Effect of size on the relative rate of degradation of rat liver soluble proteins

Question-driven stepwise experimental discoveries in biochemistry: two case studies

Philosophers of science diverge on the question what drives the growth of scientific knowledge. Most of the twentieth century was dominated by the notion that theories propel that growth whereas experiments play secondary roles of operating within the theoretical framework or testing theoretical predictions. New experimentalism, a school of thought pioneered by Ian Hacking in the early 1980s, challenged this view by arguing that theory-free exploratory experimentation may in many cases effectively probe nature and potentially spawn higher evidence-based theories. Because theories are often powerless to envisage workings of complex biological systems, theory-independent experimentation is common in the life sciences. Some such experiments are triggered by compelling observation, others are prompted by innovative techniques or instruments, whereas different investigations query big data to identify regularities and underlying organizing principles. A distinct fourth type of experiments is motivated by a major question. Here I describe two question-guided experimental discoveries in biochemistry: the cyclic adenosine monophosphate mediator of hormone action and the ubiquitin-mediated system of protein degradation. Lacking underlying theories, antecedent data bases, or new techniques, the sole guides of the two discoveries were respective substantial questions. Both research projects were similarly instigated by theory-free exploratory experimentation and continued in alternating phases of results-based interim working hypotheses, their examination by experiment, provisional hypotheses again, and so on. These two cases designate theory-free, question-guided, stepwise biochemical investigations as a distinct subtype of the new experimentalism mode of scientific enquiry.

Structure of proteins: Evolution with unsolved mysteries

Evolution of macromolecules could be considered as a milestone in the history of life. Nucleic acids are the long stretches of nucleotides that contain all the possible codes and information of life. On the other hand, proteins are their actual translated outcomes, or reflections of modifications in their structure that have occurred at a slow, but steady rate over a very long period of evolution. Over the years of research, biophysicists, biochemists, molecular and structural biologists have unfurled several layers of the structural convolutions in these chemical molecules; however evolutionists look over their structures through a different prism, which may or may not coincide with others. There remains a need to outline several well-known, but less discussed features of protein structures, like intrinsically disordered states, degron signals and different types of ubiquitin chains providing degradation signals, which help the cellular proteolytic machinery to identify and target the proteins towards degradation pathways. There are several important factors, which are critical for folding of proteins into their native three-dimensional conformations by the cytoplasmic chaperones; but in real time how the chaperones fold the newly synthesized polypeptide sequences into a particular three-dimensional shape within a fraction of second is still a mystery for biologists as well as mathematicians. Multiple similar unsolved or unaddressed questions need to be addressed in detail so that future line of research can dig deeper into the finer details of these structures of the proteins.

Protein degradation in metabolic and nutritional disorders

The increased protein degradation associated with diabetes appears to be different in many respects from protein catabolism in normal, well-nourished cells. In all normal eukaryotic cells examined, degradation of cytosolic proteins exhibits several striking features. Large proteins tend to be degraded more rapidly than small proteins, acidic proteins tend to be degraded more rapidly than neutral or basic proteins, and glycoproteins are degraded more rapidly than non-glycoproteins. Furthermore, a general correlation exists between protein half-life in vivo and susceptibility to proteolytic attack in vitro. In streptozotocin-diabetic rats the relationships between degradative rate and protein size, net charge, and carbohydrate content are absent or markedly reduced among cytosolic proteins of the liver. However, the correlation between protein half-life and susceptibility to proteinase in vitro is unaltered. Therefore, the enhanced protein degradation in diabetes shows little or no selectivity towards large, acidic, glycoproteins but does show specificity for proteins than tend to be sensitive to proteinases. Similar studies using other tissues from diabetic rats are reported and general characteristics of the enhanced liver protein catabolism in starvation and hyperthyroidism are briefly discussed. The biochemical reasons for the increased protein catabolism in diabetes are unclear although several possible explanations are presented. The enhanced breakdown is probably not due to cellular proteins becoming more proteinase sensitive in diabetes since experiments with a variety of endoproteinases including pronase, chymotrypsin, pepsin, and lysosomal cathepsins have failed to demonstrate more rapid digestion of liver proteins from diabetic animals.

