A study on the intracellular transport of prothrombin, albumin and transferrin in rat

The intracellular transport of prothrombin in rat has been studied and compared with the transport of albumin and transferrin. The proteins were immunoisolated from plasma samples after pulse labelling with [3H]leucine and the secretion kinetics were determined. The half-times for secretion (t1/2) were approx. 30, 53 and 75 min for albumin, prothrombin and transferrin, respectively, whereas the minimal transit time for prothrombin was approx. 30 min, and those for albumin and transferrin 15-20 min. After injection of vitamin K-1 into warfarin-treated rats, the accumulated prothrombin precursor was gamma-carboxylated and secreted with a t1/2 of 37 min. This indicates that the gamma-carboxylation of prothrombin in rough endoplasmic reticulum cannot account for the delay in the transport of prothrombin as compared to albumin. Comparison of the incorporation of [3H]leucine and [3H]glucosamine into plasma prothrombin and transferrin suggested that transferrin is secreted randomly from an intracellular pool, whereas prothrombin is transported in a more orderly sequence. Moreover, treatment of rough microsomes with 0.05% sodium deoxycholate indicated that prothrombin is more tightly associated with the membranes of rough endoplasmic reticulum than albumin and transferrin.     

Biosynthesis of Rat serum albumin

1. The labelling of intracellular and extracellular serum albumin was studied in liver slices and in whole rats by using new methods for the purification of the protein. 2. The results suggest that a polypeptide precursor is formed that is converted relatively slowly into serum albumin. 3. The effect of liver cell K(+) has been examined by a double-label method and it is shown that K(+) accelerates the rate of conversion of ;precursor' into albumin. The rate of transit of albumin across the cell membrane appears to be unrelated to the concentration of K(+) within the cell. 4. The time-course of incorporation of radioactive amino acid into albumin follows a sigmoidal mode. There is a pronounced time-lag before label starts to appear in intracellular albumin, and a further time-lag before it appears in extracellular albumin. 5. In slices the sum of intra- and extra-cellular label rises steadily from 30min after the start of labelling with a pulse of labelled leucine or valine and continues to rise for at least another 60min. This occurs whether labelling is stopped by addition of excess of carrier amino acid or with cycloheximide (100mum) or both. 6. The intracellular albumin content remains constant whether slices are maintained with low or normal intracellular K(+) concentrations. 7. Specific radioactivities of intracellular albumin (and fractions thereof) and of extracellular albumin were determined in vitro and in vivo. The results show that the intracellular albumin cannot be a precursor of extracellular albumin, unless a very small compartment is turning over much more rapidly than the bulk of the liver albumin or even of the microsomal albumin.     

Effects of starvation and a high-energy diet on rat blood compartmentation of injected radioactive alanine and glucose

The effects of starvation upon blood compartmentation and differential handling of glucose and alanine have been studied in control and cafeteria-fed rats. The injection of radioactive glucose resulted in higher specific radioactivities in intracellular glucose but lower intracellular amino acid specific radioactivities when compared with the plasma values. The cells/plasma specific radioactivity ratios increased dramatically in cafeteria rats with starvation. The injection of radioactive alanine resulted in higher cell than plasma glucose specific activities, and lower cell amino acid specific activities. All these parameters increased after a 24-hours starvation period. It is concluded that glucose synthetised by the liver is released mainly into the blood intracellular pool, being later liberated into the plasma or directly into the tissues.     

Inhibition by cyanate of the processing of lysosomal enzymes

In cultured human fibroblasts, maturation of the lysosomal enzymes beta-hexosaminidase and cathepsin D is inhibited by 10 mM-potassium cyanate. In cells treated with cyanate the two enzymes accumulate in precursor forms. The location of the accumulated precursor is probably non-lysosomal; in fractionation experiments the precursors separate from the bulk of the beta-hexosaminidase activity. The secretion of the precursor of cathepsin D, but not that of beta-hexosaminidase precursor, is enhanced in the presence of cyanate. The secreted cathepsin D, as well as that remaining within the cells, contains mostly high-mannose oligosaccharides cleavable with endo-beta-N-acetylglucosaminidase H. After removal of cyanate, the accumulated precursor forms of the lysosomal enzymes are largely released from the pretreated cells. It is concluded that cyanate interferes with the maturation of lysosomal-enzyme precursors by perturbing their intracellular transport. Most probably cyanate affects certain functions of the Golgi apparatus.     

