Total Liver Protein Synthesis Research Articles

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One of the attributes of amino acid lysine is to manufacture muscle protein, as it is a physiologically essential amino acid for immunity, development and production in birds. This study evaluates the influence of several levels of the essential amino acid lysine for chicks, with age of about 504 hours, maintained at a temperature of approximately 29 degrees Celsius. The basal diet contained 21.0% crude protein and 3,000 kcal of energy of biological transformations/kg. This formula was supplemented with levorotatory-lysine acid monohydrochloride, resulting in diets with 1.040 to 1.280% of total lysine for five experimental treatments. The treatments were found to have a mathematical expression squared influence on animals’ weight gain, conversion of diet, total liver mass and protein synthesis index. The effect of the five experimental treatments was not observed in diet consumption, fat synthesis index, absolute weights of intestines and heart, and percentage of liver mass. It was concluded that the total lysine requirements for chicks aged up to 504 h were 1.240%. Key words: Thermal environment, dietary lysine, growth performance, rearing setting.

Clinical use of albumin in hepatology.

Albumin has been the subject of much discussion and research for a long time. Hippocrates first mentioned some of its physiological properties, but albumin was not named or studied until the early 1800s. The modern use of human albumin was established during World War II due to the demand for plasma substitutes, while the first documented clinical use of human albumin occurred in 1941 in seven sailors severely burned during the attack at Pearl Harbour. The first report of administration of albumin to patients with cirrhosis was published by Janeway et al., who treated six patients with cirrhosis with 25 g albumin per day1. This protein was introduced as a treatment in the 1950s and was incorporated for many years in the management of patients with decompensated cirrhosis. There were, however, always varied opinions among hepatologists in relation to the use of albumin in liver cirrhosis. This review aims to highlight current thinking regarding albumin therapy in the setting of hepatology and also discusses its potential therapeutic applications based on the complex biochemistry of this multifunctional plasma protein. Properties and physiological functions of albumin Human serum albumin is an abundant multifunctional non-glycosylated, negatively charged plasma protein, which is synthesised primarily in the liver and is thought to be a negative acute-phase protein2. In healthy adults, albumin synthesis occurs predominantly in polysomes of hepatocytes (10–15 g/day) and accounts for 10% of total liver protein synthesis. Relatively small amounts of albumin are hepatologically stored (<2 g), the majority being released into the vascular space. Approximately 30%–40% of the albumin synthesised is retained within the plasma compartment. The remaining pool is located in tissues such as muscle and skin. Albumin is not stored hepatically and there is, therefore, no reserve for release on demand3. However, under physiological circumstances only 20–30% of hepatocytes produce albumin and synthesis can, therefore, be increased on demand by 200–300%. Synthesis is a constant process, regulated at both transcriptional and post-transcriptional levels by specific stimuli, but a change in interstitial colloid oncotic pressure is thought to be the predominant regulatory influence4. Albumin homeostasis is maintained by balanced catabolism occurring in all tissues, with most albumin (40%–60%) being degraded in muscles, the liver, and the kidneys. Studies of radiolabelled albumin catabolism in normal healthy young adult males indicate that the protein has a mean half-life of 14.8 days5. Colloid oncotic pressure Human serum albumin accounts for some 60% of the intravascular protein pool in healthy individuals, thereby being responsible for approximately 60% of plasma colloid oncotic pressure. Albumin is also responsible for water retention as the negative charges surrounding the protein molecules attract sodium ions. Its remaining contribution to colloid oncotic pressure is due to the Gibbs-Donnan effect of attracting other active positive ions, further enhancing its water-retaining effect6. In patients with hypoalbuminaemia (especially when it is associated with inflammation or sepsis) whose capillaries are known to be hyperpermeable, the leakage of albumin into the interstitial space draws water with it, producing oedema. Moreover, albumin may influence vascular integrity both directly, by binding in the interstitial matrix and subendothelium and reducing the permeability of these layers to large molecules, and indirectly, through its scavenging properties.

The synthesis rate of albumin decreases during laparoscopic surgery.

