Degradation of phosphoenolpyruvate carboxykinase (guanosine triphosphate) in vivo and in vitro

Coordination of protein synthesis and degradation

The degree of coordination between protein synthesis and protein degradation in developing and mature cels is considered. Studies on specific enzyme and general protein turnover in developing liver and differentiating mammary gland are presented. In the mature liver mitochondrion average protein degradation rates are higher for outer membrane and intermembrane space proteins than for matrix and inner membrane proteins. Significant heterogeneity of protein degradation rates was observed only in the outer mitochondrial membrane. During postnatal development the rates of degradation of proteins in many liver cellular fractions are increased. In the mitochondrion only the average rates of degradation of proteins in the outer membrane and intermembrane space fractions increase during development. Evidence for hormonally regulated changes in both protein synthesis and degradation during mammary cell differentiation is given. The data indicate that a transitory decrease in protein degradation accompanies the increase in protein synthesis on hormonal stimulation of the tissue. The results from the two model systems are collated and used to formulate a phenomenological hypothesis of protein degradation and its integration with protein synthesis in steady-state and non-steady-state conditions.

Inactivation of cytosol enzymes by a liver membrane protein

Non-proteolytic inactivation reactions have been suggested to play an important role in determining the relative rates of enzyme degradation within cells. An inactivation factor is present at high specific activity in hepatocyte plasma membrane which inactivates cytosol enzymes at rates roughly proportional to their rates of degradation in vivo. This factor has been purified about 100-fold and has similar catalytic selectivity to the crude factor. The inactivation reaction is accelerated by disulphide compounds and can be partially reversed by thiols. Furthermore, inactivation of enzymes is accompanied by a loss of measurable thiols in the enzymes. From these experiments it is concluded that the inactivation factor carries out a disulphide attack on surface thiol groups in cytosol enzymes leading to the formation of mixed disulphides which have lost catalytic activity. The inactive enzymes may be substrates for lysosomal or non-lysosomal proteolysis.

Conservation from rat to human of cytosolic phosphoenolpyruvate carboxykinase and the control of its gene expression

Structural conservation of cytosolic phosphoenolpyruvate carboxykinase protein and mRNA sequence was found in all species examined from rodents to human. The mitochondrial isoenzyme, in all species tested, represents a distinct protein. Moreover, irrespective of the ratio of cytosolic to mitochondrial isoenzyme, cytosolic phosphoenolpyruvate carboxykinase activity in the human as in the rat is controlled at the level of gene expression and through the same multiple hormonal stimulation. This evolutionary conservation of the cytosolic phosphoenolpyruvate carboxykinase structure and mode of regulation supports the enzymes' physiological importance in mammals.

[Intracellular proteolysis]

This article is intended to give an overview of the most significant facts in the area of intracellular proteolysis. It begins with general considerations on the importance and nature of the intracellular proteolytic processes and examples are given of what takes place during both the extensive proteolysis and the limited cleavage of the cellular proteins. We have mentioned the intracellular proteases that have been identified and their established role since the knowledge of the proteases involved in important to understand the mechanisms of these processes.

Intracellular degradation of hemoglobin transferred into fibroblasts by fusion with red blood cells

