THE SEQUENTIAL SYNTHESIS OF THE POLYPEPTIDE CHAIN OF SERUM ALBUMIN BY THE MICROSOME FRACTION OF RAT LIVER

Characterization and biosynthesis of cytochrome b(5) in rat liver microsomes

1. Cytochrome b(5) is released from rat liver microsomes by both proteolytic enzymes and by treatments that disrupt phospholipids. Cytochrome P-420 is only released to a marked extent by treatments that disrupt phospholipids. 2. Cytochrome b(5) was isolated in a pure state from both the rough and smooth fractions of rat liver microsomes after treatment with trypsin, and was shown to contain two cytochrome components with identical spectral properties. 3. Amino acid analyses of the two components are presented, together with peptide ;fingerprint' patterns of tryptic digests of the two components. 4. Studies based on the direct isolation of cytochrome b(5) after administration of a single dose of radioactive amino acid to rats demonstrate that the cytochrome is synthesized initially in the rough fraction of microsomes and only subsequently appears in the smooth fraction. 5. Isolated rat liver microsomes are capable of incorporating radioactive amino acids into cytochrome b(5) under standard conditions. 6. Under these conditions the amino acid is incorporated into peptide linkage in the cytochrome.

Isolation and characterization of binder IIIA, a new protein which binds corticosteroid anions

A new protein binding corticosteroid metabolites has been purified over 300-fold from liver cytosols of adrenalectomized rats, treated for 45 min in vivo with [1,2-3H]cortisol. Purification was accomplished by column chromatography on Sephadex G-25, DEAE-Sephadex A-50, Sephadex G-75, and hydroxylapatite. The protein has a Stokes radius of 2.27 nm by gel filtration and an apparent sedimentation coefficient of 3.0 S by sucrose gradient centrifugation. The calculated molecular weight is 30,700. The bound steroid was extracted and has been shown by Sephadex LH-20 chromatography to be a monosulfate derivative of cortisol. Using liver cytosol from adrenalectomized rats pretreated in vivo for 45 min with [1,2-3H]cortisol plus 1000-fold excess competing steroid, cortisol derivatives and progesterone were shown to be the most active competitors. Testosterone and 17 beta-estradiol were least active as competitors. The synthetic steroids, dexamethasone and triamcinolone, produced little or no competition. The protein has been named corticosteroid-anion binder IIIA in keeping with its elution position from a DEAE-Sephadex A-50 column, compared to other binding proteins. Binder IIIA has been separated chromatographically from the glutathione S-transferases (including ligandin) and protein z described by Arias [Levi, A.J., Gatmaitan, Z. and Arias, I.M. (1969) J. Clin. Invest. 48, 21856-21866], both of which have been shown to bind anionic metabolites. It has been resolved from the activities of transcortin, cortisone 5 beta-reductase, and 3 alpha-hydroxysteroid dehydrogenase.

The content and size distribution of membrane-bound and free polyribosomes in mouse liver during aging

The method of Ramsey and Steele [Anal. Biochem., 92 (1979) 305--313] was used to examine the size distribution of membrane-bound and free polyribosomes from the liver of male C57 BL/6 mice over the life span. Optimization of the concentration of Mg2+ and liver cell sap suppressed breakdown by ribonuclease and presumably gave preparations that approximate the integrity of native polyribosome populations. The content of polysomes per unit liver tissue from 7 groups of fed mice aged 10--35 months showed an age-related increase of subunits and monomers (+54%) and dimers-to-pentamers (+76%) in membrane-bound polyribosomes. The dimer-to-pentamer class of free polyribosomes also increased (+52%). Polysomes larger than nonamers showed 13% and 21% decreases that were not statistically significant. Total tissue ribosomes increased 15%, while the membrane-bound/free polyribosome ratio remained nearly constant at about 1.15. In a subsequent study, both 6 h-fasted and fed mice showed decreases (-20% to -33%) in membrane-bound and free polyribosomes larger than nonamers during aging from 15 to 35 months. The dimer-to-pentamer class of free polyribosomes in fed mice increased (+26%), while this class in the other groups underwent increases (+26 to +39%) that were not statistically significant. We conclude that the liver in old mice is not obviously deficient in either quantity or general quality of ribosomes. Old animals do tend to show an increase in small polysomes and a decrease in large polysomes, which is consistent with a reduction in the rate of translation.

