A method for the analysis of protein turnover characteristics. Indirect estimation of rates of protein degradation

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.

Regulation of intracellular casein degradation by secreted milk proteins

Intracellular degradation of newly synthesised casein was measured by a pulse-chase method in freshly prepared goat mammary explants. After incubation in medium containing L-[5-3H]proline, explants were washed and cultured again in unlabelled medium containing 5 mM proline; at intervals up to 24 h the amount of radiolabel incorporated in casein was measured. Tissue was obtained in week 33 of lactation after goats had been milked incompletely in one gland (the test gland) for 24 weeks; the contra-lateral (control) gland was milked normally. In explants from the control gland, casein was not degraded during or after secretion: L-[5-3H]proline incorporated in casein increased to a maximum value which was maintained through the chase period. For four out of five goats, explants from the test gland showed a decrease in total [3H]casein radiolabel at 0-4 h of the chase, indicating that a proportion of casein was degraded during secretion. Intracellular casein degradation was also observed when control gland explants were cultured in chase medium containing a goat whey fraction known to inhibit casein production and milk secretion (Wilde, C.J. et al., (1987) Biochem. J. 242, 285-288). This suggests that the greater volume of residual milk left by incomplete milking reduced secretory efficiency, rendering casein susceptible to intracellular degradation, and that this occurred through the action of a secreted milk constituent, which acts as a chemical feedback inhibitor of milk secretion.

Degradation of newly-synthesised casein in mammary explants from pregnant and lactating goats

Casein degradation was measured by a pulse-chase method in explants from freshly-prepared from goat mammary tissue. An initial increase in L-[5-3H]proline radioactivity incorporated in casein during the chase period was followed in some conditions by a decrease resulting from intracellular degradation of the major casein polypeptides. Differences in the proportion of casein degraded in explants cultured with or without prolactin suggested that the process was under hormonal control. Greater degradation of casein by explants obtained in late pregnancy and early lactation suggested that the process may affect net casein production in vivo.

Comparison of collagen gels and mammary extracellular matrix as substrata for study of terminal differentiation in rabbit mammary epithelial cells

Mammary epithelial cells were prepared by collagenase digestion of tissue from mid-pregnant rabbits and cultured for up to 6 days on either collagen gels or an extracellular matrix prepared from the same tissue. The behaviour of the cells in serum-supplemented medium containing combinations of insulin, prolactin, hydrocortisone, estradiol and progesterone were monitored by measuring rates of casein synthesis, lactose synthesis, DNA synthesis and protein degradation. After 6 days, epithelial cells on floating collagen gels showed substantial increases in casein synthesis and DNA synthesis over freshly-prepared cells, following a decline during the first 3 days when the collagen gels are contracting. The optimum hormone combination for casein synthesis was insulin + prolactin + hydrocortisone, whereas for optimum DNA synthesis the additional presence of estradiol and progesterone was required. Cells on extracellular matrix showed increased rates of both casein synthesis and DNA synthesis by day 6 in the presence of insulin + prolactin + hydrocortisone, with additional estradiol + progesterone having an inhibitory effect. Whereas on day 2 rates of intracellular protein degradation were generally lower in cells on extracellular matrix, by day 6 rates of protein degradation were lowest in cells cultured on collagen gels with insulin + prolactin + hydrocortisone. In all cases, rates of lactose synthesis fell to low levels as the culture proceeded. Pulse-chase labelling of freshly-prepared cells with [32P]orthophosphate in medium containing serum and insulin + prolactin + hydrocortisone demonstrated that newly-synthesized casein was degraded during its passage through the epithelial cell. The influences of the collagen gels and extracellular matrix and of the hormone combinations on epithelial cell differentiation and secretory activity are discussed.

