General characteristics of protein degradation in diabetes and starvation

Asialoorosomucoid degradation by normal and diabetic rat hepatocytes

Using high concentrations of extracellular [3H]asialoorosomucoid we obtained the steady-state level of [3H]asialoorosomucoid endocytosis by isolated hepatocytes from normal and streptozotocin diabetic rats. At the steady-state of the overall reaction, the intracellular amount of [3H]asialoorosomucoid did not change with time, the apparent overall rate of [3H]asialoorosomucoid degradation was close to that of [3H]asialoorosomucoid internalization, in both normal and diabetic rat hepatocytes. Although in diabetic cells the intracellular amount of [3H]asialoorosomucoid was about three-times lower than in normal cells, the same fraction of intracellular asialoorosomucoid was degraded per time interval by both normal and diabetic cells. The apparent first-order rate constant of steady-state degradation was about 0.011 min-1 for both normal and diabetic cells. In diabetic rat hepatocytes, the decrease of the clearance of serum asialoglycoproteins was directly correlated to the variation of cell surface receptor number, without any modification of internalization and degradation steps.

The ubiquitin-mediated proteolytic pathway and mechanisms of energy-dependent intracellular protein degradation

In this review we briefly describe the lysosomal system, consider the evidence for multiplicity of protein degradation pathways in vivo, discuss in detail the ubiquitin-mediated pathway of intracellular ATP-dependent protein degradation, and also the possible significance of ubiquitin-histone conjugates in chromatin. For detailed discussions of the various characteristics and physiological roles of intracellular protein breakdown, the reader is referred to earlier reviews [1-7] and reports of recent symposia [8-10]. Information on the ubiquitin system prior to 1981 was described in an earlier review [11]. Hershko has briefly reviewed more recent information [12].

Erythrocyte-mediated microinjection, a technique to study protein degradation in muscle cells

The technique of erythrocyte-mediated microinjection has been successfully adapted for use with cultured muscle cells. Erythrocytes were fused with primary chick myotube cultures with poly(ethylene glycol), and fluorescent antibodies to haemoglobin demonstrated that this protein was injected into the sarcoplasm of myotubes. The microinjection treatment did not significantly alter protein metabolism in the muscle cells as monitored by rates of synthesis and degradation of muscle proteins. 125I-labelled ribonuclease A and bovine serum albumin were degraded with the expected exponential decay kinetics after microinjection into muscle cells, and the half-life of ribonuclease A (40 h) was approximately twice that of bovine serum albumin (17 h). The degradation of ribonuclease A in the muscle cells was enhanced 1.6-fold in the absence of horse serum and chick-embryo extract, whereas the degradation of bovine serum albumin was not altered during deprivation. These results are characteristic of the breakdown of microinjected ribonuclease A and bovine serum albumin in other cell types. Therefore, our experiments indicate the erythrocyte-mediated microinjection is a valid technique to study protein degradation in primary chick muscle cultures.

The molecular basis of the selectivity of protein degradation in stressed senescent barley (Hordeum vulgare cv. Proctor) leaves

The molecular basis for the selectivity of protein degradation has been examined in osmotically stressed and senescent barley leaves. Relatively weak correlations between the in-vivo rate of loss of enzyme activity, and the charge and molecular weight of the enzymes ahve been detected. We interpret these correlations as supporting the view that the selectivity of enzyme degradation is the result of the physical properties of the enzymes being degraded. The weakness of the correlates is taken to mean that a number of properties which contribute to the selectivity are independent of one another. Under in-vitro conditions (autolysis at 0° C), the loss of enzyme activity was weakly correlated with the charge of the enzymes. However, there was a general similarity between the in-vivo pattern of loss of enzyme activity and the in-vitro patterns under a number of conditions. Furthermore double-isotope experiments demonstrated that the in-vivo degradation of soluble protein was reflected by in-vitro degradation under a number of conditions. Consequently we conclude that the selectivity of protein degradation is largely independent of the nature of the proteolytic system.

