Turnover of Several Glycolytic Enzymes in Rat Liver

Myelination and the trophic support of long axons

In addition to their role in providing myelin for rapid impulse propagation, the glia that ensheath long axons are required for the maintenance of normal axon transport and long-term survival. This presumably ancestral function seems to be independent of myelin membrane wrapping. Here, I propose that ensheathing glia provide trophic support to axons that are metabolically isolated, and that myelin itself might cause such isolation. This glial support of axonal integrity may be relevant for a number of neurological and psychiatric diseases.

Lysosomal sequestration of cytosolic enzymes and lysosomal thiol cathepsins

Cytoplasmic proteins are degraded with different half-lives in vivo. Large parts of proteins are believed to be degraded primarily in autophagic vacuoles-lysosomal system. However, the mechanism by which cell proteins are delivered to lysosomes and whether such a process might be selective for certain cell proteins are still unresolved. We examined the mechanism of autophagy with isolated autophagic vacuoles. Administration of leupeptin, a inhibitor of lysosomal thiol proteinases, induced the accumulation of numerous autophagic vacuoles in the liver. Highly purified preparation of autophagic vacuoles was isolated by Percoll density gradient equilibrium fractionation of crude lysosomal fractions. When cytosolic enzyme activities in autophagic vacuoles were measured, tyrosine aminotransferase and tryptophan oxygenase with short half-lives, and lactic dehydrogenase and aspartate aminotransferase with long half-lives were detected at similar ratios of enzymes in autophagic vacuoles/cytosol. During the time that cathepsin B plus L activities in autophagic vacuoles are inhibited by the injection of leupeptin, cytosolic enzymes are being accumulated in autophagic vacuoles suggesting that leupeptin blocks intralysosomal proteolysis, and that cytosolic enzymes are sequestered continuously into autophagosomes. Administration of glucocorticoid, which induces the synthesis of tyrosine aminotransferase, tryptophan oxygenase and cytosolic aspartate aminotransferase, selectively increased the sequestration of these enzymes to proportional degrees. Dietary manipulation and administration of insulin, which inhibit the formation of autophagic vacuoles, suppressed completely the accumulation of autophagic vacuoles in liver by administration of leupeptin. Results indicate that there is no selective uptake of cytosolic enzymes into autophagosome. When distribution of lysosomal cathepsin B and L in liver, which are inhibited strongly by leupeptin, was examined immunohistochemically, cathepsin L is found only in hepatocytes, but cathepsin B is localized in sinusoidal cells rather than in hepatocytes, suggesting that cathepsin L plays a most important role in intralysosomal proteolysis in hepatocytes.

Protein hydrophobicity and stability support the thermodynamic theory of protein degradation

Rat liver enzymes were used to study the relationship between their in vivo half-lives and their apparent hydrophobicity or their resistance to inactivation by mechanical shaking. The apparent hydrophobicity of these enzymes, measured as the percent of the protein recovered from an octyl-Sepharose column, is correlated with their known half-lives (r = 0.75, P less than 0.01). The presence of specific ligands which are known to increase compactness by impeding unfolding of proteins decreased the apparent hydrophobicity of fructose-1,6-bisphosphatase, glucose-6-phosphate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase. Resistance of enzymes to inactivation by mechanical shaking correlated well with their in vivo half-lives (r = 0.90, P less than 0.01). When the shaking experiments were done in the presence of substrates, fructose-1,6-bisphosphatase, glucose-6-phosphate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase and lactate dehydrogenase were protected from inactivation.

Intracellular protein degradation: basis of a self-regulating mechanism for the proteolysis of endogenous proteins

The intracellular basal proteolysis system, as distinct from the lysosomal system, is important in sustaining a high flux of proteins required for maintenance, growth and adaptability of cells. Its activity automatically fluctuates with changes in protein synthetic activity, but with a considerably slower response time, since the two processes are only indirectly or passively linked. Since as much as one-third of intracellular proteolysis in mammalian cells is directed as nascent proteins, the consequences are more fully discussed in relation to cell growth state. During rapid growth, cells have to accumulate more than double their original protein mass in order to achieve a 100% increase between divisions. The effects of reducing protein synthesis by inducing quiescence, serum step-down or cycloheximide treatment on intracellular proteolysis are considered, and the possibility that this leads to enhanced degradation of existing proteins has been explored. No substantial evidence was found to support this latter notion. The basal proteolysis system is seen as a constitutive, pervasive and broad-spectrumed collection of hydrolytic enzymes. It destroys proteins randomly, having no means of distinguishing young from old, aberrant from normal. The rate of demise of protein substrates depends on two factors, the ease of access of the hydrolytic enzymes to their peptide bonds, and the length of time that any species of protein remains at risk to this hydrolytic potential. While the former has long been recognized, the importance of the second factor in relation to the ability of proteins to become integrated in the living fabric of the cell is only beginning to be appreciated. The discussion also suggests elaborate regulatory mechanisms akin to those for protein synthesis would be unnecessary for protein degradation, especially if it can now be substantiated that substrate availability determines the turnover rates of proteins by a pervasive and relatively unlimited proteolytic system (Grisolía, 1964).