The effect of phenobarbital on protein metabolism of liver. Results with isolated hepatocytes

A technique has been devised which makes use of an amino acid alternatively labeled with either [14C] or [3H], and permits the simultaneous evaluation of synthesis, catabolism and secretion by the same sample of isolated rat liver cells during the same time-period of incubation. This technique has been used to study protein metabolism of liver cells isolated from rats treated with 4 doses of phenobarbital (8 mg/100 g body weight) given in the 4 days before killing the animals. Total protein synthesis and secretion do not change in phenobarbital-treated rats; albumin represents 40% of secreted protein in both normal and treated rats. On the contrary, the parameters which indicate protein degradation are lower in phenobarbital-treated than in normal rats, showing that protein catabolism is appreciably reduced in the liver cells obtained from rats treated with phenobarbital.

Thioacetamide effects on protein metabolism in the liver: lessons from isolated hepatocytes

The effects of a short-term treatment with thioacetamide have been studied in isolated hepatocytes obtained from intoxicated rats. A technique has been developed which utilizes leucine alternatively labeled with either [14C] or [3H] and permits the simultaneous evaluation of protein synthesis, catabolism and secretion in the same cell during the same incubation period. The results indicate that short-term thioacetamide treatment causes an overall slowing-down of protein metabolism. Protein synthesis, however, decreases less than protein degradation and total protein secretion; albumin secretion, which is also less than normal, seems to be less compromised than total protein secretion.

Extraction of glycoproteins during tissue preparation for electron microscopy

The extraction of glycoproteins labelled with 3H or 14C precursors: fucose and glucosamine, has been compared during processing of rat liver for transmission electron microscopy. Using 14C labelled fucose, approximately 6% of labelled macromolecules were extracted during processing including 3% during glutaraldehyde fixation, 1% during post-fixation and 2% during dehydration. Greater extraction (8%) occurs with glucosamine as a precursor, which may be attributed to hydrolysis of the more labile sialic acid residues of the glycoprotein molecules. Tritium labelled glycoproteins were more susceptible to extraction than 14C-labelled glycoproteins. The extraction of 3H-labelled glycoproteins as measured by liquid scintillation counting may prove difficult to interpret owing to the quenching from processing solutions.

Effects of neuraminidase on lectin binding by wild-type and ricin-resistant strains of hamster fibroblasts

The nature of cell surface receptors for ricin on wild-type and ricin-resistant variants of baby hamster kidney fibroblasts has been studied. Neuraminidase stimulated ricin binding threefold by wild-type cells, and increased their susceptibility to ricin toxicity as measured by inhibition of [3H]leucine uptake (LD30 fell from 5.0 to 0.5 microgram/mL). Basal ricin binding by ricin-resistant variants (10-300% that of wild type) was also stimulated (2- to 17-fold) by neuraminidase in all seven clonal strains examined; susceptibility to ricin was greatly increased by neuraminidase in these variants. Neuraminidase did not affect the binding of concanavalin A by wild type or a ricin-resistant variant, but decreased the binding of wheat-germ agglutinin by 90% in both cell types. The trivial binding of peanut agglutinin by wild type and a ricin-resistant variant was markedly enhanced (14- to 22-fold) by neuraminidase. Neither collagenase (50 U/mL) nor Pronase (0.0001%) affected ricin binding by wild type or a ricin-resistant variant. These data suggest the existence of "exposed" and "cryptic" oligosaccharide receptors for ricin on the cell membrane glycoproteins of baby hamster kidney fibroblasts. The cryptic ricin receptors probably include at least the sequence D-galactosyl-beta-(1 replaced by 3)-N-acetylhexosamine substituted by sialic acid residues. Exposed and cryptic ricin receptors appear to be different and under separate genetic control.