The plasma transport and metabolism of retinoic acid in the rat

The transport of retinoic acid in plasma was examined in vitamin A-deficient rats maintained on small doses of radioactively labelled retinoic acid. After ultracentrifugation of serum adjusted to density 1.21, most of the radioactivity (83%) was associated with the proteins of density greater than 1.21, and not with the serum lipoproteins. Gel filtration of the labelled serum on Sephadex G-200 showed that the radioactive label was associated with protein in the molecular-weight range of serum albumin. On polyacrylamide-gel electrophoresis almost all of the recovered radioactivity migrated with serum albumin. Similar esults were obtained with serum from a normal control rat given a single oral dose of [(14)C]retinoic acid. These findings indicate that retinoic acid is transported in rat serum bound to serum albumin, and not by retinol-binding protein (the specific transport protein for plasma retinol). Several tissues and the entire remaining carcase of each rat were extracted with ethanol-acetone to determine the tissue distribution of retinoic acid and some of its metabolites. The total recover of radioactive compounds in in the entire body of the rat was about 7-9mug, representing less than 5% or 10% respectively of the total administered label in the two dosage groups studied. The results confirm that retinoic acid is not stored in any tissue. Most of the radioactive material was found in the carcase, rather than in the specific tissues analysed. Two-thirds of the radioactivity in the carcase appeared to represent unchanged retinoic acid. Of the tissues examined, the liver, kidneys and intestine had relatively high concentrations of radioactive compounds, whereas the testes and fat-pads had the lowest concentrations.     

Cholesterol-rich intracellular membranes: a precursor to the plasma membrane

The disposition of newly synthesized sterols in cultured human fibroblasts has been examined in this study. We began by demonstrating that cholesterol mass and exogenously added [3H]cholesterol both are markers for the plasma membrane, perhaps better than 5'-nucleotidase. Cells were incubated with radioactive acetate to label their endogenous sterols biosynthetically, treated with cholesterol oxidase to convert plasma membrane cholesterol to cholestenone, and then homogenized and spun to equilibrium on sucrose gradients. The density gradient profiles of the various organelles were monitored using these markers: plasma membrane, radioactive cholestenone; smooth endoplasmic reticulum, 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase); and Golgi apparatus, galactosyltransferase. The buoyant density profiles of radioactive intracellular cholesterol and lanosterol both had a peak at 1.12 g/cm3, similar to 5'-nucleotidase and galactosyltransferase but not to HMG-CoA reductase. This result suggests that cholesterol biosynthesis is not taken to completion in the endoplasmic reticulum. Digitonin treatment shifted the profiles of both plasma membrane and intracellular cholesterol to higher densities. Pretreatment of intact cells with cholesterol oxidase abolished the digitonin shift of plasma membranes but not the intracellular cholesterol, indicating that these two membrane pools are not entirely physically associated. Because intracellular cholesterol was shifted more than any of the organelle markers, it must reside in a separate membrane. Since digitonin selectively shifts the density of membranes rich in cholesterol, we infer that newly synthesized cholesterol accumulates in such membranes prior to its delivery to the plasma membrane. Taken together, these results suggest that cholesterol may be concentrated for delivery to the plasma membrane by being synthesized from a sterol precursor such as lanosterol in a discrete but undefined intracellular membrane.     

Protein synthesis with membrane-bound polysomes and albumin messenger RNA from livers of mutant mice

Summary Investigation of deficiencies in serum protein synthesis resulting from deletion-mutations at the albino locus in mice was continued usingin vitro conditions. Previous work showed that although total protein synthesis was only slightly lower in livers from albinos, newly synthesized protein appearing in plasma was 22% of that in controls. It was thought that the disorganized endoplasmic reticulum and Golgi apparatus, characteristic for the liver (and kidney) of these mutants, might be responsible for the observed deficiencies. In the present study mebrane-bound polysomes isolated from the livers of newborn albinos were 55% (c^3H/c^3H strain) and 62% (c^14CoS/c^14CoS strain) as efficient as those from normal littermates in incorporating radioactive leucine into protein in a cell-free system. These differences could not be eliminated by the addition of excess liver mRNA, exogenous soluble factors or by the exchange of cell sap between albino and control polysomes. In an earlier study albino liver slices synthesized only 13% (or 17% per mg of total protein synthesized) as much albumin as controls. We have now found that the level of albumin poly(A)⁺-RNA isolated from albino livers and assayed with a reticulocyte lysate, was almost as high (85%) as in controls. It was concluded that the very low level of albumin synthesis in albino livers did not result from a deficiency of albumin mRNA. Whether the rate-limiting step in synthesis of albumin in mutant livers is at the level of translation or processing for secretion requires further investigation.     