Measuring synthesis rates of muscle creatine kinase and myosin with stable isotopes and mass spectrometry

We investigated a novel strategy for measuring the synthesis rate of proteins in skeletal and cardiac muscle. Mass isotopomer distribution analysis allows measurement of the isotopic enrichment of the true biosynthetic precursor for proteins (tRNA-amino acids), but cannot easily be applied to slow turnover muscle proteins due to insufficient isotope incorporation into multiply labeled species. Using a rapid turnover protein from the same tissue, however, might reveal tRNA-amino acid enrichment. We tested this strategy in rats on muscle creatine kinase (CK). A trypsinization peptide (3647u) containing 5 leucine repeats was identified by computer-simulated digestion of CK and then isolated from trypsin hydrolysates. Mass isotopomer abundances were determined by electrospray ionization-magnetic sector-mass spectrometry after in vivo administration of [(2)H(3)]leucine. Myosin heavy chain was also isolated and hydrolyzed to free amino acids. Muscle tRNA-amino acids were well labeled, by direct measurement. Enrichments of M(+1) and M(+2) mass isotopomers in the CK-peptide were measurable but low (consistent with a CK half-life of 3-10 days). Incorporation into skeletal muscle myosin indicated a half-life of 54 days. In conclusion, the general strategy of measuring protein kinetics by quantifying mass isotopomer abundances of mid-sized peptides from protein hydrolysates is effective, but CK does not turn over rapidly in muscle, contrary to previous reports. Identification of a rapid turnover muscle protein would be useful for this purpose.

Thrombolytic therapy does not change the release ratios of enzymatic and non-enzymatic myocardial marker proteins

Measurements of cardiac marker proteins in plasma from patients with acute myocardial infarction (AMI) have become important in the evaluation of recanalization therapy. The validity of this approach has however been questioned, because it was claimed that coronary reperfusion may increase the recovery in plasma of cardiac enzymes, such as creatine kinase (CK). In the present study, possible effects of thrombolytic therapy on the release of enzymatic and nonenzymatic marker proteins were investigated. Activities of CK and lactate dehydrogenase (LDH), and concentrations of myoglobin (Mb) and fatty acid-binding protein (FABP) were determined in serial plasma samples obtained from 50 patients with confirmed AMI, of whom 36 received thrombolytic therapy, and 14 did not. Treatment delay was 2.8+/-1.6 (mean+/-SD) h, and hospital delay in untreated patients was 2.7+/-1.8 h. Average infarct size, expressed in gram-equivalents of heart muscle per litre of plasma (g-eq/l), varied between 5.5 and 7.2 g-eq/l for the four marker proteins in patients treated with thrombolytic therapy, and between 4.6 and 6.4 g-eq/l in untreated patients, with a tendency to larger infarct sizes for Mb and FABP than for CK and LDH. Thrombolytic therapy, although significantly accelerating protein release rates, did not influence the release ratios. These results indicate that thrombolytic therapy has no significant effects on the recovery of cardiac marker proteins in plasma.

Expression of human aspartyl-tRNA synthetase in COS cells

Mammalian aspartyl-tRNA synthetase (DRS) occurs in a multi-enzyme complex of aminoacyl-tRNA synthetases, while DRS exists as free soluble enzymes in bacteria and yeast. The properties of human DRS transient expressed in COS cells were examined. After transfection of COS cells with the recombinant plasmids pSVL-63 that contained hDRS cDNA coding and non-coding sequences, and pSV-hDRS where the non-coding sequences were deleted, DRS in the transfected COS cells significantly increased compared to mock transfected cells. COS cells transfected with pSV-hDRS delta 32 that contained N-terminal 32 residue-coding sequence deleted hDRS cDNA showed no increase in DRS activity. Northern blot analysis showed that concentrations of corresponding mRNAs of hDRS and hDRS delta 32 were greatly enhanced in transfected cells. The increases in the level of the transcripts were much higher than those of the corresponding proteins. Gel filtration analysis showed that hDRS in pSV-hDRS transfected cells expressed as a low molecular weight form of hDRS and pSV-hDRS delta 32 transfected cells did not. Epitope tagging and indirect immunofluorescence microscopy was used to localize hDRS. Both hDRSmyc and hDRS delta 32myc were localized in the cytoplasm and showed diffused patterns. These results showed that hDRS has little tendency to aggregate in vivo and suggested that the N-terminal extension in hDRS was not involved in the expression and sub-cellular localization of hDRS, but may play a role in the maintenance of enzymatic activity of hDRS in COS cells.