In previous studies, a decline in total liver protein synthesis during elective laparoscopic surgery has been observed. However, when albumin synthesis was measured in parallel no apparent influence of the procedure was detected. The aim of the present study was to specifically investigate the effect of a laparoscopic procedure on albumin synthesis. Female (n = 9) patients scheduled for elective laparoscopic cholecystectomy as a consequence of cholecystolithiasis were investigated. The fractional synthesis rate (FSR) of albumin was investigated twice in each patient, before and during surgery (2-3 h apart), employing L-[2H5]phenylalanine and gas chromatography mass spectrometry. The FSR of albumin decreased from 7.3 +/- 1.2% per day before surgery to 6.2 +/- 1.4% per day during the procedure (P<0.01), whereas the corresponding absolute synthesis rates of albumin decreased from 114 +/- 24 to 86 +/- 16 mg kg(-1) day(-1), respectively (P<0.001). In conclusion, the synthesis rate of albumin decreased during a laparoscopic surgery procedure. However, the characteristics for this decrease differ from those previously observed for total liver protein synthesis.

Oral Administration of Leucine Stimulates Ribosomal Protein mRNA Translation but Not Global Rates of Protein Synthesis in the Liver of Rats

The objective of the current study was to examine the role of the branched-chain amino acid (BCAA) leucine in the regulation of hepatic protein synthesis and ribosomal protein (rp) mRNA translation in vivo. Food-deprived (18 h) male rats (200 g) were orally administered saline (control) or 270 mg leucine, isoleucine or valine and killed 1 h later. Administration of any BCAA resulted in enhanced phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein-1 (4E-BP1) compared with controls. However, leucine was the most effective at stimulating phosphorylation of 4E-BP1 as well as the 70-kDa ribosomal protein S6 kinase (S6K1). Despite these effects on components of the translation initiation process, there were no differences in total protein synthesis rates among treatment groups. The distribution of rp (S4, S8, L26) and non-rp (albumin, beta-actin) mRNAs across sucrose density gradients showed that the preponderance of hepatic rp mRNAs in control rats was unloaded from polysomes. Of the BCAA, only leucine was the most effective in causing a shift in the distribution of rp mRNA to polysomes compared with controls. Non-rp transcripts remained mainly polysome-associated under all conditions. These results suggest that leucine is most effective among the BCAA in its ability to stimulate translation of rp mRNA in liver. Furthermore, the translation of rp mRNA is disjointed from rates of total protein synthesis in liver and related to the degree of S6K1 phosphorylation.

Synthesis rate of plasma albumin is a good indicator of liver albumin synthesis in sepsis.

Plasma albumin is well known to decrease in response to inflammation. The rate of albumin synthesis from both liver and plasma was measured in vivo by use of a large dose of L-[(2)H(3)-(14)C]valine in rats injected intravenously with live Escherichia coli and in pair-fed control rats during the acute-phase period (2 days postinfection). The plasma albumin concentration was reduced by 50% in infected rats compared with pair-fed animals. Infection induced a fall in both liver albumin mRNA levels and albumin synthesis relative to total liver protein synthesis. However, absolute liver albumin synthesis rate (ASR) was not affected by infection. In plasma, albumin fractional synthesis rate was increased by 50% in infected animals compared with pair-fed animals. The albumin ASR estimated in the plasma was similar in the two groups. These results suggest that hypoalbuminemia is not due to reduced albumin synthesis during sepsis. Moreover, liver and plasma albumin ASR were similar. Therefore, albumin synthesis measured in the plasma is a good indicator of liver albumin synthesis.

Hepatic protein synthesis in the sheep: effect of intake as monitored by use of stable-isotope-labelled glycine, leucine and phenylalanine.