Starvation and hypothyroidism exert an overlapping influence on rat hepatic messenger RNA activity profiles

To assess the effect of starvation and to explore the potential interrelationship of starvation and thyroid status at the pretranslational level, we have analyzed by two-dimensional gel electrophoresis, the hepatic translational products of starved and fed euthyroid and hypothyroid rats. 5 d of starvation resulted in a statistically significant change in 27 of 240 products visualized, whereas hypothyroidism caused a change in 20, both in comparison with the fed euthyroid state. Of considerable interest was that 68% of all changing messenger (m)RNA sequences were common to the hypothyroid and starved groups and showed the same directional shift. Further, both starvation and hypothyroidism yielded comparable decreases in total hepatic cytoplasmic RNA content. Although it has been well established that the level of circulating triiodothyronine (T3) and the level of hepatic nuclear receptors fall in starvation, this reduction cannot account for the observed decrease of total hepatic RNA nor for all of the alterations in the concentrations of specific mRNA sequences. Thus, administration of T3 to starved animals in a dose designed to occupy all nuclear T3 receptors fails to prevent the fall in total RNA and the majority of starvation-induced changes in the level of mRNA sequences. Moreover, starvation of athyreotic animals results in a further decrease in total RNA and in a further change in the level of individual mRNA species. We conclude, therefore, that although the reduced levels of circulating T3 and the nuclear T3 receptors can contribute to the observed results of starvation, the starvation-induced changes are not exclusively mediated by this factor. The striking overlap in the genomic response between hypothyroid and starved animals raises the possibility that those biochemical mechanisms regulated at a pretranslational level by T3 are either not helpful or injurious to the starving animal. The reduction in circulating T3 and nuclear receptor sites together with T3-independent mechanisms initiated in the starved animal may constitute redundant processes designed to conserve energy and substrate in the nutritionally deprived organism.

Regulation of catabolism of microinjected ribonuclease A requires the amino-terminal 20 amino acids

RNase A introduced into the cytoplasm of IMR-90 human diploid fibroblasts by erythrocyte-mediated microinjection is degraded with a half-life of approximately equal to 75 hr in the presence of fetal bovine serum. In response to serum deprivation the degradative rate of microinjected RNase A is enhanced 2-fold. RNase S protein (amino acids 21-124) is degraded with a half-life similar to that of RNase A in the presence of serum, but its catabolism is not increased during serum withdrawal. Reconstitution of RNase S protein with RNase S peptide (amino acids 1-20) restored full enzymatic activity to the S protein as well as the ability of fibroblasts to increase its catabolism during serum deprivation. Finally, RNase S peptide microinjected alone shows the full 2-fold increase in degradative rate during serum withdrawal. These results show that recognition of RNase A for enhanced breakdown during serum deprivation is based on some feature of its amino-terminal 20 amino acids. Furthermore, our results indicate that the enhanced protein catabolism during serum deprivation can be highly selective.

Similarities in properties, content, and relative rates of synthesis of fructose-P2 aldolase in livers of fed and starved rats

The present work gives evidence that, in contrast to the situation reported by Pontremoli et al. for the rabbit (Proc, Natl. Acad. Sci. U.S.A. 76, 6323-6325, 1979; Arch. Biochem. Biophys. 203, 390-394, 1980; Proc. Natl. Acad. Sci. U.S.A, 79, 5194-5196, 1982), starvation for as long as 3 days does not cause intracellular covalent modification and inactivation of fructose-P2 aldolase molecules in rat liver cells. This conclusion is based on our observations that liver aldolase molecules isolated from fed and starved rats in the presence of proteolytic inhibitors were not distinguished on the basis of specific catalytic activity, electrophoretic mobility, subunit molecular weight, NH2-terminal structure, or COOH-terminal structure. Further, the approximate 40% loss in rat liver mass which occurred during the 3-day fast was not associated with appreciable changes in the content of aldolase and most other abundant cytosolic proteins per gram of rat liver, as judged by electrophoretic analysis of 100 000-g soluble fractions of liver extracts. Finally, a 3-day fast had no appreciable effect on the relative rates of synthesis of aldolase and most other abundant cytosolic proteins in rat liver. Our findings suggest that nutrient deprivation has no preferential effect on the concentration or metabolism of aldolase in rat liver cells.

Effect of alloxan diabetes on a Ca2+-activated proteinase in rat skeletal muscle

The effect of making rats diabetic by alloxan injection on activity of the muscle Ca2+-activated proteinase (CAF) was investigated. Groups of four to seven control or alloxan-injected rats were killed 10 min (0 day) and 10, 17, and 24 days after a second alloxan injection. The second alloxan injection was given 3 days after the first. CAF activity was assayed in fractions precipitated between 0 and 45% ammonium sulfate saturation (P0-45 crude CAF fractions) that had been prepared so as to remove the protein inhibitor of CAF. Gel permeation, followed by DEAE-cellulose chromatography of pooled P0-45 crude CAF fractions from each time-treatment group, demonstrated that the assays used in this study were specific for CAF activity. Muscle CAF activity was up to 50% higher in alloxan-injected rats than in control rats, regardless of whether activity was expressed per gram sarcoplasmic protein, per gram contractile protein, or per gram skeletal muscle fresh weight. Alloxan injection diminished rate of muscle mass accumulation but did not change the proportion of sarcoplasmic or contractile protein in skeletal muscle. Hence, alloxan injection decreased the rate of contractile protein deposition. The elevation of muscle CAF activity in alloxan-injected rats is consistent with the proposed role of CAF in initiating metabolic turnover of myofibrillar proteins but does not prove this role nor exclude participation of other proteinases.

