Analysis by flow cytometry of rat hepatocytes from different acinar zones

Many functional, morphological and biochemical differences among hepatocytes from different acinar zones have been described. Therefore, it will facilitate studies on liver metabolism rapid, non-destructive procedures to isolate hepatocytes from these zones. Flow cytometry is a new powerful tool which, however, has not been used thus far to accomplish the separation of hepatocytes from different acinar zones. We describe here various cytometric parameters which characterize hepatocyte populations, separated by isopycnic centrifugation in Percoll gradients. The intraacinar origin of the different hepatocytes was assessed by enzymatic and morphological measurements.     

Spermidine and spermine stimulate the transport of the precursor of ornithine carbamoyltransferase into rat liver mitochondria

We have examined the effect of low molecular weight components of the transport mixture generally used for the import of rat liver pre-ornithine carbamoyltransferase by isolated rat liver mitochondria. These studies revealed that spermidine and spermine, at physiological concentrations, stimulate the transport of the precursor of ornithine carbamoyltransferase into mitochondria. This stimulatory effect of spermidine and spermine is concentration-dependent and is completely inhibited at higher than physiological concentrations (20 mM for spermidine and 4 mM for spermine). Magnesium ions, which also have a stimulatory effect, inhibit the stimulatory effect of spermidine.     

Cloning and sequencing of the peroxisomal amine oxidase gene from Hansenula polymorpha

We have cloned the AMO gene, encoding the microbody matrix enzyme amine oxidase (EC 1.4.3.6) from the yeast Hansenula polymorpha. The gene was isolated by differential screening of a cDNA library, immunoselection, and subsequent screening of a H. polymorpha genomic library. The nucleotide sequence of a 3.6 kilobase stretch of DNA containing the amine oxidase (AMO) gene was determined. The AMO gene contains an open reading frame of 692 amino acids, with a relative molecular mass of 77,435. The 5' and 3' ends of the gene were mapped and show that the transcribed region measures 2134 nucleotides. The derived amino-acid sequence was confirmed by sequencing an internal proteolytic fragment of the purified protein. Amine oxidase contains the tripeptide sequence Ser-Arg-Leu, located 9 residues from the carboxy terminus, which may represent the topogenic signal for protein import into microbodies.     

Increased susceptibility of carbamylated glutamate dehydrogenase to proteolysis

Glutamate dehydrogenase is very susceptible to carbamylation which results in loss of activity. The effect of a number of proteolytic enzymes (pronase, trypsin and chymotrypsin) on native and carbamylated glutamate dehydrogenase was tested. In all cases, the carbamylated enzyme was at least twice as susceptible to proteolysis as the native enzyme. Antibodies were prepared against glutamate dehydrogenase and carbamylated glutamate dehydrogenase; the carbamylated enzyme was antigenically indistinguishable from the native enzyme. Preliminary experiments indicate that the carbamylated glutamate dehydrogenase is taken up by ascites tumor cells while glutamate dehydrogenase is not. It seems possible that the effects described can be extrapolated to degradation by lysosomes and to other covalently modified enzymes.     

Evidence pointing to the main role of lysosomes in mitochondrial proteolysis at neutral pH

Recombination experiments using radioactive mitochondria and mitoplasts, and nonradioactive lysosomes or digitonin-soluble fraction of mitochondria, show equal rates of proteolysis and of inactivation of carbamyl phosphate synthetase; the amount of lysosomal protein was equal in both cases on the basis of N-acetyl-beta-glucosaminidase activity. Therefore, lysosomes seem to be responsible for all the proteolytic activity exhibited by the digitonin soluble fraction of mitochondrial preparations. Since this fraction contains ca. 90% of the proteolytic activity present in mitochondrial preparations, most of the proteolysis can be attributed to lysosomal contamination. These findings and stability characteristics "in vitro" and "in vivo" of some matrix enzymes are presented and discussed in relation to protein turnover.     