Rates of protein synthesis for the liver, plasma albumin and total plasma protein were quantified in sheep either offered a supra-maintenance intake or fasted for 3 d. The technique of continuous infusion over a 12 h period was employed with the simultaneous infusion of [1-13C]glycine, [1-13C]leucine and [2H5]phenylalanine. Blood and plasma samples were removed at timed intervals from the hepatic portal and hepatic veins plus the aorta. Enrichments of the free amino acids (AA) were determined in all blood and plasma samples as was the protein-bound AA in an apolipoprotein B100 extract. Protein-bound phenylalanine enrichments were also measured in albumin and total protein from plasma plus samples from liver biopsies. The apolipoprotein B100 enrichments agreed well with those of the free AA in hepatic (and hepatic portal) plasma but were lower than for arterial free AA and greater than liver homogenate free AA. This adds support to the concept that export proteins may preferentially use AA directly from extracellular sources. Intake had no significant effect on constitutive liver protein synthesis and the values agreed well with those obtained by other isotopic approaches. There were, however, significant declines, based on hepatic venous free phenylalanine enrichment, at the lower intake in both the fractional (3.4 v. 4.7% per d; P = 0.024) and absolute (2.4 v. 4.2 g/d; P = 0.011) synthesis rates of albumin, which matched the estimated decrease in total plasma albumin content (52 v. 67 g, P < 0.01). In contrast, there was a smaller reduction in total plasma protein mass (145 v. 151 g, P = 0.035) with no observed significant difference in kinetic parameters. Albumin synthesis was calculated to account for a maximum of 17% of total liver protein synthesis in the fed condition and this may fall to 8% during moderate fasts.

The synthesis rates of total liver protein and plasma albumin determined simultaneously in vivo in humans

Although the metabolism of liver-derived plasma proteins such as albumin has been extensively studied, human hepatic protein synthesis as a whole has not been well characterized, because a reproducible model for obtaining human liver tissue has not been available. In this study, the fractional synthesis rates of total liver protein and albumin in vivo were determined simultaneously in nine subjects undergoing elective laparoscopic cholecystectomy. L-[2H5]phenylalanine (45 mg/kg body wt) was administered for 10 minutes intravenously. Blood samples were collected at regular intervals for 90 minutes and a liver biopsy specimen was taken at 35 +/- 7 minutes. The enrichments of plasma free phenylalanine, plasma albumin, and total liver protein were measured with gas chromatography mass spectrometry (GC-MS). The fractional synthesis rate (FSR) of total liver protein was 24.7 +/- 3.1 %/d (mean +/- SD), and that of albumin was 5.9 +/-1.2%/d. The amount of albumin synthesized per day (absolute synthesis rate, ASR) was 109 +/- 21 mg/kg body wt. No correlation between FSR of total liver protein and ASR of albumin was found. It is concluded that the technique of obtaining liver tissue specimens during laparoscopic surgery may serve as a human in vivo model to study total liver protein synthesis. The fractional synthesis rate of total liver proteins (stationary and exported), equals approximately 25% of the liver protein content daily. Within the range of values of this study, the absolute synthesis rate of albumin was not correlated to the fractional synthesis rate of total liver protein.(Hepatology 1997 Jan;25(1):154-8)

The synthesis rates of total liver protein and plasma albumin determined simultaneously in vivo in humans.

Although the metabolism of liver-derived plasma proteins such as albumin has been extensively studied, human hepatic protein synthesis as a whole has not been well characterized, because a reproducible model for obtaining human liver tissue has not been available. In this study, the fractional synthesis rates of total liver protein and albumin in vivo were determined simultaneously in nine subjects undergoing elective laparoscopic cholecystectomy. L-[2H5]phenylalanine (45 mg/kg body wt) was administered for 10 minutes intravenously. Blood samples were collected at regular intervals for 90 minutes and a liver biopsy specimen was taken at 35 +/- 7 minutes. The enrichments of plasma free phenylalanine, plasma albumin, and total liver protein were measured with gas chromatography mass spectrometry (GC-MS). The fractional synthesis rate (FSR) of total liver protein was 24.7 +/- 3.1 %/d (mean +/- SD), and that of albumin was 5.9 +/-1.2%/d. The amount of albumin synthesized per day (absolute synthesis rate, ASR) was 109 +/- 21 mg/kg body wt. No correlation between FSR of total liver protein and ASR of albumin was found. It is concluded that the technique of obtaining liver tissue specimens during laparoscopic surgery may serve as a human in vivo model to study total liver protein synthesis. The fractional synthesis rate of total liver proteins (stationary and exported), equals approximately 25% of the liver protein content daily. Within the range of values of this study, the absolute synthesis rate of albumin was not correlated to the fractional synthesis rate of total liver protein.