Calculation of half-lives of proteins in vivo. Heterogeneity in the rate of degradation of yeast proteins

A method is given for the calculation of half-lives of proteins in vivo from the measurement of the decrease of radioactivity in pulse-labelled proteins with time. This method could be particularly useful for the study of the degradation of proteins in cells that have a low growth rate. The method applied to growing yeast indicates that there are two major classes of protein. The class with low turnover constitutes the bulk of yeast protein and has a half-life of 160 h in a medium with glucose or galactose and of 50 h in a medium with ethanol. The class of proteins with high turnover (half-life between 0.8 and 2.4 hours) represents from 1% of total protein in yeast growing on glucose to 7% in yeast growing on ethanol. It is shown that some proteins which are depressed during growth on ethanol or induced during growth on galactose are particularly susceptible to degradation in a medium which contains glucose. It is proposed that protein degradation is regulated by a coarse control at the level of protease activity and a fine control on the susceptibility of individual proteins to proteases.

Degradation of intra- and extrahepatic protein by livers of normal and diabetic mice: differential responses to starvation

Rates of total hepatic proteolysis were measured in normal and streptozotocin-diabetic mice during feeding and over 48 hr of starvation, during which livers in the two groups lost 37% and 54% of their protein content, respectively. Measurements were made in 15-min in situ cyclic perfusions from the linear accumulation of free valine in the presence of cycloheximide; rates were corrected for turnover of short-lived proteins because these components contribute negligibly to alterations in liver protein content. During deprivation, corrected rates, expressed on a per liver basis, remained constant in normal mice but increased markedly in the diabetic group, attaining twice prestarvation values by 48 hr. By contrast, degradation rates of long-lived intracellular proteins, calculated from the sum of their synthesis and the linear decrease in protein content, decreased predictably in both groups and in parallel with absolute rates of protein synthesis. The extra proteolysis, representing the difference between total and long-lived protein degradation, was small in fed animals but increased progressively during starvation. With diabetic mice, however, the increase was approximately 5 times that of the normal and, in absolute terms, roughly equaled the total loss of liver protein. We suggest that this fraction arose from intrahepatic breakdown of proteins that were ultimately derived from sources outside the liver. Acceleration of this novel process could play an important interim role in providing endogenous glucogenic substrate under conditions in which the demand for this substrate is high.

Degradation of proteins microinjected into IMR-90 human diploid fibroblasts

Erythrocyte ghosts loaded with 125I-labeled proteins were fused with confluent monolayers of IMR-90 fibroblasts using polyethylene glycol. Erythrocyte-mediated microinjection of 125I-proteins did not seriously perturb the metabolism of the recipient fibroblasts as assessed by measurements of rates of protein synthesis, rates of protein degradation, or rates of cellular growth after addition of fresh serum. A mixture of cytosolic proteins was degraded after microinjection according to expected characteristics established for catabolism of endogenous cytosolic proteins. Furthermore, withdrawal of serum, insulin, fibroblast growth factor, and dexamethasone from the culture medium increased the degradative rates of microinjected cytosolic proteins, and catabolism of long-lived proteins was preferentially enhanced with little or no effect on degradation of short-lived proteins. Six specific polypeptides were degraded after microinjection with markedly different half-lives ranging from 20 to 320 h. Degradative rates of certain purified proteins (but not others) were also increased in the absence of serum, insulin, fibroblast growth factor, and dexamethasone. The results suggest that erythrocyte-mediated microinjection is a valid approach for analysis of intracellular protein degradation. However, one potential limitation is that some microinjected proteins are structurally altered by the procedures required for labeling proteins to high specific radioactivities. Of the four purified proteins examined in this regard, only ribonuclease A consistently showed unaltered enzymatic activity and unaltered susceptibility to proteolytic attack in vitro after iodination.