Biogenesis of plasmalemmal glycoproteins. Intracellular site of synthesis of mouse liver plasmalemmal 5'-nucleotidase as determined by the sub-cellular location of messenger RNA coding for 5'-nucleotidase

1. Free and membrane-bound mouse liver polyribosomes were separated by prolonged density-gradient centrifugation of the post-mitochondrial supernatant. RNA was extracted from free and membrane-bound polyribosomes and mRNA purified by oligo(dT)-cellulose column chromatography. 2. Antisera against purified mouse liver plasma membrane 5'-nucleotidase and moust albumin were prepared and characterized. 3. Microinjection of equivalent amounts of mRNA from free and membrane-bound liver polyribosomes into Xenopus laevis oocytes indicated by immuno precipitation and sodium dodecylsulphate gel electrophoresis a higher proportion of mRNA coding for 5'-nucleotidase and serum albumin in membrane-bound polyribosomes than free polyribosomes. 4. Although small, significant amounts of serum albumin and 5'-nucleotidase were also coded for by mRNA purified from free polyribosomes. The results suggest that in vivo, mRNA in mouse liver membrane-bound polyribosomes codes for the synthesis of 17 times more 5'-nucleotidase than does the mRNA in free polyribosomes.

The isolation of large polysomes in high yield from unfractionated tissue homogenates

It was found that if large quantities of both exogenous RNA and Mg-2+ were present during gentle tissue homogenization, the subsequent addition of deoxycholate to the whole homogenate produced a viscous mass from which polysomes could be isolated in large yields. These polysomes were substantially less degraded than those isolated by previous methods. In the case of rat liver, 15 ribosomes per mRNA was the species present in highest concentration. The parameters of this method were investigated and optimized. About 80 percent of the rRNA in the homogenates was recovered in the polysomes. Omission of deoxycholate permitted the isolation of less-degraded free polysomes as well. In the liver of fed rats these represented one-fourth of the total polysomes, in good agreement with results obtained by an independent approach. Using the method to isolate polysomes from the liver of starving rats, it was found that only about one percent of the large amount of monomers and dimers present resulted from polysome breakdown during isolation. It was further shown that random RNAase hydrolysis of polysomes could not produce the patterns of liver polysomes seen during starvation. Polysomes isolated by this procedure were quite stable in solution and were very active in cell-free protein synthesis. Application of this method without adaptation to eight other tissues also permitted the isolation of large polysomes in high yields.

Synthesis of plasma haemopexin, albumin and fibrinogen by the regenerating rat liver

The concentration of plasma haemopexin was significantly increased 24 and 48 h following both partial hepatectomy and laparotomy in the rat. Increased haemopexin synthesis was demonstrated by studies on the in vivo incoporation of isotope labelled amino acids and in vitro perfusion of the isolated liver. In vivo studies also indicated increased synthesis of plasma fibrinogen. No change was found in the incorporation of amino acids into plasma albumin apart from a transient rise 4 h after laparotomy but albumin production by the perfused rat liver 48 h after partial hepatectomy appeared enhanced. The observed changes in rat haemopexin and fibrinogen synthesis may represent a nonspecific acute phase response to surgical trauma.

The presence of N-terminal methionine on nascent protein of rat liver and rabbit reticulocytes and its cleavage during polypeptide-chain elongation

1. The size of nascent globin peptides from which the N-terminal methionine residue is cleaved has been investigated by comparing the proportion of N-terminal methionine and valine in short and long chains. Nascent chains were labelled in rabbit reticulocyte lysates, fractionated according to length by chromatography on Sephadex G-50, and analysed by the Edman degradation of selected pooled fractions. It was found that different peptide fractions contained either methionine or valine, but not both, as the N-terminal residue. Methionine was present at the N-terminus of globin chains containing up to approx. 50 amino acids whereas valine was found to be the N-terminal amino acid of longer peptides. 2. In similar experiments with nascent proteins of rat liver, labelled either in vivo or in a cell-free system containing microsomal material and cell sap, evidence was obtained for the presence of methionine at the N-terminus of nascent chains up to approx. 65 amino acid residues long. Thus protein synthesis in liver appears to be initiated also by methionine, but in this case cleavage takes place somewhat later during peptide elongation than in globin synthesis.