Secretion of proalbumin by canavanine-treated Hep-G2 cells

The two processing sites in the conversion of preproalbumin to albumin are marked by arginine residues. Therefore, to study the mechanisms of albumin processing and secretion, the arginine residues of nascent albumin were replaced with canavanine by the incubation of Hep-G2 cells with this arginine analog. During a 4-h interval, canavanine inhibited (67%) the secretion of nascent albumin and increased the intracellular transit time of albumin secretion from 24 to 39 min. At 1 h, canavanine inhibited total protein synthesis by 19% and albumin synthesis by about 40%. Both the intracellular and secreted albumin produced by canavanine-treated cells were analyzed by DEAE-cellulose chromatography and were found to be more acidic than normal proalbumin and albumin. Further analysis on sodium dodecyl sulfate polyacrylamide gel electrophoresis indicated that the albumin produced and secreted by canavanine-treated cells appeared to have a larger molecular weight (by 4000) than serum albumin. The canavanine-treated cells were incubated with L-[3H]leucine and L-[3H]phenylalanine and the location of radioactive L-leucine and L-phenylalanine in the 30 NH2-terminal amino acid residues of secreted albumin was determined. The results indicated that canavanine-treated cells secreted proalbumin (79%) and also some fully processed albumin (21%). Preproalbumin was not secreted. Untreated Hep-G2 cells mostly secreted fully processed serum albumin (93%) with only traces of proalbumin (7%).     

Secretion of a major phosphorylated glycoprotein by hepatocytes. Characterization of specific antibodies and investigations of the processing, excretion kinetics, and phosphorylation

Isolated rat hepatocytes secreted a major phosphorylated glycoprotein (PP63) with apparent Mr = 63,000 and isoelectric point ranging from 4.8 to 5.3. Specific antibodies were raised in a rabbit using material obtained from plasma as an antigen. The biosynthesis of PP63 was studied in vitro in a cell-free system and in intact hepatocytes incubated with or without tunicamycin. The mRNA translation product had a Mr = 43,000 and was of the same size as the major unglycosylated precursor found in intact cells. This precursor was rapidly processed into two major intracellular forms of Mr = 53,000 and 56,000. These species were insensitive to neuraminidase but susceptible to endoglycosidase H, indicating that they contained oligosaccharide side chains of the high mannose-type. Terminal glycosylation gave rise to the mature Mr = 63,000 protein that contained sialic acid and fucose. This species represented the exportable form of the protein and was the only one to be phosphorylated. The charge heterogeneity observed for the mature protein already existed in all the precursors, indicating that it could not be ascribed to sialylation or to phosphorylation. However, these covalent modifications were mainly responsible for the acidic character of PP63. PP63 secretion was altered by tunicamycin. Pulse-chase experiments showed that the phosphorylated glycoprotein was secreted according to kinetics similar to that described for other liver glycoprotein, with slower kinetics than albumin. Permanent phosphorylation did not appear mandatory for excretion since the dephosphorylated PP63 was excreted with an efficacy comparable to that of the phosphorylated protein. Phosphorylation of PP63 was shown to occur on a single tryptic peptide, at a serine residue.     

Transhepatic transport of taurocholic acid in normal and mutant analbuminemic rats

To elucidate a possible function of plasma albumin in vectorial transport of various cholephilic organic anions, such as bile acids, plasma clearance and transhepatic transport of radioactive taurocholate were studied in vivo in normal and mutant analbuminemic rats. Intravenous administration of taurocholate was followed by its rapid disappearance from the circulation in both animal groups. However, plasma clearance of taurocholate was significantly larger in analbuminemic (68.3 ml/min per kg of body weight) than in normal rats (29.8 ml/min per kg of body weight) at a dose of 8 mumol/kg of body weight. The increased plasma clearance in analbuminemic rats was accompanied by a more prompt biliary secretion of the ligand than occurred in normal animals; 79 and 42% of the injected dose was recovered in analbuminemic and normal rat bile, respectively, within 10 min after administration. Ultrafiltration analysis revealed that the binding of taurocholate to serum protein(s) was significantly lower in analbuminemic rats as compared with that in normal rat serum; 24 and 76% of taurocholate bound to protein fractions of analbuminemic and normal rat serum, respectively, at 0.5-mM ligand concentration. Binding of taurocholate to cytosolic proteins of normal and analbuminemic liver were similar; 23 and 28% of taurocholate bound to protein fractions from analbuminemic and normal rat, respectively, at 10 mg protein/ml and 20-microM ligand concentration. These results indicate that plasma albumin does not play a role in directing circulating taurocholate to the liver and that transhepatic transport of the bile acid increases with the increase in concentration of unbound ligand in the circulation.     