Quantitative relationship between autophagy and proteolysis during graded amino acid deprivation in perfused rat liver

Fractional volumes of lysosomal-vacuolar elements and long lived protein degradation were quantitatively correlated in rat livers perfused in the single pass mode with varying levels of plasma amino acids. Volumes were determined stereologically; degradation was measured in a second stage cyclic perfusion from the linear accumulation of valine in the presence of cycloheximide and was corrected for loss of short lived proteins. Livers exhibited a high degree of amino acid responsiveness; total protein degradation decreased sharply from 4.5 to 1.5%/h (basal) over an amino acid range of 0-10 times (10X) normal plasma concentrations; near basal values were achieved at 1X. Vacuoles containing undegraded cytoplasm (AVi) appeared immediately following stringent deprivation and by 7.5 min were converted to degradative forms (AVd); both autophagic populations attained steady state volumes by 20 min. With amino acid additions, AVi formation virtually ceased and AVd regressed rapidly (0.087 min-1). Cytoplasmic turnover, calculated from this rate constant and the increases over basal in fractional volumes of either AVi or degradative components, agreed quantitatively with corresponding rates of protein turnover. Predictions from these findings, together with evidence for intralysosomal protein pools in both deprived and basal states, account fully for the accelerated proteolysis and suggest that cytoplasm is also internalized by lysosomes under basal conditions.

Decreased turnover of soluble liver proteins in mice with alloxan-induced diabetes

Mice with alloxan-induced diabetes were used as a model to assess whether the synthesis and/or degradation of soluble liver proteins in general is affected in vivo by the diabetic state. Protein turnover was measured 2-3 wk after diabetes was induced. Degradation of liver cytosol proteins was decreased in diabetic mice as measured by the loss of protein radiolabeled with [14C]bicarbonate. The incorporation of radiolabeled amino acids into protein was also decreased in diabetic mice. When [3H]leucine was administered as the precursor for protein synthesis, the radiospecific activity of leucine derived from leucyl-tRNA in livers was similar in control and diabetic mice. Thus, the rate of protein synthesis appears to be decreased. There was no indication that diabetes affected the turnover of long- or short-lived proteins differentially. The activities of several cellular proteinases were unaffected or slightly decreased in livers of diabetic mice. These data indicate that protein turnover is decreased in this chronic form of diabetes.

Selective depletion of small basic non-glycosylated proteins in diabetes

Degradative rates of small basic non-glycosylated proteins are preferentially enhanced in rat liver cytosol during severe streptozotocin-induced diabetes. Synthetic rates of these classes of proteins are not selectively enhanced in diabetes, so small basic non-glycosylated proteins should be depleted from liver cytosol as a consequence of this disease. To test this hypothesis, proteins were analysed from normal animals, from diabetic animals receiving insulin and from diabetic animals after insulin withdrawal for 3 days. The proteins were separated according to subunit molecular weight by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, according to isoelectric point by isoelectric focusing and according to carbohydrate content by affinity chromatography with concanavalin A linked to agarose. Severe uncontrolled diabetes is associated with the predicted depletion of small basic non-glycosylated proteins from liver cytosol. The preferential degradation and loss of these protein classes may be of considerable physiological importance to the animal.

General characteristics of normal and stress-enhanced protein degradation in Lemna minor (duckweed)

The general features of protein degradation in Lemna minor were studied by using a double-isotope technique. In common with several animal systems, there are correlations between the relative rate of protein degradation on the one hand and molecular weight, charge and carbohydrate content on the other. Large proteins, acidic proteins and non-glycosylated proteins are degraded relatively more rapidly than small or basic proteins, or glycoproteins. The correlations with size and carbohydrate content are explicable on the basis of differential susceptibility to Pronase, whereas the charge correlation cannot be explained on the basis. In addition, acidic proteins are not generally of higher molecular weight than neutral or basic proteins. The correlations are found in fronds growing in normal complete medium and in fronds transferred to medium lacking nitrate of made 50% (v/v) with respect to deuterium oxide, both of which are conditions that induce a large increase in protein breakdown in Lemna. Thus basal protein degradation and enhanced degradation do not appear to differ fundamentally in their general characteristics. The results are discussed in relation to the reported features of normal and enhanced proteolysis in animal tissues and to the possible mechanism of protein degradation in Lemna.

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.