Expression and turnover of acylphosphatase (muscular isoenzyme) in L6 myoblasts during myogenesis

Acylphosphatase (muscular isoenzyme) levels have been measured in L6J1 myoblasts either proliferating or differentiating into myotubes. Results indicated that the increase in enzyme levels during differentiation is very similar to that of creatine kinase, a specific muscular enzyme. The half-lives of acylphosphatase in myoblasts and myotubes were also determined; t1/2 values of 3 h 30 min (myoblasts), and 2 h 18 min (myotubes) were found. These results indicate that acylphosphatase could be considered a short-lived muscle-specific protein and that its increase in myotubes must be accompanied by an activation of its breakdown.

Proteolytic signal sequences (PEST) in the mammalian aminoacyl-tRNA synthetase complex

Eight aminoacyl-tRNA synthetases together with three unidentified proteins are associated as a multi-enzyme complex in mammalian cells. Partial peptide sequences for lysyl- and aspartyl-tRNA synthetases are determined and no highly hydrophobic peptides are found. The partial amino acid sequences for two of the unidentified proteins in the complex are shown to have substantial homology and each has a number of unique sequences. The results suggest that the two unidentified proteins are fragments of synthetases. The partial sequences revealed the presence of PEST sequences in at least three proteins. Inasmuch as PEST sequences are signals for intracellular degradation, the mammalian synthetase complex may have evolved to protect these synthetases against intracellular proteolysis.

Intrinsic physical instability of the proteins with short half-time: possible role in aging and in carcinogenesis

Different proteins have vastly different lifetime in cells, some survive as long as or even longer than the cell does, whereas others survive for just minutes. Proteins showing short half-times play an extremely important role as cellular regulators, in particular in controlling cell mitosis and in carcinogenesis. These proteins probably possess an intrinsic physical instability, i.e. they are biologically active only while being in a meta-stable non-equilibrium state resulting from the very mechanisms of protein biosynthesis on ribosomes. Aging of these proteins consists of internal equilibration through protein chain folding rather than in accumulation of errors due to intermolecular interactions. We want to attract more attention to such fast (short-lived) proteins. The question of what determines these protein lifetimes is of practical as well as theoretical interest.

The breakdown of the individual neurofilament proteins by cathepsin D

In a continuing study of proteolysis of CNS proteins by CNS enzymes, neurofilament proteins (210 K, 155 K, 70 K) and desmin were separated, and the breakdown of individual proteins by purified brain cathepsin D was measured and compared to breakdown by plasma thrombin. With both cathepsin D and thrombin, the rate of breakdown of the 70 K protein was the highest, followed by the 155 K, and that of the 210 K was the lowest. With each substrate cathepsin D breakdown was the highest at pH 3; small but significant breakdown could be seen at pH 6. The pattern of intermediate breakdown products depended on pH, with greater amounts of fragments detected at higher pH, and the patterns with the two enzymes were different. We showed that differences exist in cleavage sites and breakdown rates of the neurofilament proteins. The capacity of the cathepsin D present in the tissue to hydrolyze these substrates was high, even at pH close to neutral, and was greatly in excess of that needed for physiological neurofilament turnover.

Amino acid sequences common to rapidly degraded proteins: the PEST hypothesis

The amino acid sequences of ten proteins with intracellular half-lives less than 2 hours contain one or more regions rich in proline (P), glutamic acid (E), serine (S), and threonine (T). These PEST regions are generally, but not always, flanked by clusters containing several positively charged amino acids. Similar inspection of 35 proteins with intracellular half-lives between 20 and 220 hours revealed that only three contain a PEST region. On the basis of this information, it was anticipated that caseins, which contain several PEST sequences, would be rapidly degraded within eukaryotic cells. This expectation was confirmed by red blood cell-mediated microinjection of 125I-labeled caseins into HeLa cells where they exhibited half-lives of less than 2 hours. The rapid degradation of injected alpha- and beta-casein as well as the inverse correlation of PEST regions with intracellular stability indicate that the presence of these regions can result in the rapid intracellular degradation of the proteins containing them.

