A noncleavable signal for mitochondrial import of 3-oxoacyl-CoA thiolase

Rat 3-oxoacyl-CoA thiolase, an enzyme of the fatty acid beta-oxidation cycle, is located in the mitochondrial matrix. Unlike most mitochondrial matrix proteins, the thiolase is synthesized with no transient presequence and possesses information for mitochondrial targeting and import in the mature protein of 397 amino acid residues. cDNA sequences encoding various portions of the thiolase were fused in frame to the cDNA encoding the mature portion of rat ornithine transcarbamylase (lacking its own presequence). The fusion genes were transfected into COS cells, and subcellular localization of the fusion proteins was analyzed by cell fractionation with digitonin. When the mature portion of ornithine transcarbamylase was expressed, it was recovered in the soluble fraction. On the other hand, the fusion proteins containing the NH2-terminal 392, 161, or 61 amino acid residues of the thiolase were recovered in the particulate fraction, whereas the fusion protein containing the COOH-terminal 331 residues (residues 62-392) was recovered in the soluble fraction. Enzyme immunocytochemical and immunoelectron microscopic analyses using an anti-ornithine transcarbamylase antibody showed mitochondrial localization of the fusion proteins containing the NH2-terminal portions of the thiolase. These results indicate that the NH2-terminal 61 amino acids of rat 3-oxoacyl-CoA thiolase function as a noncleavable signal for mitochondrial targeting and import of this enzyme protein. Pulse-chase experiments showed that the ornithine transcarbamylase precursor and the thiolase traveled from the cytosol to the mitochondria with half-lives of less than 5 min, whereas the three fusion proteins traveled with half-lives of 10-15 min. Interestingly, in the cells expressing the fusion proteins, the mitochondria showed abnormal shapes and were filled with immunogold-positive crystalloid structures.     

Immunocytochemical localization of cathepsin B in rat kidney. II. Electron microscopic study using the protein A-gold technique

Thin sections of Lowicryl K4M-embedded materials were labeled with protein A-gold complex. Gold particles representing the antigen sites for cathepsin B were exclusively confined to lysosomes of each segment of the nephron. The heaviest labeling was noted in the lysosomes of the S1 segment of the proximal tubules. Labeling intensity varied considerably with the individual lysosomes. Lysosomes of the other tubular segments, such as the S2 and S3 segments of the proximal tubules, distal convoluted tubules, and collecting tubules were weakly labeled by gold particles. Quantitative analysis of labeling density also confirmed that lysosomes in the S1 segment have the highest labeling density and that approximately 65% of labeling in the whole renal segments, except for the glomerulus, was found in the S1 segment. These results indicate that in rat kidney the lysosomes of the S1 segment are a main location of cathepsin B. Further precise observations on lysosomes of the S1 segment revealed that apical vesicles, tubules, and vacuoles were devoid of gold particles, but when the vacuoles contained fine fibrillar materials, gold labeling was detectable in such vacuoles. As the lysosomal matrix becomes denser, the labeling density is increased. Some small vesicles around the Golgi complex were also labeled. These results indicate that the endocytotic apparatus including the apical vesicles, tubules, and vacuoles contains no cathepsin B. When the vacuoles develop into phagosomes, they acquire this enzyme to digest the absorbed proteins.     

The structure of hemocyanin II from the horseshoe crab, Limulus polyphemus. The amino acid sequence of the second largest cyanogen bromide fragment

Fourteen fragments have been isolated from hemocyanin component II of Limulus polyphemus by cleavage with CNBr. The amino acid sequence of the largest fragment, CNBr Ia has been reported (Yokota, E., and Riggs, A. F. (1984) J. Biol. Chem. 259, 4739-4749). The amino acid sequence of the 12 smaller fragments is reported in an accompanying paper (Moore, M. D., Behrens, P. Q., and Riggs, A. F. (1985) J. Biol. Chem. 261, 10511-10519). We have determined the amino acid sequence of the second largest fragment, CNBr Ib. The fragment contains 142 residues and has a molecular weight of 16,095.     

Purification and some properties of mitochondrial glutamate:glyoxylate aminotransferase and mechanism of its involvement in glycolate pathway in Euglena gracilis z

Euglena contains glutamate:glyoxylate aminotransferase (GGT) both in mitochondria and in cytosol. Both isoforms were separated from each other by DEAE-cellulose chromatography. The mitochondrial enzyme had an apparent Km of 1.9 mM for glutamate and the cytosolic enzyme 52.6 mM. Mitochondrial GGT was further purified by ammonium sulfate fractionation, isoelectric focusing, and gel chromatography. It had a molecular weight of 141,000 and an isoelectric point of pH 4.88; the optimum pH was 8.5. Its apparent Km values for glutamate and for glyoxylate were 2.0 and 0.25 mM, respectively. In addition to glutamate, mitochondrial GGT used 5-hydroxytryptophan, tryptophan, and cysteine as amino donors in the transamination to glyoxylate. Alanine did not support the activity. The relative activity of the enzyme for amino acceptors on the transamination from glutamate was 4-hydroxyphenylpyruvate greater than phenylpyruvate greater than glyoxylate greater than hydroxypyruvate. Pyruvate and 2-oxoglutarate were not used in the reaction. Evidence that GGT functions mainly in the irreversible transamination between glutamate and glyoxylate is presented. The functional significance of GGT in the glycolate pathway of Euglena is also discussed.     