Hamster fibroblast protein and rabbit hemoglobin were labelled by incubation of fibroblasts (BHK21) or reticulocytes with [3H]leucine. Alternatively, human or rabbit hemoglobin was labelled by carbamoylation of erythrocytes with K14CNO. The labelled hemoglobins were introduced into fibroblasts by virus-mediated fusion between the blood cells and fibroblasts. The hemoglobins became uniformly distributed throughout the cytoplasm. Degradation was assessed from release of acid-soluble radioactivity into the medium. Radioactivity from [14C]-carbamoylhemoglobin was released as carbamoylvaline and homocitrulline, and these compounds were not metabolized or reincorporated by the cells. Intermediate degradation products could not be detected. The degradation of hemoglobin followed first-order kinetics. The half-life of both carbamoylated and native rabbit hemoglobin in hamster fibroblasts was 28 h, and the half-life of carbamoylated human hemoglobin was about 150 h in fibroblasts from hamster (BHK21), mouse (Balb/3T3), and man (MRC 5), corresponding to that of the more stable endogenous proteins. Phenylhydrazine increased the intracellular degradation of carbamoylated human hemoglobin about 13 times, whereas the degradation of endogenous proteins was little affected. Hemoglobin was degraded in homogenates at 31% h-1 at pH 5 and 0.3% h-1 at pH 7.4. Phenylhydrazine increased these rates to 45% h-1 and 9.7% h-1, respectively. Growing hamster fibroblasts, which are brought into quiescence by serum deprivation or by high culture density, increase the degradation of endogenous protein and of hemoglobin in parallel.

Protein turnover in retina

Rabbit retinas were exposed in vitro to 0.5-h pulses of [3H]leucine or [14C]leucine. Some retinas were harvested promptly after labeling to measure synthesis. These were combined, in double-labeling experiments, with retinas that had been returned to unlabeled medium for a subsequent 1 h or 3.75 h to measure degradation. All of the proteins were solubilized, and separated according to size by gel electrophoresis. The gels were cut into 95 slices, and each slice was differentially counted. The amount of protein in the slice was estimated from the Coomassie blue staining, and its molecular weight from the distribution of molecular weight (MW) standards. Turnover rates of the various sizes of proteins were calculated from these data using certain well-defined assumptions. Retinal protein contained about 32 X 10(3) nmol of polypeptide per g, with a median MW of 27,000. Total synthesis was at the rate of 103 nmol/g of protein/h, with the most rapid synthesis in the 33,000--43,000 MW range, at 2 nmol/g/h for every 1000 increment in MW. Protein renewal averaged 0.52%/h, but varied directly (p < 0.0001) with MW, so that proteins of 10,000 MW were being renewed at about 0.1%/h and proteins of 140,000 MW at about 1.4%/h. Taken together, the measurements of fractional renewal and the measurements of degradation of the newly synthesized proteins demonstrated that each slice contained proteins with markedly different breakdown coefficients, and provided enough information to characterize the proteins in the slice in terms of a fast and slow subgroup. This analysis indicated that: breakdown coefficients varied much more than rates of synthesis and were therefore the prime determinant of the amount of each protein that was present; as MW increased, breakdown coefficients of the long-lived proteins increased (p < 0.0001), accounting in major part for the correlation between size and turnover; most staining bands were due to proteins with peculiarly long lifespans; the proteins with the slowest turnover of all appeared to be histones; there was an unusually rapid synthesis of a 138,000 MW polypeptide with a moderately short half-life (about 3 h).

Enzyme inactivation via disulphide-thiol exchange as catalysed by a rat liver membrane protein

1. The inactivation of cytosol enzymes by a rat liver membrane protein was studied with crude microsomal fraction, plasma membranes or a partially purified preparation of inactivation factor. 2. Complete inactivation of (125)I-labelled glucose 6-phosphate dehydrogenase (EC 1.1.1.49) by membranes did not result in any detectable change in molecular weight when the products were analysed by gradient polyacrylamide-gel electrophoresis. 3. Inactivation of radioactive enzyme was not accompanied by extensive binding to the membrane surface. The maximum extent of binding was 15% of the total enzyme labelled, and bound radioactivity was released only slowly, mainly as trichloroacetic acid-insoluble material. 4. Treatment of membranes with dithiothreitol destroyed the inactivation capacity, whereas the thiol-alkylating agent iodoacetamide had no effect. Partial restoration of the inactivation capacity of reduced membranes after exposure to air was prevented by membrane alkylation with iodoacetamide. 5. Modification of enzyme thiol groups during inactivation was determined by measuring a decrease in iodoacetamide-reactive groups in purified glucose 6-phosphate dehydrogenase. 6. Incubation of membrane-inactivated glucose 6-phosphate dehydrogenase with dithiothreitol resulted in a partial restoration of enzyme activity. 7. As a result of these experiments it is concluded that inactivation proceeds by a disulphide-thiol exchange mechanism. The proposal that this reaction could be involved in the initial step of enzyme degradation is discussed.