Measurement of steady state protein degradation in cultured human muscle cells

Double label techniques for measurement of protein turnover in cultured cells are described. In the isotope withdrawal method protein in cultured muscle is labeled with two isotopes of the same amino acid for 24 to 100 h, followed by exposure to fresh medium containing one isotope only at the same specific activity for an additional 24 to 48 h. In the isotope addition method the order of addition of single and double-labeled media is reversed. After incubation the ratio of the two isotopes in the cell protein is a function of the incubation time and the degradation rate constant KD; KD can readily be calculated using a graphical or iterative method. In mixed cultures of human muscle with initial incubation ranging 24 to 159 h, the KD's obtained from various incubation times were similar. Both the isotope withdrawal and the isotope addition methods gave a KD value of 0.018 h-1 similar to values obtained by two different single isotope methods which monitor the appearance of free isotope in the medium of previously labeled cells. There were no differences of KD values obtained in cultures of muscle from normal patients and those with denervation, inflammatory myopathies, or nonspecific myopathic biopsy changes. When proteins were separated by gel electrophoresis, those of molecular weight greater than 60,000 had higher average KD values as compared to lower molecular weight proteins. The double isotope labeling method has the advantage of being easily applied to cultures with small numbers of cells and is potentially useful in obtaining the degradation rates of individual cellular proteins. The major disadvantages are (1) in their present form the methods can be used only in steady state cultures and (2) they require rather long (24 h) labeling times.

Extensive destruction of newly synthesized casein in mammary explants in organ culture

1. Explants of mammary glands of mid-pregnant rabbits that had been cultured for 18h in the presence of insulin, prolactin and cortisol were incubated at 37 degrees C for 2h in Medium 199 containing l-[4,5-(3)H]leucine. After a wash procedure at 4 degrees C, explants were re-incubated at 37 degrees C in fresh medium and the radioactivity of casein polypeptides isolated by isoelectric focusing (at pH 4.6) was followed with time. Casein radioactivity rose during the first hour of re-incubation, but fell markedly during the subsequent hour. 2. Loss of radioactivity represented casein degradation, since less than 10% of newly synthesized casein was found in the incubation medium. 3. Such a loss of radioactivity was not due solely to hydrolysis of signal peptides, since similar results were obtained when l-[5-(3)H]proline, which is not part of casein signal peptides, was the radiolabelled precursor. 4. A dual-isotope experiment using l-[U-(14)C]proline and N-[(3)H]acetyl-d-mannosamine gave similar profiles of radioactivity loss from isoelectrically focused casein, indicating that degradation of mature casein was occurring. 5. Analysis of total pellet and particle-free-supernatant fractions prepared by centrifugation of explant homogenates at 115000g(av.) for 1h did not show loss of radioactivity on re-incubation. Total pellet-protein radioactivity remained constant, whereas total soluble-protein radioactivity increased during the 2h re-incubation period. 6. Radioactivity in a specific particle-free-supernatant polypeptide, the subunit of fatty acid synthetase, mimicked that of the total soluble protein. 7. Addition of cycloheximide (20mug/ml) during the re-incubation period completely blocked the incorporation of radioactivity from l-[5-(3)H]proline into casein and the subsequent fall, indicating that observations were being made on newly synthesized casein. 8. Addition of chloroquine (50mum) did not prevent the increase in radioactivity from l-[5-(3)H]proline into casein during the first hour of re-incubation, but did prevent the loss of radioactivity in the second hour. 9. The intracellular degradation of a newly synthesized milk protein is discussed in relation to the known intracellular degradation of other secretory polypeptides.

Turnover of mouse intestinal brush border membrane proteins and enzymes in organ culture. A direct evaluation from studies on the evolution of enzyme activities during the culture

The turnover of mouse intestinal brush border membrane enzymes has been studied by kinetic analysis of the evolution of enzyme activities during organ culture. By comparing the results obtained in these studies with the predictions from a mathematical model of enzyme synthesis and degradation in organ cultures, it has been possible to reach the following conclusions: (1) There is no degradation of brush border membrane enzymes during culture and the rate of synthesis of each enzyme is directly measurable from the kinetics of total enzyme accumulation (tissue + media). (2) Brush border membrane enzymes are released in culture media by two complementary processes. The first one involves a differential solubilization of enzymes but its exact nature cannot be exactly stated. The second one involves a microvesiculation of brush border membranes, the importance of which in vivo is seen in the possible conciliation between urinary membrane synthesis and heterogeneous turnover of membrane components.