Effect of local anesthetics on plasma protein secretion by rat hepatocytes

The effects of some local anesthetics on plasma protein secretion by rat liver slices have been studied and have been compared with those of colchicine. Rat liver slices were pulse-labelled with l-[¹⁴C]leucine for 9 min at 37°C, collected on filter paper, washed with non-radioactive leucine and reincubated in the presence or absence of the drug to be tested. The radioactive plasma proteins produced were obtained by immunoprecipitation from either the chase medium or from the washed slices. Chlorpomazine, (3 · 10^?5 M), dibucaine (10^?5 M), lidocaine (10^?3 M) and procaine (5 · 10^?5 M) inhibited both the synthesis and secretion of plasma protein but did not affect the uptake of l-leucine into the slices nor the incorporation of phosphate into intracellular nucleotide phosphates or into phopholipids. The inhibition of secretion elicited by these drugs is probably not due to the inhibition of protein synthesis since cycloheximide, when added to the chase medium at a concentration which completely inhibits protein synthesis, did not inhibit plasma protein secretion, while cycloheximide plus procaine did inhibit secretion and also caused a retention of non-secreted plasma proteins within the slices. Unlike colchicine, howover, procaine did not cause the retained plasma proteins to accumulate in Goli-derived secretory vesicles, but showed a more general effect causing a distribution among several cell fractions.     

Monoclonal antibody DH12 reacts with a cell surface and a precursor form of the beta subunit of the human fibronectin receptor

Monoclonal and polyclonal antibodies were raised against a placenta plasma membrane protein preparation, which was obtained by fractionation on Blue B dye matrix and by HPLC-anionexchange, and which was shown to contain fibronectin receptors. Immunochemical and functional evidence showed that monoclonal antibody DH12 recognized the beta subunit of the human fibronectin receptor on fibroblasts. This monoclonal antibody reacted with two proteins in Western blots and in double immune precipitations of whole cell preparations. Only the higher Mr protein became labeled by surface iodination of intact fibroblasts. The lower Mr protein is thought to be an intracellular precursor of the beta subunit of the fibronectin receptor.     

Secretion of C-reactive protein becomes more efficient during the course of the acute phase response

We studied the kinetics of synthesis and secretion of the acute phase plasma protein, C-reactive protein, in primary hepatocyte cultures prepared from rabbits manifesting differing degrees of the acute phase response to inflammatory stimulus. In cultures prepared from progressively more responsive animals, rate of C-reactive protein secretion increased to a much greater degree than did intracellular C-reactive protein content, resulting in a progressive decrease in the ratio of intracellular content to rate of secretion. This ratio, which represents the time required to secrete the amount of C-reactive protein contained within the intracellular pool, decreased from 18 h in cultures from unstimulated rabbits to 2.5 h in cells from highly responsive animals. In contrast, these ratios for albumin were short and fell within a narrow range (0.8-2.1 h). In pulse-chase labeling experiments, the time required for secretion of 50% of pulse-labeled C-reactive protein varied markedly, ranging from well over 6 h in cells from a minimally responsive animal to about 75 min in cells from a highly responsive rabbit. In contrast, the half-time for secretion of albumin was consistently about 45 min in the same cultures. Taken together, these findings indicate that the process by which C-reactive protein is secreted becomes more efficient during the course of the acute phase response. Recent studies have indicated that secretory proteins pass from the rough endoplasmic reticulum to Golgi at different and characteristic rates, possibly by a receptor-mediated process in which rate of transfer is determined by receptor affinity. We postulate that C-reactive protein secretion is regulated, during the course of the acute phase response, either by alterations in availability of specific receptors or by competition between different secretory proteins for a common receptor.     