The control of the form and function of the ribosomes in androgen-dependent tissues by testosterone

1. The ribosome content of the rat ventral prostate gland is controlled by the concentrations of circulating androgens and the polyribosomal complement of the total population of ribosomes is acutely dependent on androgenic stimulation. After the administration of testosterone to castrated rats in vivo, there is a pronounced increase in the amounts of heavy (150-240S) polyribosomes. 2. These results are consistent with a pronounced increase in the mRNA and rRNA content of the prostate gland after the administration of testosterone in vivo. 3. From studies conducted both in vitro, the heavy prostate polyribosomes formed after androgenic stimulation are particularly active in protein synthesis. 4. The androgen-stimulated increase in the formation of prostate polyribosomes has a mandatory requirement for sustained RNA and protein synthesis. 5. Since the androgen-mediated increase in prostate polyribosomes may also be suppressed by the concomitant administration of certain anti-androgenic steroids in vivo, the response in polyribosome formation is probably initiated by the binding of a metabolite of testosterone, 5alpha-dihydrotestosterone, in the prostate gland. 6. The relevance of these findings to the pronounced increase in protein synthesis in androgen-dependent tissues after hormonal stimulation is discussed.

Synthesis of a brain-specific protein (S100 protein) in a homologous cell-free system programmed with cerebral polysomal messenger RNA

Polyribosomes, carrying nascent polypeptide chains, were prepared from whole brain, cortex, and hindbrain-medullary white matter of young adult rats. In a homologous cell-free system, a brain-specific protein (S100 protein) was identified in the mixture of polypeptides released from the polyribosomes during incubation for 1 hr at 37 degrees . De novo synthesis of the S100 protein was achieved in a reconstituted cerebral cell-free system containing polysome-derived mRNA and 40S + 60S subunits. The radioactively labeled S100 protein synthesized in vitro was identified by precipitation with antibody to S100 after addition of purified S100 as a carrier, and migration of the solubilized precipitate on acrylamide gels in the presence of sodium dodecyl sulfate. In vitro synthesis of the S100 protein did not occur in analogous cell-free systems derived from hepatic tissue or in a heterologous system containing liver polyribosomes and cerebral enzymes.

Albumin synthesis by the perfused rat liver. A comparison of methods with special reference to the effect of dietary protein deprivation

1. Isolated perfused rat livers were used to study synthesis of albumin after donors had been fed on normal or protein-free diets. 2. Methods of determining the liver's ability to produce albumin included incorporation of [(14)C]carbonate, [(3)H]lysine and [(14)C]arginine, as well as a direct method based on a heterologous perfusing system of rat erythrocytes and rabbit plasma. 3. Livers from protein-deprived rats were found to form extremely little urea and not to incorporate (14)CO(2) into [(14)C]urea, but they were capable of producing [(14)C]urea from [(14)C]arginine and of incorporating the latter and [(3)H]lysine into albumin. 4. By immunological means these lives were found to synthesize less albumin than normal, but their ability was only slightly impaired when related to body weight or liver weight. 5. These findings are consistent with a block in urea-cycle enzymes with relative integrity of arginase activity and of amino acid activation.