Selective denaturation of several yeast enzymes by free fatty acids

The denaturation of eight purified yeast enzymes, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, alcohol dehydrogenase, beta-fructosidase, hexokinase and glucose-6-phosphate isomerase, promoted under controlled conditions by the free fatty acids myristic and oleic, is selective. Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1 oxidoreductase, EC 1.1.1.49) is extremely sensitive to destabilization and was studied in greater detail. Results show that chain length and degree of unsaturation of fatty acids are important to their destabilizing effect, and that ligands of the enzyme can afford protection. The denaturation process results in more than one altered form. These results can be viewed in the perspective of the possibility that amphipathic substances, and in particular free fatty acids, may play a role for enzyme degradation in vivo, by initiating steps of selective denaturation.

Subcellular distribution of a factor inactivating tyrosine aminotransferase. Study of its mechanism and relationship to different forms of the enzyme

The subcellular distribution of a tyrosine aminotransferase inactivating factor in rat liver has been investigated. Most of its activity is associated with plasma membranes, with minor amounts in mitochondria and endoplasmatic reticulum. The factor is also found in kidney and inactivates the enzyme reversibly in presence of cysteine, most likely by modification of -SH groups. ATP counteracts this inactivation only, when crude enzyme extracts are inactivated by purified subcellular fractions or when the purified enzyme is inactivated in presence of liver or kidney cortex homogenates. The relationship of this inactivation to reported different forms of the enzyme has been investigated. Form I of three different forms, that can be obtained by hydroxyl-apatite chromatography, is readily inactivated, form III can be partly converted to form I by incubation in presence of purified plasma membranes. The relationship of these findings to a possible multistep mechanism in the turnover of the enzyme discussed.

Evidence for the autophagy of microinjected proteins in HeLA cells

Rhodamine-conjugated proteins were microinjected into living HeLa cells. Fluorescence microscopy was then employed to study their segregation from the cytoplasm into lysosomes. Results obtained in this way were verified when the corresponding unconjugated proteins were localized by autoradiographic, histological, and antibody-staining methods after their microinjection. Most injected proteins were segregated into cytoplasmic granular structures during their removal from cells. As evidence that these were autophagic vacuoles, they were found to contain no detectable acid phosphatase activity upon formation, after which they moved to the juxtanuclear position of lysosomes and appeared to fuse with them. The segregation of microinjected proteins exhibited a high degree of selectivity. The half-times of placement of individual exogenous proteins into cytoplasmic granules varied from 3 h to nearly 3 days, and one protein, hemoglobin, was never observed to enter them. Furthermore, endogenous HeLa proteins in a size fraction near 200,000 daltons were segregated much more rapidly than those in a fraction near 40,000 daltons. In these studies, rapid protein segregation appeared to take place by a mechanism of exclusion of the injected protein from numerous cytoplasmic domains.

The effect of insulin and glucocorticoids on the synthesis and degradation of phosphoenolpyruvate carboxykinase (GTP) in rat adipose tissue cultured in vitro

1. Epididymal adipose tissue from the rat was maintained in culture for periods of up to 96h. 2. After an initial decrease in protein synthesis during the first 24h of culture, the adipose tissue recovered its capacity to synthesize and accumulate proteins of a relatively large size. 3. The activity of phosphoenolpyruvate carboxykinase decreased in a parallel manner, but increased again after 24h of incubation of the tissue in culture, to a value twice that noted in the tissue in vivo. This increase in enzyme activity was due to an increase in its rate of synthesis. 4. Both insulin and dexamethasone (9alpha-fluoro-16alpha-methyl-11beta,17,-21-trihydroxypregna-1,4-diene-3,20-dione) inhibited phosphoenolpyruvate carboxykinase synthesis, but dexamethasone also decreased total protein synthesis. 5. The half-life of phosphoenolpyruvate carboxykinase in adipose tissue cultured in vitro was 5--7h and was not altered by insulin or dexamethasone. 6. It is concluded that both insulin and glucocroticoids lower the activity of phosphoenolpyruvate carboxykinase in rat adipose tissue by decreasing its rate of synthesis.