Protein degradation during terminal cytodifferentiation. Studies on mammary gland in organ culture

1. In mammary gland explants subjected to experimental manipulation, average rates (during 24 h periods) of degradation of fatty acid synthase, casein and cytosol-fraction proteins were measured by a double-isotope method. Rates of degradation of fatty acid synthase were also computed from measurements of changing enzyme amount and rate of synthesis. 2. During the period of most rapid enzyme accumulation there is a transient decrease in the computed rate of degradation of fatty acid synthase. Removal of hormones produces a rapid increase in the computed rate of degradation of the enzyme. 3. The average rate of degradation of fatty acid synthase measured by the double-isotope method is low in the presence of hormones, and increases on hormone removal. This increase in degradation rate is inhibited by adrenaline and further blocked by insulin. NH4Cl (10 mM) also partially inhibits the increase in protein degradation on hormone removal. 4. The pattern of changes in the average rate of degradation of cytosol-fraction proteins is similar to that for fatty acid synthase alone. There is no relationship between subunit molecular weight and rate of degradation under all experimental conditions. 5. Isotope ratios for resolved cytosol protein mixtures are transformed logarithmically to make the standard deviations an estimate of heterogeneity of degradation rates. By this analysis, in some conditions there appears to be significant measureable heterogeneity of degradation rates. 6. Little degradation of casein is measured in the presence of hormones, but a marked increase in the rate of degradation can be measured when hormones are removed. Whereas at 24-48h NH4Cl (10 mM) has little effect on this enhanced rate of degradation, at 48-72h it causes a large decrease in degradation rate. 7. Results are discussed in terms of a two-component degradation system in mammary gland explants.

Protein degradation in rat liver during post-natal development

Protein degradation rates for liver subcellular and submitochondrial fractions from neonatal (8-day), weanling (25-day) and adult rats were estimated by the double-isotope method with NaH14CO3 and [3H] arginine as the radiolabelled precursors [Dice, Walker, Byrne & Cardiel (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 2093-2097]. Decreased protein degradation rates were found during post-natal development for homogenate, nuclear, mitochondrial, lysosomal and microsomal proteins. A decrease in degradation rates for the immunoisolated subunits of monoamine oxidase and pyruvate dehydrogenase was also observed in neonatal and weanling rats respectively. The results suggest coordinate degradation of the subunits of the multi-subunit enzyme pyruvate dehydrogenase. Pyruvate dehydrogenase has a faster rate of degradation in adult rat liver than does cytochrome oxidase. Data analysis suggests heterogeneity of protein degradation rates in the mitochondrial outer membrane and intermembrane space fractions at each developmental stage but not in the mitochondrial inner membrane or matrix fractions. Results obtained for protein degradation rates in adult rat liver by the method of Burgess, Walker & Mayer [(1978) Biochem. J. 176, 919-926] in general confirmed the results obtained for the adult rat liver by the above method. No evidence of a subunit-size relationship for protein degradation was found for proteins in any subcellular or submitochondrial fraction.

Lipogenic enzymes in rat maternal adipose tissue in the perinatal period

1. The specific activities of fatty acid synthetase, acetyl-CoA carboxylase and pyruvate dehydrogenase were measured in rat adipose-tissue extracts in pregnancy and lactation. Fatty acid synthetase specific activity correlates very closely with the rate of fatty acid synthesis, the enzyme specific activity decreasing after mid-pregnancy in a manner very similar to the rate of fatty acid synthesis. Acetyl-CoA carboxylase specific activity also decreases dramatically after mid-pregnancy. Initial pyruvate dehydrogenase specific activity shows a decrease between 2 days pre partum and 2 days post partum, but total enzyme activity shows no significant change in the same period. 2. Immunotitrations of fatty acid synthetase and pyruvate dehydrogenase activities were carried out; the titrations showed that the change in the fatty acid synthetase activity is due to a change in the enzyme amount; the amount of pyruvate dyhydrogenase does not change. Therefore the decrease in fatty acid biosynthesis in subcutaneous and parametrial adipose tissue in late pregnancy and early lactation is associated with a decrease in the amount of at least one of the enzymes involved in fatty acid biosynthesis. The correlation of these events with known hormonal changes is discussed.