The effect of age on protein synthesis in mouse liver

1. A system of microsomes and 105000g supernatant from livers of old mice is less able to promote the incorporation of [(14)C]phenylalanine into protein than a similar system from livers of young animals. 2. The decrease in [(14)C]phenylalanine incorporation is attributable to changes in microsomes from old animals rather than in the cell-sap fraction. 3. Decreased synthetic ability is found in various classes of microsomes from older animals, namely unfractionated, light and heavy microsomes, but not in detergent-washed ribonucleoprotein particles. 4. Deletions of certain detergent-soluble microsomal proteins accompany the decreased synthetic ability of microsomes from older animals. 5. Microsomes from old mice are less responsive to a synthetic messenger RNA, polyuridylic acid, and this is partly due to a higher rate of hydrolysis in the presence of cell sap from animals of extreme age. 6. Other more direct evidence, from the priming of a cell-free protein-synthesizing system from bacteria and the examination of ribonucleoprotein particles on sucrose density gradients, suggests that senescence is accompanied by a decrease in messenger RNA content.

Preferential synthesis of ferritin and albumin by different populations of liver polysomes

Free polysomes and a mixture of free and membrane-attached polysomes were isolated separately from rat liver, and each was added to a cell-free, protein-synthesizing system. The free polysomes showed a greater capacity than the mixed polysome population for incorporation of (14)C-leucine into ferritin, whereas the reverse was true for (14)C-leucine incorporation into albumin.

Biosynthesis of microsomal nicotinamide-adenine dinucleotide phosphate-cytochrome c reductase by membrane-bound and free polysomes from rat liver

1. NADPH-ferricytochrome c oxidoreductase (EC 1.6.2.3) was purified from the endoplasmic reticulum of rat liver cells. The methods, which involved digestion of membrane with Steapsin, a crude pancreatic extract containing diastase and trypsin, gel filtration and preparative electrophoresis on polyacrylamide, provided an enzyme with a high specific activity in good yield. 2. The incorporation of (14)C-labelled amino acids into the purified reductase by the incubation of various subcellular fractions was studied. The microsome fraction, bound polysomes, free polysomes and detergent-treated polysomes effected the synthesis of the enzyme. 3. The reductase that had been synthesized by the polysomes was tightly bound to preparations of smooth-surfaced endoplasmic reticulum that were added to the incubation medium. 4. Reductase activity could be detected on both free and detergent-treated polysomes. Evidence is presented to show that this activity was due, at least in part, to the presence on the ribosomes of nascent enzyme. The association of enzyme with detergent-treated polysomes did not appear to be due to contamination of the ribosomes with either membrane or cell sap but it is possible for such ribosomes to adsorb some enzyme. 5. The amount of reductase activity associated with the detergent-treated polysomes was increased when the rats from which the polysomes were derived had been previously injected with phenobarbitone. 6. The results are discussed with respect to their relevance for the question of the existence of two functionally different groups of polysomes in the liver and for current ideas on the biogenesis of membranes.

Studies on the biosynthesis of protein by lactating guinea-pig mammary gland. Characteristics of the synthesis of alpha-lactalbumin and total protein by slices and cell-free systems

1. Slices of lactating guinea-pig mammary gland were incubated with radioactive amino acids and the various subcellular fractions separated by centrifugation after disruption of the cells by mincing and homogenization. The most active fraction for protein synthesis appeared to be the ;mitochondrial'. 2. When the subcellular fractions were prepared without previous incubation of the cells and were then incubated with radioactive amino acid and an energy-generating system, the ;mitochondrial fraction' was at least as active for protein synthesis as the ;microsomal fraction'. 3. The ribosomes in the microsomal fraction are mainly unattached to membrane whereas those in the mitochondrial fraction are probably attached to fragments of the rough-surfaced endoplasmic reticulum. This latter fraction contains few mitochondria. 4. The combined mitochondrial and microsomal fractions incorporated radioactive amino acids into alpha-lactalbumin. 5. The radioactive leucine isolated from tryptic and chymotryptic peptides of alpha-lactalbumin synthesized in the cell-free system was not of uniform specific radioactivity. This was consistent with the polypeptide being assembled by the sequential addition of amino acids. 6. Evidence is presented for the polypeptide chain of alpha-lactalbumin being assembled from the N-terminus and for chain initiation in the cell-free system. 7. It is concluded that cell-free extracts of lactating mammary gland synthesize alpha-lactalbumin.