Crystallization and properties of cathepsin B from rat liver

Cathepsin B from rat liver was purified to apparent homogeneity by cell-fractionation, freezing and thawing, acetone treatment, gel filtration, DEAE-Sephadex and CM-Sephadex column chromatography, and was crystallized. The purified enzyme formed spindle-shaped crystals and its homogeneity was proved by disc gel electrophoresis in the presence of sodium dodecyl sulfate and by ultracentrifugal analysis. Its s20,w value was 2.8 S and its relative molecular mass was calculated to be 22,500 (+/- 900) by sedimentation equilibrium analysis. Crystalline cathepsin B was shown to consist of four isozymes with isoelectric points between pH 4.9 and 5.3, the main isozyme having an isoelectric point of pH 5.0. The enzyme was irreversibly inactivated by exposure to weak alkali. The pH optimum was 6.0 with alpha-N-benzoyl-DL-arginine-4-nitroanilide as substrate. Amino acid analysis showed that the enzyme contained hexosamine, glucosamine and galactosamine. Cathepsin B inactivated aldolase, glucokinase, apo-ornithine aminotransferase, and apo-cystathionase, but the rates of inactivation of glucokinase, apo-ornithine aminotransferase, and apocystathionase were lower than that of aldolase. Studies by polyacrylamide gel electrophoresis in the presence and absence of sodium dodecyl sulfate showed that cathepsin B degraded apo-ornithine aminotransferase to two polypeptide chains differing in relative molecular mass and electrophoretic mobility.

Turnover rates of hexokinase I, phosphofructokinase, pyruvate kinase and creatine kinase in slow-twitch soleus muscle and heart of the rabbit

Hexokinase I was purified from rabbit heart to a specific activity of 70 U/mg protein. The purified enzyme was electrophoretically homogeneous with an apparent molecular weight of 102,000. Purified immunoglobulins from sheep were used to titrate the percentage of hexokinase I in various tissues of the rabbit. Precipitating antibodies from sheep were also prepared against rabbit muscle MM-creatine kinase, phosphofructokinase and pyruvate kinase. Apparent turnover rates of these phosphotransferases and of hexokinase I were determined in rabbit heart and soleus muscle by means of the immunoprecipitation technique after single pulse labelling with [U-14Cl]leucine in vivo. Apparent half-lives of phosphofructokinase, pyruvate kinase and hexokinase I were 0.56 d, 0.73 d and 0.93 d in rabbit heart. In slow-twitch soleus muscle half-lives of phosphofructokinase, pyruvate kinase, hexokinase II and creatine kinase were 0.63 d, 0.72 d, 0.85 d and 0.82 d. The similarity of the rate constants of degradation of these enzymes is interpreted as an indication that different tissue concentrations result primarily from different rates of synthesis.

Acid inactivation of short-lived rat liver enzymes

The stabilities of nine rat liver cytosol enzymes were compared at a variety of pH values. The cytosol enzymes studied were (a) those with half-lives in vivo of 3 days or longer: lactate dehydrogenase, arginase, glyceraldehyde phosphate dehydrogenase and alanine aminotransferase, (b) those with half-lives in vivo shorter than 2 days; glucokinase, dihydroorotase, serine dehydratase and tyrosine aminotransferase and (c) catalase, which has an intermediate half-life of 2.5 days for the protein protion. All the enzymes were stable in vitro at neurtal and alkaline pH values. However, at acidic pH values (pH 4): the long-lived enzymes (a) were stable; the short-lived enzymes (b) were completely inactivated with one exception; and catalase was partially inactivated. Tyrosine aminotransferase was the exception in that it is a short-lived enzyme in vivo but stable under all conditions tested in vitro. The finding that long-lived enzymes are stable in an acid milieu and short-lived enzymes are generally unstable was only observed if certain ligands (NAD+, pyridoxal 5'-phosphate, Mn2+, amino acids) were added to the invitro system. Lysosomal extracts did not accelerate the rate of inactivation of any cytosol enzyme in acidic solutions. These results indicate that if degradation of intracellular enzymes occurs in lysosomes, acid inactivation and denaturation of enzymes may be the initial event in determining the functional half-lives of the enzymes in vivo.

The influence of insulin on various enzyme activities in human and rat hepatoma cells

1. Incubation of human and rat hepatoma cells with insulin (1 mU/10(6) cells) decreases their content of adenosine 3':5'-monophosphate by more than half after 1 h and by about a quarter after 4 h. 2. The activities of the ATP-metabolising enzymes, adenylate kinase and Mg2+-adenosine triphosphatase are significantly increased by insulin within 1 h and after 4 h. Activity of succinate dehydrogenase and lactic dehydrogenase showed no change at either time interval. 3. Insulin markedly stimulated glucose 6-phosphate dehydrogenase activity within 1 h but by 4 h the increase was less apparent. Glutamate dehydrogenase activity by contrast was not increased by 1 h but was elevated at 4 h.

On the application of affinity chromatography to turnover studies on the lactate dehydrogenase isoenzymes

1. The suitability of a combined application of the techniques of affinity chromatography, double labelling and gel electrophoresis in the determination of the turnover characteristics of the lactate dehydrogenase isoenzymes (L-lactate:NAD+ oxidoreductase EC 1.1.1.27) in rat tissues has been studied. 2. Affinity chromatography was established as affording the advantages of rapidity, high yield and purity to such studies, and the double-labelling procedure was modified to encompass the differential decay kinetics in the separate rat tissues. In addition, a convenient method for the resolution and separate collection of radioactively labelled isoenzymes has been described. 3. Using this methodology, comparative turnover values for the isoenzymes of lactate dehydrogenase and for total soluble protein have been determined. 4. The comparability of these results with other methodologies, and the advantages of this approach in facilitating broad comparative studies on turnover are discussed.