Down-regulation of albumin synthesis in the rat by human recombinant interleukin-1 beta or turpentine and the response to nutrients.

Inflammatory stimuli and provision of nutrients are important factors regulating both total liver protein synthesis and albumin synthesis. The influence of interleukin-1 beta or turpentine injection and a 2-hour infusion of a hypocaloric mixture of glucose and amino acids on the synthesis of total liver protein and albumin was investigated in rats. Total liver protein synthesis was measured by an i.v. flooding dose of L-[2,6(3)H]phenylalanine and albumin synthesis was determined from the labeling of immunoprecipitated albumin and expressed both as a fraction of total liver protein synthesis and as an absolute rate. Interleukin-1 beta or turpentine injection stimulated total liver protein synthesis compared with controls, whereas albumin synthesis, both as a fraction of total liver protein synthesis and as an absolute synthesis rate, decreased. In control animals, the 2-hour i.v. infusion with glucose and amino acids resulted in a significant increase of total liver protein synthesis. Albumin synthesis as a fraction of total liver protein synthesis was not altered, but increased when expressed as an absolute rate in control animals. However, interleukin-1 beta or turpentine injection abolished this response of albumin synthesis to nutrient supply. Total liver protein synthesis increases under inflammatory conditions and remains responsive to nutrient supply. In contrast, albumin synthesis decreases under the same conditions and does not seem to be responsive to short-term i.v. nutrients.

Characterization of mechanisms causing hypoalbuminemia in rats with long-term bile duct ligation

Albumin kinetics and albumin synthesis were studied in rats with chronic bile duct ligation and compared with pair-fed control rats. The plasma albumin concentration was significantly reduced in bile duct ligated rats as compared to control rats, averaging 35±1 vs 40±2 g/l after 2 weeks and 28±3 vs 38±5 g/l after 4 weeks of bile duct ligation. Two weeks after the bile duct ligatio, the transcapillary escape rate of albumin was increased by 60% in bile duct ligated rats, whereas the plasma volume was unchanged. Albumin synthesis expressed as a fraction of total liver protein synthesis, as assessed by the ‘flooding’ dose method using [ 3H]phenylalanine, was decreased by 46% in bile duct ligated rats. However, absolute albumin synthesis expressed per 100 g body weight was not different from control rats. Four weeks after bile duct ligation, the transcapillary escape rate of albumin was no longer different, whereas the plasma volume was increased by 38% in bile duct ligated rats. At this time point, albumin synthesis as a fraction of total liver protein synthesis was decreased by 60% in bile duct ligated rats, and absolute albumin synthesis expressed per 100 g body weight averaged 80±8 vs 53±12 mg/(day×100 g) in control and bile duct ligated rats ( p<0.05). The hepatic steady-state levels of albumin mRNA determined by Northern blot analysis were decreased in bile duct ligated rats at both 2 and 4 weeks after surgery. The studies suggest that reduced plasma albumin concentrations in bile duct ligated rats are caused by increased capillary permeability and lack of compensatory increase in albumin synthesis 2 weeks, and by increased plasma volume and decreased albumin synthesis 4 weeks after surgery.

