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PubMed1973

Localization of Na + , K + -ATPase and other enzymes in teleost pseudobranch. II. Morphological characterization of intact pseudobranch, subcellular fractions, and plasma membrane substructure.

The pseudobranch of the pinfish Lagodon rhomboides is an unusually homogeneous and structurally simple tissue, well suited to cell fractionation studies. Its principal cell type, closely related to the chloride cells of teleost gill, is characterized by numerous mitochondria in close association with abundant tubular invaginations of the plasma membrane. Other cytoplasmic organelles are rarely encountered. In broken fresh pseudobranch cells negatively stained with ammonium molybdate, a 40 A particulate layer was observed on the intracellular surface of the tubular plasma membrane fragments. Nuclear (N), mitochondrial-light mitochondrial (M+L), and microsomal (P) fractions, obtained by differential centrifugation, were characterized by examination of fixed, embedded pellets and unfixed preparations negatively stained with ammonium molybdate and potassium phosphotung-state. Mitochondria, in orthodox configuration and retaining their outer membranes, were observed in M+L and N. Significant amounts of tubular, sheetlike, or vesicular membrane fragments were observed in all three fractions. Many such fragments, when negatively stained, showed the 40 A particulate surface layer characteristic of plasma membrane invaginations, and in some cases 20-A projections could be resolved on the opposite (extracellular) surface. Since these morphological observations, together with previously presented biochemical data, suggest a plasma membrane localization of Na(+), K(+)-ATPase, the possible association of the enzyme with membrane projections is discussed.

Adenosine Triphosphatases↗
PubMed1983

Analytical subcellular fractionation of normal human skeletal muscle by sucrose density gradient centrifugation.

The principle organelle marker enzymes and various adenosine triphosphatase (ATPase) activities were studied in human skeletal muscle. The reproducibility of each assay was established under optimal and linear assay conditions. Whole homogenates of normal human quadriceps muscle were fractionated by centrifugation on a continuous sucrose density gradient. Gradient fractions were assayed for organelle marker enzymes and frequency-density histograms were constructed for each enzyme. Good resolution of the principal organelles was obtained. Adenosine triphosphatase (ATPase) was assayed under conditions of maximal stimulation by Ca2+, or Mg2+ or Na2+, K+ + Mg2+. The distribution of these activities was compared with those of the organelle marker enzymes. Both Ca2+-ATPase and Mg2+-ATPase were distributed to both the mitochondrial and myofibrillar fractions but could be distinguished by the inhibition of mitochondrial ATPase with sodium azide. The distribution of Na+, K+-activated, Mg2+-dependent ATPase (Na+, K+ ATPase) activity suggested a sarcolemmal localization. The results of electron microscopy of gradient fractions were consistent with the organelle content of the fractions as determined by enzymic analyses. These studies provide reference information for the subsequent investigation of organelle pathology of human muscle disorders.

5'-Nucleotidase↗
PubMed1982

Reconstitution of a tandem Co- and post-translational processing pathway with rat liver subcellular fractions.

Previously we showed that smooth microsomes from a variety of tissues effectively cleaved, sequestered, and "core" glycosylated nascent chains of secretory proteins. To further characterize the role of smooth membranes in the biosynthesis of secretory polypeptides, rat liver smooth microsomes were separated into smooth endoplasmic reticulum and Golgi fractions. Membranes of the smooth endoplasmic reticulum cleaved the signal peptide of pre-placental lactogen, attached the high mannose core to the alpha subunit of chorionic gonadotropin, and sequestered the processed proteins. None of these processing steps were performed by Golgi membranes. However, processing of asparagine-linked oligosaccharides and the coincident addition of terminal sugars was performed by Golgi but not by smooth endoplasmic reticulum membranes. The properties of this post-translational reaction are very similar to those described for the reactions in vivo. These observations demonstrate that the enzymes for co-translational (pre-protein processing) and posttranslational (oligosaccharide maturation) processing events are localized in the endoplasmic reticulum and Golgi apparatus, respectively. This functional differentiation of Golgi and endoplasmic reticulum membranes is an important feature of the secretory process in eukaryotic cells. Restriction of the recognition and transport of nascent secretory proteins to the endoplasmic reticulum establishes the polarity necessary for the ordered sequence of post-translational steps involved in the synthesis and maturation of secretory proteins.

Animals↗
PubMed1985

Effects of metabolic inhibitors on the distribution of 45-Ca in subcellular fractions from pancreatic islets.

It is well established that intracellular calcium buffering is of vital importance for the regulation of insulin release. The metabolic inhibitors, cyanide and N-ethylmaleimide, both decreased the 45-Ca content of a mitochondrial fraction isolated from glucose-stimulated pancreatic islets. We suggest that the main mechanism behind this decrease is an impaired uptake of the isotope, since cyanide had no effect on the 45-Ca wash-out from the mitochondrial fraction of preloaded islets.

Animals↗