetyl-coA in rat-liver mitochondria

The ratio of the specific radioactivities of 3-hydroxybutyrate: citrate was determined in rat liver mitochondria which were incubated in the presence of [1-14C]palmitate, pyruvate, bicarbonate, ATP, phosphate and malonate. Without compartmentation this ratio would maximally be 2, however, under our conditions values of 2.5-3.7 were observed. In further experiments with mitochondria, the sensitivity of pyruvate carboxylase for acetyl-CoA produced from various precursors was tested. It was found that acetyl-CoA produced from L-acetylcarnitine or by oxidation from either pyruvate, octanoate or palmitylcarnitine but not from leucine led to a stimulation of pyruvate carboxylation. These results demonstrate a compartmentation of acetyl-CoA in liver mitochondria. The further finding that different mitochondrial fractions showed varying ratios of specific radioactivities of 3-hydroxybutyrate:citrate indicates that the observed compartmentation may be explained by the existence of different types of mitochondria with varying enzyme patterns and acetyl-CoA pools.

The diagnostic significance of liver cell inhomogeneity: serum enzymes in patients with central liver necrosis and the distribution of glutamate dehydrogenase in normal human liver

16 Patients with acute right-sided cardiac failure associated with a high pressure of the central venous system, exhibited a marked increase in glutamate dehydrogenase activity in serum. This increase was 40-fold higher than in patients with acute viral hepatitis. Histological examination of seven deceased patients revealed central necrosis within the liver lobule. This observation led us to determine glutamate dehydrogenase activity in microdissected peripheral and central portions from the unchanged liver lobule. A 1.7-fold higher glutamate dehydrogenase activity was found in the central part of the liver lobule than in the peripheral portion. The diagnostic significance of the glutamate dehydrogenase activity distribution along the cords of liver cells is discussed in view of liver diseases with central necrosis.