Is albumin synthesis regulated by the colloid osmotic pressure? Effect of albumin and dextran on albumin and total protein synthesis in isolated rat hepatocytes

In order to study the influence of the colloid osmotic pressure on albumin and total liver protein synthesis, rat hepatocytes were isolated by collagenase perfusion and incubated in Krebs-Ringer-buffer for 4 h. The colloid osmotic pressure produced by different bovine serum albumin (BSA) or dextran 60 concentrations varied from 3 to 80 mm Hg. A physiological colloid osmotic pressure of 20 mm Hg was obtained with 5.7 g BSA or 3.7 g dextran 60 per 100 ml of buffer. Albumin synthesis was measured by Laurell rocket immunoelectrophoresis. Total liver protein and total secretory protein synthesis were determined by the measurement of 1-14C-leucine incorporation. Albumin synthesis was not primarily regulated by the colloid osmotic pressure as was demonstrated by a lack of inhibition after addition of BSA. There was no significant influence of the oncotic pressure on the incorporation of 14C-leucine into total liver proteins. The incorporation into total secretory proteins was inhibited by an increasing colloid osmotic pressure, mediated either by BSA or dextran, suggesting an inhibition of the secretion of plasma proteins other than albumin.

Diabetes-induced alterations in liver protein synthesis. Changes in the relative abundance of mRNAs for albumin and other plasma proteins.

Mechanisms responsible for diabetes-induced alterations in liver protein synthesis were investigated in vivo and in perfused liver using Bio-Breeding Worcester (BB/W) control rats, spontaneously diabetic BB/W rats maintained on insulin therapy, and diabetic BB/W rats withdrawn from insulin therapy for 48 h. Withdrawal of insulin therapy in the diabetic rats resulted in marked alterations in a number of parameters related to liver protein synthesis compared to BB/W control or insulin-maintained diabetic rats. Alterations seen in vivo following withdrawal of insulin included changes in the relative concentrations of several plasma proteins, a 40% reduction in total liver RNA relative to DNA, a 5-fold reduction in albumin synthesis relative to the synthesis of total liver proteins, a 5-fold reduction in albumin mRNA relative to total RNA, reductions in the relative abundance of mRNAs for at least four plasma proteins other than albumin, and a relative increase in mRNA for at least one plasma protein. Alterations observed in perfused liver included reductions in total liver protein synthesis (60% of control), albumin production (24% of control), and total secretory protein production (44% of control). All parameters studied were essentially unchanged from BB/W control values when the diabetic rats were maintained on insulin therapy. The results indicate that insulin deficiency leads to marked reductions in liver protein synthesis, particularly the synthesis of albumin and other plasma proteins. The mechanisms responsible for these alterations include changes in the relative abundance of specific mRNAs and a decrease in total cellular RNA.

Regulation of N‐Acetylglutamate Synthetase in Mouse Liver

N‐Acetyl‐l‐glutamte synthetase catalyzes the synthesis of N‐Acetyl‐l‐glutamte, an allosteric and essential activator of carbamoyl‐phosphate synthetase I in the liver of uretelic animals. The enzyme is activated specifically by arginine. Mice were fed laboratory chow (protein content, 21%) for 3 h and, after various times, N‐Acetyl‐l‐glutamte synthetase activity in the sonicated mitochondria was measured both in the presence and absence of 1 mM l‐arginine. The activity in the absence of arginine did not significantly change after the feeding, while the activity in the presence of arginine increased with time and reached a maximum (fivefold increase) 9 h after the start of feeding, then decreased gradually. The ratio of the activity with arginine to that without arginine was 1.6 at the start of feeding and 5.5 at 9 h. Similar posprandial changes were observed with the mice on a protein‐free diet, when these mice had previously been on a protein diet. No such change was seen with a protein‐free diet when the mice previously given a protein‐free diet for a few days. Cycloheximide injection, which inhibited the total protein synthesis in the mouse liver by 90%, did not inhibit the postprandial increase in arginine activation of the synthetase. Thus, protein synthesis is presumably not required for the change. The enzymes with a low and a high arginine sensitivity were purified over tenfold from mitochondrial extracts of fased and fed animals respectively. The extent of arginine activation of the enzymes remained unchanged, low or high, throughout the purification. In Sephacryl S‐300 chromatography both types of the enzyme were eluted as a single peak and corresponded to a Mr of 220000–250000. the change in arginine activation may be due to a modification of the enzyme molecule without a gross change in the molecular weight. The diet‐dependent change in sensitivity of the synthetase to arginine presents a new type of regulation of this enzyme and this regulation may contribute to the prevention of a rapid decrease in heptic acetylglutamate levels after food ingestion.

