Effects of protein-degradation inhibitors on the inactivation of tyrosine aminotransferase, tryptophan oxygenase and benzopyrene hydroxylase in isolated rat hepatocytes

Nonselective autophagy of cytosolic enzymes by isolated rat hepatocytes

Seven cytosolic enzymes with varying half-lives (ornithine decarboxylase, 0.9 h; tyrosine aminotransferase, 3.1 h; tryptophan oxygenase, 3.3 h; serine dehydratase, 10.3 h; glucokinase, 12.7 h; lactate dehydrogenase, 17.0 h; aldolase, 17.4 h) were found to be autophagically sequestered at the same rate (3.5%/h) in isolated rat hepatocytes. Autophagy was measured as the accumulation of enzyme activity in the sedimentable organelles (mostly lysosomes) of electrodisrupted cells in the presence of the proteinase inhibitor leupeptin. Inhibitors of lysosomal fusion processes (vinblastine and asparagine) allowed accumulation of catalytically active enzyme (in prelysosomal vacuoles) even in the absence of proteolytic inhibition, showing that no inactivation step took place before lysosomal proteolysis. The completeness of protection by leupeptin indicates, furthermore, that a lysosomal cysteine proteinase is obligatorily required for the initial proteolytic attack upon autophagocytosed proteins. The experiments suggest that sequestration and degradation of normal cytosolic proteins by the autophagic-lysosomal pathway is a nonselective bulk process, and that nonautophagic mechanisms must be invoked to account for differential enzyme turnover.

Compounds that inhibit chymotrypsin and cell replication

Several compounds have been tested for their ability to inhibit bovine pancreatic alpha-chymotrypsin (Ki) and their ability to inhibit cell replication (IC50). There is good agreement over three orders of magnitude between the Ki and the IC50 values of these compounds. The data support the hypothesis that a cellular, chymotrypsin-like activity is necessary for cell replication.

The 1.8 A structure of the complex between chymostatin and Streptomyces griseus protease A. A model for serine protease catalytic tetrahedral intermediates

The naturally occurring serine protease inhibitor, chymostatin, forms a hemiacetal adduct with the catalytic Ser195 residue of Streptomyces griseus protease A. Restrained parameter least-squares refinement of this complex to 1.8 A resolution has produced an R index of 0 X 123 for the 11,755 observed reflections. The refined distance of the carbonyl carbon atom of the aldehyde to O gamma of Ser195 is 1 X 62 A. Both the R and S configurations of the hemiacetal occur in equal populations, with the end result resembling the expected configuration for a covalent tetrahedral product intermediate of a true substrate. This study strengthens the concept that serine proteases stabilize a covalent, tetrahedrally co-ordinated species and elaborates those features of the enzyme responsible for this effect. We propose that a major driving force for the hydrolysis of peptide bonds by serine proteases is the non-planar distortion of the scissile bond by the enzyme, which thereby lowers the activation energy barrier to hydrolysis by eliminating the resonance stabilization energy of the peptide bond.

Effect of puromycin on cartilage proteoglycan structure and capacity to bind hyaluronic acid

Rat chondrosarcoma chondrocytes were cultured in the presence of puromycin to induce premature termination of core protein precursor. The structure and function of intracellular and extracellular proteoglycans were assessed by molecular sieve chromatography and polyacrylamide gel electrophoresis. [3H]Serine incorporation was maximally inhibited by 3 X 10(-4) M puromycin but unaffected by 10(-5) M puromycin. Proteoglycans synthesized in the presence of puromycin exhibited increased monomer size due to increased chondroitin sulfate chain size, typical of proteoglycans synthesized in the presence of protein synthesis inhibitors, but no loss in ability to bind to hyaluronic acid; and no loss in core protein size was observed after treatment with chondroitinase. These data suggest that chondrocytes select only completed or nearly completed core protein molecules to process into proteoglycans.

Drug metabolism activities of isolated rat hepatocytes in monolayer culture

The levels of cytochrome P-450 in hepatocytes cultured as monolayers for 22 hrs in Dulbecco's modified Eagle medium supplemented with serum and insulin was reduced to approximately 40% of initial values of freshly isolated hepatocytes. In correspondence with this the activities of the cytochrome P-450 monooxygenases aryl hydrocarbon (benzo(a)pyrene) hydroxylase (AHH) and ethylmorphine (EM) N-demethylase were reduced to 40 and 22% of their initial activities, respectively. Modifying the culture medium through omission of cysteine and cystine, and adding dexamethazone and delta-amino levulinic acid, increased the content of cytochrome P-450 to 59% and EM N-demethylase to 46% of initial values, but was without effect on AHH activity. However, further modifications by adding high concentrations of asparagine and leucine increased AHH activity to 62% of initial values, but did not further enhance the total content of cytochrome P-450 or the EM N-demethylase activity. The activities of cytochrome P-450 reductase, flavin containing monooxygenase, epoxide hydrolase and glutathione S-transferase decreased less (to about 70-80% of initial values) than cytochrome P-450 associated monooxygenase activities, whereas UDP-glucuronyl transferase decreased to about 50% of initial values. In contrast to what was observed regarding cytochrome P-450 and associated monooxygenase activities, modification of the incubation conditions did not affect the non-cytochrome P-450 enzymatic activities.

Tyrosine aminotransferase activity of frog (Rana esculenta) liver--II. Comparative aspects of intracellular distribution

1. A subcellular fractionation procedure for frog liver is reported and validated by the distribution pattern of several marker enzymes, also in comparison with rat liver. 2. The subcellular distribution of tyrosine aminotransferase was investigated in frog liver as compared to rat liver: a different distribution of the enzyme was observed, being the activity mostly recovered in mitochondrial and cytosolic compartments. 3. Results indicate that mitochondrial tyrosine aminotransferase of both frog and rat liver is a matrix enzyme, even if differences are observed concerning its release from the organelles upon detergent treatment.