Amino acid incorporation in relation to molecular weight of proteins in young and adult brain

Rates of protein synthesis were studied in immature and adult rat brain tissue. After an amino acid incorporation period, in vivo or in incubated slices from brain, the soluble protein was fractionated according to molecular weight by column chromatography. In examining soluble whole proteins, no direct correlation between molecular weights and synthesis rates could be established; the highest synthesis rates were found in fractions around 70,000 MW and below 10,000. Incorporation into the subunits after fractionation by SDS gel electrophoresis was proportional to subunit molecular weight, with rates of incorporation into the largest subunits being the highest. The results suggest a relationship between turnover rate and structure of subunits of brain proteins.

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.

Turnover of adenosine triphosphatase from rat liver mitochondria. Effect of high-protein and low-protein diets

The half-life of mitochondrial adenosine triphosphatase and the relative rate constants of protein degradation for several fractions of rat liver have been measured by the double-isotope technique. It has been shown that the apparent turnover rates of some mitochondrial enzymes, far apart in size, such as carbamoyl phosphate synthetase, glutamate dehydrogenase and malate dehydrogenase, are not related to molecular weight or to size of subunits. In view of the possibility that mitochondrial proteins are degraded by different mechanisms, it was of interest to determine the half-life of a protein tightly bound to the inner membrane such as adenosine triphosphatase. The rate constants of degradation for rats fed a basal diet and injected at three-day intervals with isotopic leucine were: homogenate, kd = 0.195 days-1; mitochondria, kd = 0.135 days-1; cytosol, kd = 0.140 days-1; microsomes, kd = 0.28 days-1; ATPase, kd = 0.275 days-1. The rate constants of the cellular fractions of liver of rats fed a high protein diet did not change or showed a small increase, compared with those of animals fed the basal diet, while those from rats on the protein-free diet showed a decrease. The rate constant for adenosine triphosphatase showed an increase with high-protein and a decrease with protein-free diet. A procedure for the purification of ATPase from a single liver of a rat is described.

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.

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.

The effect of protease inhibitors and decreased temperature on the degradation of different classes of proteins in cultured hepatocytes

Leupeptin, chymostatin and antipain inhibited the degradation of long-lived proteins in cultured rat hepatocytes by 20-30%, probably by inhibiting lysosomal proteases: (1) Leupeptin and chymostatin decreased to a similar extent the degradation of an exogenous protein 125I-asialo fetuin, a process known to occur within lysosomes. (2) In extracts of cells treated with leupeptin, cathepsin B activity was inhibited by 35-50%. (3) Leupeptin, chymostatin and antipain inhibited proteolysis by homogenates of liver lysosomes but not by the supernatant fraction. These agents, however, do not appear to rapidly permeate the membrane of isolated lysosomes. Leupeptin, chymostatin and antipain did not inhibit the breakdown of short-lived normal cell proteins, and ones containing amino acid analogs. Even when the amount of abnormal proteins was increased, such that it comprised a large fraction of cell protein, the degradation of these polypeptides was still very rapid and not affected by these inhibitors. The pathway for the degradation of short-lived cell proteins thus appears distinct from that responsible for degradation of long-lived cell proteins. In accord with this conclusion, reduction of the temperature of cultures inhibited the breakdown of long-lived proteins to a much greater extent than it affected the breakdown of short-lived ones. Treatment of cultured hepatocytes with glucagon, or deprivation for serum or amino acids stimulated the degradation of the more stable cell proteins but did not affect the breakdown of 125I-asialo-fetuin. Under these conditions leupeptin and chymostatin inhibited the breakdown of long-lived cell proteins to the same extent as in control cultures. Thus, lysosomal enzymes seem to play an important role in protein breakdown both in fed hepatocytes and in cells where proteolysis is accelerated.

The age dependence of intracellular proteolysis: changes of the substrate proteins

Liver cytosol proteins of young (4--6 months) and old (18--27 months) rats were degraded in vitro by papain, pronase, trypsin, pepsin, cathepsin D from rat liver and a soluble lysosomal enzyme mixture from rat liver. We could demonstrate the capability of the latter enzyme mixture to degrade proteolytically the cytosol proteins of young animals about 20% faster than those of the older animal group. Digesting radioactive labelled "young" cytosol in the presence of unlabelled "old" cytosol the possibility could be excluded, that this effect was due to an inhibitor of macromolecular size present in the "old" cytosol.