Localization and biosynthesis of NADH-cytochrome b5 reductase, an iontegral membrane protein, in rat liver cells. III. Evidence for the independent insertion and turnover the enzyme in various subcellular compartments

The biosynthesis and turnover of rat liver NADH-cytochrome b(5) reductase was studied in in vivo pulse-labeling and long-term, double-labeling experiments. Rats under thiopental anesthesia were injected into the portal vein with [(3)H]L-leucine and sacrificed at various times after the injection. NADH-cytochrome b(5) reductase was extracted from liver cell fractions by cathepsin D-catalyzed cleavage and was then immunoadsorbed onto antireductase-bearing affinity columns in the presence of excess unlabeled rat serum. After elution of the enzyme from the columns with a pH-2.2 buffer, the amount of the reductase protein in the samples was determined by radioimmunoassay, and the radioactivity in reductase was determined on SDS polyacrylamide gel reductase bands. The specific radioactivity of the reductase extracted from the homogenate as well as from rough and smooth microsomal, mitochondrial, and Golgi fractions, estimated at the end of the pulse (10 min after the injection) and at various time points thereafter, remained approximately constant over a 6-h period. These data suggest tha tth eenzyme is independently inserted into the various membranes where it is located. Moreover, the specific radioactivity of the mitochondrial reductase was lower than that of the other fractions, suggesting that it turns over at a slower rate. The lower turnover rate of the mitochondrial enzyme was confirmed by long-term, double-labeling experiments carried out according to the technique of Arias et al. (J. Biol. Chem. 244: 3303-3315.). The relevance of these findings in relation to the understanding of membrane biogenesis and turnover 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.

A releationship between protein-degradation rates in vivo, isoelectric points, and molecular weights obtained by using density labelling

1. Half-lives of five plant enzymes were estimated by rate-labeling with 2H2O on the assumption that loss of catalytic activity is equivalent to protein degradation. 2. This involved measuring band-broadening of activity profiles after isopycnic centrifugation. 3. Isoelectric points were determined by isoelectric focusing, and molecular weights were estimated by gel filtration. 4. The conclusion is drawn from the experimental evidence presented that a weak correlation exists between rates of degradation and isoelectric points (r = 0.699; P greater than 0.10; not significant). 5. A highly significant relationship exists between the logarithm of subunit size and half-life (r = -0.939; P greater than 0.02). 6. A literature survey confirmed the trends observed.

Degradation of proteins microinjected into cultured mammalian cells

Iodinated proteins were degraded after injection into HeLa cells at first-order rates with half-lives varying from three hours for the trout monhistone chromosomal protein, HMG-T, -to 60 hours for whale myoglobin. Fluoresceinated-bovine serum albumin (fl-BSA) was degraded almost twice as fast as unmodified BSA. The rate of degradation of 125I-BSA was very similar in eight cell lines of mouse, human, monkey and rat origin. Microinjected proteins were analyzed on SDS-acrylamide gels after injection, and for BSA and immunoglobin G, all remaining intracellular 125I migrated at the molecular weight of the injected proteins. By contrasting, more than 80% of the extracellular 125I chromatographed as iodotyrosine. With the exception of fl-BSA, which exhibited perinuclear accumulation in approximately one-half of the injected cells, autoradiography showed that throughout the period of study the injected proteins remained dispersed in the cytoplasm.

Studies on the relationship between the degradative rates of proteins in vivo and their isoelectric points

Acidic proteins tend to be degraded more rapidly than neutral or basic proteins in rat liver, skeletal muscle, kidney and brain and in mouse liver and skeletal muscle. We now report a similar relationship among soluble proteins from rat lung, heart and testes, and from human fibroblasts and mouse-embryo cells grown in culture. These findings indicate that the correlation between protein net charge and degradative rate is a general characteristic of intracellular protein degradation in mammals. This relationship between isoelectric point and half-life appears to be distinct from the previously reported correlation between subunit molecular weight and protein half-lives. The more rapid degradation of acidic proteins does not result from their being of larger molecular weight than neutral or basic proteins. Furthermore, proteins within specific isoelectric point ranges still exhibit a relationship between subunit size and half-life. Finally, a group of membrane or organelle-associated proteins that are insoluble in phosphate-buffered saline and water but soluble in 1% Triton X-100 exhibit a correlation between size and half-life, but not between net charge and half-life. The biochemical reasons for the relationship between protein isoelectric point and half-life are unclear, although several possible explanations are presented. It is not due to a greater sensitivity of acidic proteins to proteolytic attack since experiments with a variety of endoproteinases, including trypsin, chymotrypsin, Pronase, papain, chymopapain, Staphylococcus aureus V8 proteinase, pepsin and lysosomal cathepsins from rat liver, have failed to demonstrate more rapid digestion of acidic proteins.