Degradation of abnormal proteins in HeLa cells

Canavanine, an arginine analog, is incorporated into HeLa cell protein when cells are incubated in the absence of arginine, and this incorporation can result in the production of nonfunctional enzymes or abnormal proteins. The cells degrade these abnormal proteins up to three times more rapidly than normal cell proteins. The capacity for selective degradation of abnormal proteins is not limited to HeLa cells since human fibroblasts also showed increased degradative rates following exposure to canavanine. In addition, enhanced degradation is not a peculiar property of canavanine incorporation since other amino acid analogs also promoted protein degradation. Thus, mammalian cells have the capacity to recognize and selectively degrade abnormal proteins.

Selective control of the degradation of normal and aberrant proteins in Reuber H35 hepatoma cells

1. Rates of degradation of normal and abnormal protein were measured in hepatoma cells after labelling first for 16h with [14C]leucine plus L-arginine and then for 3h with [3H]-leucine plus the arginine analogue, L-canavanine. 2. Over the first 2h of the degradation period, canavanine-containing proteins were degraded at approximately 5 times the average degradation rate of normal proteins. 3. Degradation of normal proteins was inhibited by about 30% by insulin, cycloheximide, puromycin, leupeptin, antipain and foetal calf serum, whereas these agents had a negligible effect on the breakdown of canavanine-containing proteins. 4. Other compounds inhibited degradation of both classes of protein to equal extents. 5. Combination experiments showed no additional inhibitory effects on the degradation of normal proteins over degradation measured in the presence of a single selective inhibitor. 6. In contrast with the results with a 16 h labelling period, the degradation of normal proteins labelled for only 3 h was not inhibited by insulin. 7. These results are explained by a model with two distinct pathways of protein turnover. The first of these pathways involves the formation of autophagic vacuoles and would be completely inhibited by each of the selective inhibitors. Normal and canavanine-containing proteins would be catabolized by this pathway at equal rates. We propose that degradation by a second pathway is not regulated by the agents tested, but by the inherent stability of each protein.

Inactivation of phosphoenolypyruvate carboxykinase (GTP) by liver extracts

1. The inactivation of phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) in liver extracts was catalysed by the microsomal fraction, and led to the enzyme becoming bound to the microsomal membranes. 2. Inactivation by microsomal fraction, typsin or heating at 48degreesC was accelerated by L-cystine, D-cystine and oxidized glutathione and decreased by dithiothreitol. 3. MnC1(2) and CoC1(2) protected the enzyme from inactivation by heat or microsomal fraction, but did not affect the inactivation caused by trypsin. 4. Several proteinase inhibitors had no effect on the microsomal inactivation reaction, suggesting that proteolysis was not involved. 5. It is argued that the initial step in the degradation of phosphoenolpyruvate carboxykinase (GTP) is an inactivation reaction, perhaps involving oxidized thiol compounds.

Studies of new intracellular proteases in various organs of rats. Participation of proteases in degradation of ornithine aminotransferase in vitro and in vivo

Homogenates of the muscle layer of rat small intestine irreversibly inactivated endogenous ornithine aminotransferase at 37 degrees C. Addition to the homogenate of coenzymes and the various keto-acids which act as substrate inhibited conversion of the holoenzyme to the apoenzyme and its subsequent degradation. Addition of protease inhibitors including soybean trypsin inhibitor, chymostatin and phenylmethylsulfonyl fluoride almost completely prevented inactivation of he enzyme. Immunological activity decreased during inactivation of the enzyme, but its rate of decrease was much slower than that of loss of enzyme activity. Antigen-antibody precipitates from homogenates containing inactivated enzyme, were separated by electrophoresis on sodium dodecylsulfate-polyacrylamide gels. In this way breakdown products of the enzyme were found, indicating that the enzyme in homogenates was inactivated by limited proteolysis. These results obtained in vitro support our previous suggestion (1975) of a stepwise mechanism for degradation of pyridoxal enzymes.