Ferritin phenotypes: structure and metabolism

Recent advances in our understanding of the biochemistry of ferritin have provided new insights into its role in iron metabolism. Findings of multiple structural forms in many tissues may have important consequences for ferritin's function and metabolism. This article reviews the molecular basis of apoferritin heterogeneity and discusses mechanisms operating in the phenotypic expression of ferritin in normal and malignant cells.

Decreased dissociation of the 3-methylcrotonyl-CoA carboxylase complex from Achromobacter in the presence of 3-methylcrotonyl-CoA. A possible regulatory mechanism for the intracellular degradation of the enzyme

By inactivation of different concentrations of 3-methylcrotonyl-CoA carboxylase from Achromobacter IVS with a fixed concentration of iodoacetamide, it was demonstrated that the degree of dissociation of the complex is considerably lower in the presence of 3-methylcrotonyl-CoA. ATP did not produce this effect. This property could serve to regulate the intracellular degradation of the enzyme, if the dissociated subunits were attacked preferentially.

Control of fatty-acid synthetase biosynthesis in Saccharomyces cerevisiae

143 out of 308 fas1 mutants (47%) and 139 out of 443 fas2-mutants (32%) genetically studied in this laboratory fail to complement with any other fas-mutant (deficient in fatty acid synthetase) of the same gene locus. From these noncomplementing fas-mutants no mutant fatty acid synthetase can be isolated using the wild-type enzyme purification procedure. Furthermore the noncomplementing fas-mutants generally contain no material immunologically crossreacting with a specific fatty acid synthetase antiserum. However, subunits obtained after dissociation of the complex with sodium dodecylsulfate still cross react with this antiserum. Therefore, it is concluded that noncomplementing fas-mutants contain no fatty acid synthetase component proteins, though one of the two fas-loci is mutationally unaffected. This conclusion was further confirmed by 14C-labeled amino acid incorporation studies which indicated that in noncomplementing fas-mutants, other than in wild type and complementing fas-mutant cells, no label was incorporated into fatty acid synthetase subunits or precursor proteins. At nonpermissive temperature, the same biochemical and immunological characteristics were observed with temperature-sensitive non-complementing fas-mutants. These results suggest that noncomplementing fas-mutants either represent regulatory mutants unable to induce the mutationally unaffected other fas-gene locus or that they are association-defective mutants. In both cases the resulting individual subunits of the complex may be rapidly degraded by intracellular proteases.

Relationship between in vivo degradative rates and isoelectric points of proteins

Previous studies have shown that in mammalian cells proteins of large molecular weight are degraded more rapidly than small ones. Evidence is presented here that half-lives of proteins are also related to their isoelectric points. A double-isotope method was used to compare degradative rates of soluble proteins separated by isoelectric focusing. In rat liver, skeletal muscle, kidney, and brain, more rapid rates of catabolism were found for acidic protein fractions than for neutral or basic ones. Acidic proteins also tended to be degraded faster in several mouse tissues. A literature survey confirmed this trend. For 22 proteins from rat liver, a highly significant correlation was found between rates of degradation and isoelectric points (r = 0.824; P less than 0.01). This relationship between isoelectric point and half-life appears to be distinct from that between protein size and half-life.

Studies on the synthesis of estrogen-specific uterine proteins. Comparison of methods of quantitative evaluation of double-isotope peaks

Double-isotope techniques are frequently used to provide a sensitive means of detecting specific biosynthetic responses to experimental manipulations, usually protein or nucleic acid synthesis. The qualtative basis for the detection of changes in biosynthetic rates in these double-isotope experiments involves measuring changes in the ratio of the isotopes to each other after fractionation of the albeled extract. Changes in the isotope ratios have been used directly in quantitative interpretations also, but such quantitative interpretations of changes in isotope ratios are potentially erroneous since the magnitude of the change in ratio of one isotope to the other will depend upon the total number of counts within the area in question (the 'baseline'). The results of experiments on an estrogen-specific uterine protein are presented in this report. It is demonstrated that under conditions in which the 'baseline' incorporation of isotope varies within an experiment, such as in purification or pulse-chase studies, the use of isotope ratios to quantify the data results in erroneous values. In these instances computations using the delta 14C method described by Mayol and Sinsheimer (1970) provides for what appear to be valid conclusions. However, under conditions in which the baseline incorporation of isotope of the control and experimental groups does not vary, as in studies of the temporal course of synthesis and dose-response relationships, the use of increases in isotope ratios is adequate for quantitative interpretations.