Dissociation of intracellular lysosomal rupture from the cell death caused by silica

The relationship between intracellular lysosomal rupture and cell death caused by silica was studied in P388d(1) macrophages. After 3 h of exposure to 150 μg silica in medium containing 1.8 mM Ca(2+), 60 percent of the cells were unable to exclude trypan blue. In the absence of extracellular Ca(2+), however, all of the cells remained viable. Phagocytosis of silica particles occurred to the same extent in the presence or absence of Ca(2+). The percentage of P388D(1) cells killed by silica depended on the dose and the concentration of Ca(2+) in the medium. Intracellular lyosomal rupture after exposure to silica was measured by acridine orange fluorescence or histochemical assay of horseradish peroxidase. With either assay, 60 percent of the cells exposed to 150 μg silica for 3 h in the presence of Ca(2+) showed intracellular lysosomal rupture, was not associated with measureable degradation of total DNA, RNA, protein, or phospholipids or accelerated turnover of exogenous horseradish peroxidase. Pretreatment with promethazine (20 μg/ml) protected 80 percent of P388D(1) macrophages against silica toxicity although lysosomal rupture occurred in 60-70 percent of the cells. Intracellular lysosomal rupture was prevented in 80 percent of the cells by pretreatment with indomethacin (5 x 10(-5)M), yet 40-50 percent of the cells died after 3 h of exposure to 150 μg silica in 1.8 mM extracellular Ca(2+). The calcium ionophore A23187 also caused intracellular lysosomal rupture in 90-98 percent of the cells treated for 1 h in either the presence or absence of extracellular Ca(2+). With the addition of 1.8 mM Ca(2+), 80 percent of the cells was killed after 3 h, whereas all of the cells remained viable in the absence of Ca(2+). These experiments suggest that intracellular lysosomal rupture is not causally related to the cell death cause by silica or {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187. Cell death is dependent on extracellular Ca(2+) and may be mediated by an influx of these ions across the plasma membrane permeability barrier damaged directly by exposure to these toxins.     

Mechanism of carbamoyl-phosphate synthetase. Binding of ATP by the rat-liver mitochondrial enzyme.

This paper demonstrates, by pulse-chase techniques, the binding to rat liver mitochondrial carbamoyl phosphate synthetase of the ATP molecule (ATPB) which transfers its gamma-phosphoryl group to carbamoyl phosphate. This bound APTB can react with NH3, HCO-3 and ATP (see below) to produce carbamoyl phosphate before it exchanges with free ATP. Mg2+ and N-acetylglutamate, but not NH3 or HCO-3, are required for this binding; the amount bound depends on the concentration of ATP (Kapp = 10--30 microns ATP) and the amount of enzyme. At saturation at least one ATPB molecule binds per enzyme dimer. Binding of ATPB follows a slow exponential time course (t1/2 8--16 s, 22 degrees C), independent of ATP concentration and little affected by NH3, NCO-3 or by incubation of the enzyme with unlabelled ATP prior to the pulse of [gamma-32P]ATP. Formation of carbamoyl phosphate from traces of NH3 and HCO-3 when the enzyme is incubated with ATP follows the kinetics expected if it were generated from the bound ATPB, indicating that the latter is a precursor of carbamoyl phosphate ('Cbm-P precursor') in the normal enzyme reaction. This indicates that the site for ATPB is usually inaccessible to ATP in solution but becomes accessible when the enzyme undergoes a periodical conformational change. Bound ATP becomes Cbm-P precursor when the enzyme reverts to the inaccessible conformation. Pulse-chase experiments in the absence of NH3 and HCO-3 (less than 0.2 mM) also demonstrate binding of ATPA (the molecule which yields Pi in the normal enzyme reaction), as shown by a 'burst' in 32Pi production. Therefore, (in accordance with our previous findings) both ATPA and ATPB can bind simultaneously to the enzyme and react with NH3 and HCO-3 in the chase solution before they can exchange with free ATP. However, at low ATP concentration (18 micron) in the pulse incubation, only ATPB binds since ATP is required in the chase (see above). Despite the presence of two ATP binding sites, the bifunctional inhibitor adenosine(5')pentaphospho(5')adenosine(Ap5A) fails to inhibit the enzyme significantly. A more detailed modification of the scheme previously published [Rubio, V. & Grisolia, S. (1977) Biochemistry, 16, 321--329] is proposed; it is suggested that ATPB gains access to the active centre when the products leave the enzyme and the active centre is in an accessible configuration. The transformation from accessible to inaccessible configuration appears to be part of the normal enzyme reaction and may represent to conformational change postulated by others from steady-state kinetics. The properties of the intermediates also indicate that hydrolysis of ATPA must be largely responsible for the HCO-3-dependent ATPase activity of the enzyme. The lack of inhibition of the enzyme by Ap5A indicates substantial differences between the Escherichia coli and the rat liver synthetase.