Properties of phosphoenolpyruvate carboxykinase (guanosine triphosphate) synthesized in hepatoma cells in the presence of amino acid analogues

Phosphoenolpyruvate carboxykinase (GTP) was induced by a combination of dibutyryl cyclic AMP, theophyline and dexamethasone in Reuber H35 hepatoma cells under conditions where an amino acid in the medium was replaced by an appropriate analogue. 2. With canavanine replacing arginine or with 5-fluorotryptophan or 6-fluorotryptophan replacing tryptophan the induced enzyme had a lower catalytic activity-relative to antibody reactivity. 3. These aberrant enzyme molecules were heat-labile in vitro. 4. Measurements of enzyme degradation in vivo indicated that the canavanine-containing enzyme and the 6-fluorotryptophan-containing enzyme were degraded more rapidly than the enzyme containing all natural amino acids.

Synthesis of phosphoenolpyruvate carboxykinase (guanosine triphosphate) by isolated liver polyribosomes

1. Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) was synthesized by postmitochondrial supernatants of rat liver in the presence of appropriate salts, an energy supply and [(3)H]leucine. Synthesis of enzyme released from polyribosomes was detected by immunoprecipitation with specific antibody followed by electrophoresis of the dissolved antibody-antigen precipitates on sodium dodecyl sulphate-polyacrylamide gels in the presence of a (14)C-labelled enzyme marker. 2. Enzyme synthesis in vitro occurs predominantly on free rather than bound polyribosomes. 3. Starved animals in which de-induction of phosphoenolpyruvate carboxykinase (GTP) had been initiated by re-feeding for 2h had a markedly decreased rate of enzyme synthesis, whether the measurements were made after injection of radioactive leucine into the intact animal or if synthesis was determined in vitro. 4. The low rate of enzyme synthesis by liver polyribosomes from re-fed animals was not due to the absence of soluble factors, nor could it be increased by the addition of cyclic AMP to the protein synthesis system. 5. Phosphoenolpyruvate carboxykinase (GTP) synthesis in vitro is diminished relative to total protein synthesis when the postmitochondrial supernatant is kept at 0 degrees C for several hours before measurement of protein synthesis. Since this effect is blocked by heparin, it is probably caused by selective ribonuclease attack on enzyme mRNA. 6. De-induction of phosphoenolpyruvate carboxykinase (GTP) is tentatively explained as being due to a transcriptional block in specific mRNA synthesis, followed by rapid degradation of existing message.

The relative stability of liver cytosol enzymes incubated in vitro

1. Relative rates of enzyme inactivation were measured in liver slices, homogenates and cytosol fractions as well as in the presence of trypsin and at acid pH. The enzymes chosen are all present in the cytosol fraction of rat liver, and have widely different degradation rate constants in vivo. 2. The inactivation rates of lactate dehydrogenase, fructose bisphosphate aldolase, glucose 6-phosphate dehydrogenase, glucokinase, phosphoenolpyruvate carboxykinase (GTP), l-serine dehydratase and thymidine kinase in liver preparations at neutral pH are in a similar order to the rate constants of degradation of these enzymes in the intact animal. 3. The two exceptions of this general correlation were tyrosine aminotransferase, which was stable in vitro but not in vivo, and glyceraldehyde phosphate dehydrogenase, which shows the reverse pattern. 4. These findings generally support the concept that the same factors are responsible for enzyme inactivation in vitro as occur in the intact tissue.