A method for the estimation of esterase synthesis and degradation and its application to evaluate the influence of insulin and glucagon

The irreversible reaction between liver esterases and the active-site-directed inhibitor bis(4-nitrophenyl)phosphate can be used in vivo both for the estimation of the esterase contents and for the measurement of the esterase degradation rates. A method based on this reaction is described which allows the simultaneous estimation of the rate constants of degradation and synthesis of esterases during a period of change in protein concentration. Rat liver was found to contain about 1 mg of organophosphate-binding esterases per g of fresh tissue while the microsomal fraction contains about 30 mg of esterases per g of microsomal protein. Esterase degradation and de novo synthesis were shown to remain in equilibrium for a period of at least five days following the injection of 10 mg bis(4-nitro-[14C]phenyl)phosphate per kg. The decrease of the relative amount of labeled esterases with time was found to follow first-order kinetics yielding an average esterase degrading constant of 0.0165 h-1 which corresponds to a half-life of 42 h. These data were confirmed by an independent experiment using one of the standard procedures for the estimation of degradation rates: [14C]leucine was incorporated and one of the esterases was subsequently isolated by immuno-precipitation. Using isoelectric focussing and dodecyl sulfate electrophoretic methods, the various esterase isoenzymes appeared to have very similar, if not identical turnover rates. This method for the estimation of the turnover characteristics was applied to evaluate hormone effects on liver esterases. The time course of the contents and the turnover of liver esterases was measured under the influence of glucagon treatment in diabetic rats and under the influence of high doses of insulin. The esterase content decreased faster than the average content of microsomal protein under the influence of glucagon. The reverse effect was observed with insulin-treated rats. Both insulin and glucagon apparently reduced the intracellular esterase turnover in rat liver. Kinetic analysis of the results revealed that insulin mainly lowered the esterase degradation rate, though the rate of esterase synthesis might also have been restricted. In the glucagon-treated rats the de novo synthesis of esterases was strongly reduced.

Casein turnover in rabbit mammary explants in organ culture

1. Explants of mammary gland from mid-pregnant rabbits were cultured in medium 199 containing insulin, prolactin and cortisol, and specific anti-casein immunoglobulin G was used to measure the amount, rate of synthesis and rate of degradation of casein in the explants in the presence of hormones and after removal of hormones from previously stimulated tissue. 2. The amount of casein in particle-free supernatants prepared from mammary explants was measured by ;rocket' immunoelectrophoresis. 3. The rate of incorporation of l-[4,5-(3)H]leucine into casein was measured after isolation of the casein by immunoadsorbent chromatography and polyacrylamide-gel electrophoresis in the presence of urea and sodium dodecyl sulphate. 4. Casein accumulates in mammary explants in the presence of insulin, prolactin and cortisol, but not in the absence of hormones. Removal of hormones after 24h in culture results in a decrease in the rate of accumulation of casein in the explants. 5. Casein-synthetic rate increases in mammary explants in the presence of insulin, prolactin and cortisol, but not in the absence of hormones. Removal of hormones after 24h in culture results in continued casein synthesis at approx. 30% of the rate in the presence of hormones. The synthetic rate does not decrease to values observed in explants cultured throughout in the absence of hormones. 6. Casein is not degraded in mammary explants during a phase of rapid casein accumulation (36-72h) in the presence of hormones. Furthermore casein is not degraded when hormones are removed from the tissue after between 36 and 72h in culture. 7. Casein is glycosylated in mammary explants; the extent of glycosylation parallels the rate of synthesis. The glycosylated protein is rapidly secreted from the tissue. 8. The results are consistent with the notion that after hormonal stimulation mammary explants from mid-pregnant rabbits synthesize, glycosylate and rapidly secrete casein. Removal of hormones decreases the synthetic rate of casein, but does not cause the accumulation of a pool of degradable casein in the lobuloalveolar cells.