Biogenesis of the mitochondrial enzyme, carbamyl phosphate synthetase Appearance during fetal development of rat liver and rapid repression in freshly dispersed hepatocytes

Synthesis of carbamyl phosphate synthetase was undetectable in fetal rat liver at 16 days gestation but by 4–5 days after birth (11–12 days later), this single protein accounted for approx. 5% of total liver protein and roughly 1% of total liver protein synthesis. Likewise, translatable mRNA coding for the enzyme was absent from 16-day fetal livers but then rapidly accumulated reaching maximum levels just after birth. The in vitro primary translation product of carbamyl phosphate synthetase mRNA corresponded to a higher molecular weight biosynthetic precursor of the enzyme; peptide maps obtained from the precursor synthesized both in vivo and in vitro and from the mature enzyme made in vivo were the same. When livers of neonatal rats were perfused with collagenase and further treated to yield a preparation of freshly dispersed hepatocytes highly active in general protein synthesis, a procedure which took about 45 min to complete, biosynthesis of carbamyl phosphate synthetase was found to be completely absent in these cells. The mRNA coding for the enzyme, however, could be extracted from the dispersed hepatocytes and was actively translatable in vitro, at levels approximately 75% of those for mRNA obtained from intact liver. Repression of biogenesis of carbamyl phosphate synthetase in dispersed hepatocytes, therefore, must involve a mechanism which shifts the mRNA coding for the enzyme out of active polysomal complexes and renders it further untranslatable in vivo but not in vitro.

The effect of starvation on the rate of protein synthesis in rat liver and small intestine

1. A method is described that allows for measurement of protein synthesis in liver and intestine in the rat. By injecting a massive amount of [14C]leucine (100 mumol/100 g body wt.) an attempt has been made to over come problems of precursor specific radioactivity and problems arising from the breakdown of labelled protein that are encountered when tracer amounts of amino acids are used. 2. Starvation for 2 days resulted in decline in the rate of total liver protein synthesis from 87%/day to 62%/day. 3. In jejunal mucosa the rate of protein synthesis was 136%/day. This declined to 105%/day after 2 days of starvation.

PROTEIN SYNTHESIS IN LIVER, MUSCLE AND GILL OF MULLET (MUGIL CEPHALUSL.)IN VIVO

1. Protein synthesis in mullet was measured by use of pulse injections of 14Cleucine into the hepatic portal circulation and by 4-hr continuous infusion of 14Ctyrosine into the dorsal aorta. 2. Liver protein synthesis was 37 mg/day per g wet weight of tissue. Plasma proteins accounted for about 25% of total liver protein synthesis. Replacement rate for intrahepatic proteins was 20%/day. Average polypeptide chain synthesis time was 3 min. 3. Gill protein synthesis was comparable to that in liver with a replacement rate of 23%/day. 4. Muscle protein synthetic rate was 0.54%/day, comparable to rates in rabbit and sheep when temperature difference is taken into account. 5. Constant infusion is shown to be a feasible technique for measurement of protein metabolism of fish in vivo.

Effect of Low Lysine Diet on Rat Protein Metabolism

Prolonged lysine deficiency resulted in inhibition of growth of the rat and in lowered levels of albumin and beta-globulins, but not of total liver proteins. The apparent incorporation of [3H]lysine into total and chromosomal liver proteins, however, was increased in lysine deficient rats relative to pair-fed controls. The concentration of free lysine in neither serum nor liver was altered in lysine deficient rats. The specific radioactivity of liver free lysine 2 hours after administration of [3H]lysine was noticeably elevated in the deficient rats. However, when the pool of liver free lysine is considered, synthesis of all proteins studied was inhibited. The largest inhibitory effect was seen in serum albumin and total liver protein synthesis. Inhibition of snythesis of nuclear chromatin protein was less pronounced. The data indicate an adaptive mechanism operates to preserve liver lysine in rats fed lysine deficient diets for a prolonged period. In spite of retention of lysine, the synthesis of liver and serum proteins was inhibited.