Constitutive secretion of serum albumin requires reversible protein tyrosine phosphorylation events in trans-Golgi

Serum albumin secretion from rat hepatocytes proceeds via the constitutive pathway. Although much is known about the role of protein tyrosine phosphorylation in regulated secretion, nothing is known about its function in the constitutive process. Here we show that albumin secretion is inhibited by the tyrosine kinase inhibitor genistein but relatively insensitive to subtype-selective inhibitors or treatments. Secretion is also blocked in a physiologically identical manner by the tyrosine phosphatase inhibitors pervanadate and bisperoxo(1,10-phenanthroline)-oxovanadate. Inhibition of either the kinase(s) or phosphatase(s) leads to the accumulation of albumin between the trans-Golgi and the plasma membrane, whereas the immediate precursor proalbumin builds up in a proximal compartment. The trans-Golgi marker TGN38 is rapidly dispersed under conditions that inhibit tyrosine phosphatase action, whereas the distribution of the cis-Golgi marker GM130 is insensitive to genistein or pervanadate. By using a specifically reactive biotinylation probe, we detected protein tyrosine phosphatases in highly purified rat liver Golgi membranes. These membranes also contain both endogenous tyrosine kinases and their substrates, indicating that enzymes and substrates for reversible tyrosine phosphorylation are normal membrane-resident components of this trafficking compartment. In the absence of perturbation of actin filaments and microtubules, we conclude that reversible protein tyrosine phosphorylation in the trans-Golgi network is essential for albumin secretion and propose that the constitutive secretion of albumin is in fact a regulated process. vesicular trafficking; liver; genistein; pervanadate     

Biosynthesis of properdin

Properdin (P) is synthesized by the human promyelocytic cell line, HL-60, after differentiation with DMSO. The secreted P was physiochemically indistinguishable from purified plasma P. It was polymerized and able to bind to C3IBb-Sepharose but not to C3i-Sepharose. No extracellular precursor was present. The intracellular form, detected between 1 and 4 h after labeling, was similar but had a slightly lower Mr. It also bound reversibly to C3iBb-Sepharose, and polymers could be demonstrated by cross-linking. Pulse-chase experiments suggested the existence of an earlier, but undetectable, intracellular precursor(s). This form could not be immunoprecipitated even when harsh solubilization conditions and/or antibodies against reduced and denatured P were utilized. Studies with endoglycosidases F and H and tunicamycin indicated that the detectable intracellular precursor contains high mannose N-linked carbohydrate that is processed to the complex form before secretion. The sugars are not required for polymerization, secretion, or functional activity, or responsible for the electrophoretic heterogeneity. Polymerization of P is therefore an early intracellular event, perhaps carefully controlled to prevent anomalous aggregation.     

Castanospermine inhibits glucosidase I and glycoprotein secretion in human hepatoma cells.

We studied the effect of the plant alkaloid castanospermine on the biosynthesis and secretion of human hepatoma glycoproteins. The HepG-2 cells, grown in the presence or absence of the alkaloid, were labelled with [2-3H]mannose and then the labelled glycopeptides were prepared by Pronase digestion. This material was analysed by gel filtration on Bio-Gel P-4 before and after treatment with endo-beta-N-acetylglucosaminidase H. Castanospermine caused an accumulation of high-mannose oligosaccharides, by 70-75% over control. The major accumulated product, which could also be labelled with [3H]galactose and was only partially susceptible to alpha-mannosidase digestion, was identified by h.p.l.c. as a Glc3Man9GlcNAc. Thus the alkaloid inhibits glucosidase I in the human hepatoma cells. Analysis of total glycoproteins secreted by the cells into the medium revealed the presence of only complex oligosaccharides in both control and treated cultures, and the amount of the oligosaccharides labelled with radioactive mannose, galactose or N-acetylmannosamine, secreted by treated cells, was decreased by about 60%. The rate of secretion of total protein labelled with [35S]methionine and precipitated from the medium with trichloroacetic acid was inhibited by up to 40% in the presence of castanospermine. Pulse-chase studies utilizing [35S]methionine labelling were performed to study the effect of the alkaloid on secretion of individual plasma proteins. Immunoprecipitation at different chase times with monospecific antisera showed that castanospermine markedly decreased the secretion rates of alpha 1-antitrypsin, caeruloplasmin and, to a lesser extent, that of antithrombin-III. Secretions of apolipoprotein E, a glycoprotein containing only O-linked oligosaccharide(s), and albumin, a non-glycosylated protein, were not affected by the drug. It is suggested that castanospermine inhibits secretion of at least some glycoproteins containing N-linked oligosaccharides, owing to the inhibition of oligosaccharide processing.     