Amplification and expression of a cellular oncogene (c-myc) in human gastric adenocarcinoma cells.

Three of 16 human gastric adenocarcinoma samples, maintained as solid tumors in nude mice, were found to carry amplified c-myc genes. In two samples with a high degree of c-myc DNA amplification (15- to 30-fold), double minute chromosomes were observed in karyotype analysis. The level of c-myc RNA was markedly elevated in a rapidly growing and poorly differentiated tumor, whereas it was only slightly elevated in a slowly growing and more differentiated tumor.     

Enrichment and characterization of clonogenic epithelial cells from adult rat liver and initiation of epithelial cell strains

Summary A highly efficient method is described for obtaining prolifertive epithelial cells from adult rat livers for the reproducible establishment of liver epithelial cell strains. When cells were isolated from livers of 10-to 15-wk-old male Fischer 344 rats by a collagenase-perfusion method, collected by centrifugation at 50×g for 5 min, and cultured in Williams' medium E containing fetal bovine serum and dexamethasone, colonies of epithelial cells different in size and morphology from hepatocytes were obtained. Sequential perfusion with collagenase and dispase yielded numerous epithelial cell colonies. When isolated cells were fractionated by differential centrifugation, the great majority of hepatocytes were sedimented at 50 ×g for 1 min, whereas many non-hepatocytic cells remiined in the supernatant and could be sedimented by a second centrifugation at 50×g for 5 min. Culture of the two fractions revealed that almost all the epithelial cell colonies were derived from cells in the non-hepatocytic cell fraction. The epithelial cells were cytochemically negative for γ-glutamyl transpeptidase activity, whereas an increase in the activity was detected in hepatocytes with duration in culture. Ultrastructural characteristics of hepatocytes were not found in the cells of newly established cell strains. These results suggest that adult rat liver epithelial cells propagable in culture were derived from a cell type other than the hepatocyte.     

Sequence of heat-labile enterotoxin of Escherichia coli pathogenic for humans.

We determined the complete nucleotide sequence of the toxB gene (375 base pairs in length), which encodes the B subunit of heat-labile enterotoxin produced from Escherichia coli pathogenic for humans (hLT). The amino acid sequence of the B subunit of hLT was deduced from the nucleotide sequence. Consequently, it has become possible to study the homology between the B subunits of three similar toxins: hLT, LT produced from E. coli pathogenic for piglets (pLT), and cholera toxin (the latter two sequences have been reported by others). The three B subunits are all 103 amino acids in length. A comparison of the toxB gene and the eltB gene, which encodes the B subunit of pLT, showed a 98% homology at the nucleotide level and a 95% homology at the amino acid (of a precursor) level, indicating the possibility that the two genes share a common ancestor. With respect to the B-subunit sequences, the homologies between hLT and pLT, between hLT and cholera toxin, and between pLT and cholera toxin were 96, 81, and 79%, respectively. Several large common sequences are conserved by the three peptides. In contrast, no sequences are present in both pLT and cholera toxin but missing in hLT.     

Transformation of leukotriene D4 catalyzed by lysosomal cathepsin H of rat liver

Among several intracellular protease tested, cathepsin H transformed leukotriene D4 to E4 with a release of glycine in a stoichiometric quantity. Under the optimal conditions the rate of leukotriene D4 transformation by cathepsin H was about 3% of the hydrolysis rate of alpha-N-benzoyl-DL-arginine-2-naphthylamide which is commonly utilized as a very efficient substrate to test the peptidase activity of the enzyme. Leukotriene C4 was not transformed to leukotriene D4 by cathepsin H. Neither cathepsin B nor C was active with leukotrienes C4 and D4.     

Immunocytochemical localization of two peroxisomal enzymes of lipid beta-oxidation in specific granules of rat eosinophils

Peroxisomes contain a system for beta-oxidation of fatty acids which differs from the mitochondrial system and is associated with hydrogen peroxide formation. We show that two enzymes: enoyl-CoA hydratase and 3-ketoacyl-CoA thiolase of the peroxisomal system are present in specific granules of rat eosinophils. Both enzyme proteins were purified from rat liver and monospecific antibodies were raised in rabbits. Eosinophils from peripheral blood and tissue eosinophils from the wall of intestine, fixed by glutaraldehyde and embedded in Epon were investigated. The postembedding immunocytochemical procedure with protein A-gold technique was used. The gold particles representing the antigenic sites for both enzymes were present only in specific granules of eosinophils with no immune deposits in mitochondria, nucleus and the cytoplasm. Although gold particles were found over the entire domain of the granule, the electron dense paracrystalline inclusions contained more gold than the granule matrix. Control preparations incubated with nonspecific IgG and protein A-gold complex alone were negative. These findings indicate that in specific granules of eosinophils both peroxisomal and lysosomal enzymes share the same intracellular compartment. The peroxisomal lipid beta-oxidation in eosinophils may be involved in generation of hydrogen peroxide, which has a crucial role in killing of metazoon parasites.