The effect of dietary protein deficiency on albumin synthesis and on the concentration of active albumin messenger ribonucleic acid in rat liver.

In rats fed on a protein-deficient diet, albumin synthesis as a percentage of total liver protein synthesis falls from the normal value of approx. 15% to about 8%. We have extracted total cytoplasmic RNA from individual rat livers and measured the concentration of active albumin mRNA by translation in a reticulocyte lysate system from which the endogenous mRNA had been removed [Pelham & Jackson (1976) Eur. J. Biochem. 67, 247-256]. In this messenger-dependent system it is possible to measure the synthesis of albumin as a proportion of the overall protein synthesis promoted by the addition of the hepatic RNA. The results show that the concentration of translatable albumin mRNA in samples of total cytoplasmic RNA from livers of protein-deficient rats is decreased markedly. These findings suggest that dietary protein supply affects selectively the synthesis and/or functional stability of albumin mRNA in rat liver.

Synthesis and secretion of rat albumin in vivo, in perfused liver, and in isolated hepatocytes. Effects of hypophysectomy and growth hormone treatment.

The effects of hypophysectomy on albumin and total protein synthesis in rat liver were investigated in vivo, in perfused liver, and in isolated hepatocytes. In all systems, hypophysectomy resulted in about a 50% decrease in the rate of total protein synthesis and a 30 to 50% decrease in the relative rate of albumin synthesis. Albumin synthesis accounted for 11 to 13% of total protein synthesis in all normal systems, but represented only 5 to 8% of the total in all systems derived from hypophysectomized rats. Growth hormone, administered subcutaneously to hypophysectomized rats for 5 days, restored the relative rate of albumin synthesis to normal in vivo; however, only partial restoration was demonstrated in the in vitro systems. Perfused livers and isolated hepatocytes exhibited linear rates of total protein and albumin secretion for 3 h. The rate of albumin secretion by normal perfused livers was 3 times that of perfused livers from hypophysectomized animals, being 0.54 and 0.17 mg/g of liver/h, respectively. Isolated hepatocytes synthesized total protein and albumin at nearly the same rate as perfused livers. The amount of albumin secreted by cells derived from normal and hypophysectomized rats was 0.38 and 0.10 mg/ml of packed cells/h, respectively. Ribosome half-transit times for albumin and total liver protein were 1.6 to 1.7 min in isolated liver cells derived from both normal and hypophysectomized rats. Analysis by polyacrylamide gel electrophoresis showed no difference in the qualitative distribution of the proteins secreted by perfused livers and isolated hepatocytes.

Effect of hypophysectomy on the synthesis of rat liver albumin.

Hypophysectomy of adult rats results in approximately a 50% decrease in the rate of albumin synthesis relative to total liver protein synthesis. This decrease is accompanied by a proportional decline in the number of albumin-synthesizing polysomes, as determined by the binding of 125I-Labeled anti-albumin antibody, and indirect immunoprecipitation of [3H]leucine-labeled albumin-synthesizing polysomes. Furthermore, this decrease is associated with an equivalent reduction in the amount of total membrane-bound polysomes, whereas total free polysomes show little quantitative change. The size of the specific albumin-synthesizing polysomes, as well as the size of the total polysomes, however, appear to be the same following hypophysectomy as in the normal untreated animal. These observations are consistent with the finding that the relative amount of albumin mRNA activity also decreases approximately 50%, as assayed in a heterologous cell-free protein-synthesizing system using exogenous liver RNA prepared from either isolated polysomes or total liver homogenates. The decrease in albumin production in the hypophysectomized rat, therefore, is apparently the result of a reduction in the amount of active albumin mRNA. The concomitant decrease in albumin-synthesizing polysomes appears to reflect a similar reduction in the amount of total membrane bound polysomes. Thus, a major physiological defect in hypophysectomy may be a preferential decline in membrane-bount polysomes accompanied by a reduction in mRNA levels, which is represented by the decrease in albumin synthesis.