Effect of cytochalasin D on secretion by rat pancreatic acini

Using the model of isolated acini the effect of cytochalasin D (CD) on rat pancreatic secretion in vitro was studied. The influence of CD (0.01-10 micrograms/ml = 0.02-19.7 microM) on amylase secretion and 3H-pulse-labelled protein release was measured under two sets of conditions: (a) basal, and (b) stimulated by 77pM caerulein. Basal secretion was not altered, but stimulated secretion of amylase and 3H-labelled proteins were similarly inhibited by up to 45%. It is concluded that CD affects only exocytosis of zymogen granules and not intracellular transport.     

Biosynthesis of the carbohydrate portion of immunoglobins. Kinetics of synthesis and secretion of [3H]leucine-, [3H]galactose- and [3H]mannose-labelled myeloma protein by two plasma-cell tumours

The kinetics of incorporation of leucine, galactose and mannose into intracellular and secreted myeloma protein, MOPC 21 IgG(1) and MOPC 46 kappa-type light chain, by cell suspensions of two myeloma plasma-cell tumours, MOPC 21 and MOPC 46, were similar. Radioactive galactose was incorporated to over 90% into galactose residues of intracellular and secreted protein, mannose to over 90% into glucosamine and mannose residues of intracellular protein and to over 90% into glucosamine, mannose and fucose residues of secreted protein, but not into galactose residues. The results show that specific residues in the carbohydrate portion of myeloma proteins can be labelled by specific radioactive monosaccharides, and suggest that fucose residues are added, while myeloma protein is in its final stage of secretion from the plasma cell. The kinetics of incorporation indicate at least three sequential precursor-product relationships between different intracellular forms and the secreted form of myeloma protein.     

Fractionation of the rat ventral prostate with respect to isolation and exocytosis of the prostatic secretion protein

The ventral prostrate was fractionated into one mitochondrial and three microsomal fractions. The different fractions were characterized morphologically and chemically. An interesting finding was that upon homogenization the endoplasmic reticulum membranes often turned ‘inside-out’ giving rise to microsomes with ribosomes attached to the inside of the vesicles. The secretion of the protatic secretion was studied by means of isotopic pulse labeling using radioactive leucine. Peak radioactivity in the secretory fluid was obtained at 2 h after injection with a relativity rapid fall. The radioactivity in the secretory fluid displayed a continuous increase up to 8 h followed by a plateau. When prostatic secretion was purified from secretory fluid and microsomes using a Con A-Sepharose column it showed a typical precursor-product relationship with an early peak at 60 min in microsomal prosatatic secretion protein followed by a peak in secretory fluid at 4 h. Vinblastine blocked the release of labeled secretion protein into the secretory fluid, a phenomenon characteristic for secretory proteins which are exocytosed by means of fusion between secretory granules and the plasma membrane. Following intravenous injection of [³H]estramustine, accumulation was seen in the secretory fluid. Some estramustine probably binds to newly synthesized protatic secretion protein and follows the same route of intracellular transport and extracellular discharge as does prostatic secretion protein.     

The identification of the site of action of NN′-dicyclohexylcarbodi-imide as a proteolipid in mitochondrial membranes

Mitochondrial membranes were incubated with NN'-dicyclohexyl[(14)C]carbodi-imide, which irreversibly inhibited the partial reactions of oxidative phosphorylation by 95-100%. Solutions of the membranes were analysed on polyacrylamide gels. Of the radioactivity recovered from the gels 90% was shown to be associated with a single protein of molecular weight about 10000. The radioactive protein and associated phospholipid was solubilized from the membrane by extraction with chloroform-methanol mixtures and was concentrated 50-fold by solvent fractionation and adsorption chromatography on Sephadex LH-20. Several protein-radioactivity peaks were obtained by Sephadex LH-20 chromatography. However, 90-100% of the radioactivity in each peak was shown to be associated with a single protein similar to the major radioactive protein observed in electrophoretograms of the membrane solutions. It is concluded that dicyclohexylcarbodi-imide inhibits mitochondrial oxidative phosphorylation by reacting covalently with a group on this chloroform-methanol-soluble protein. The possible role of this protein in oxidative phosphorylation is discussed.