25-Hydroxylation of 1-α-hydroxyvitamin D-3 in rat and human liver

1α-Hydroxyvitamin D-3 25-hydroxylase activity was measured in subcellular fractions of rat and human liver. The formation of 1,25-dihydroxyvitamin D-3 was determined by high pressure liquid chromatography. In rat liver 1α-hydroxyvitamin D-3 25-hydroxylase activities were found in the purified nuclei, the heavy mitochondrial fraction and the microsomal fraction. The enrichment of 25-hydroxylase activity was highest in the heavy mitochondrial fraction. With this fraction a minimum amount (about 0.5 mg) of protein was required before formation of 1,25-dihydroxyvitamin D-3 could be detected. Above this amount the reaction was linear with amount of protein up to at least 2 mg/ml. The reaction was also linear with time up to 60 min. An apparent K_m value of 2·10^?5 M was found. The mitochondrial 25-hydroxylase was stimulated by addition of cytosolic protein or bovine serum albumin. The degree of stimulation was dependent on the amount of mitochondrial protein present in the incubation mixture. Maximal stimulation was seen with 0.2 mg/ml of either protein in the presence of 0.5 mg mitochondrial protein. The stimulating effect remained after heating the protein for 5 min at 100°C. The cytosolic protein did not stimulate a reconstituted mitochondrial 1α-hydroxyvitamin D-3 25-hydroxylase. The mitochondrial vitamin D-3 25-hydroxylase was inhibited both by cytosolic protein and by bovine serum albumin. Human liver revealed only one 1α-hydroxyvitamin D-3 25-hydroxylase activity located to the heavy mitochondrial fraction. The results are in agreement with previous studies on the localization of vitamin D-3 25-hydroxylase in rat and human liver. The difference in localization of the 25-hydroxylase between rat and human liver implies that studies on the regulation of the microsomal 25-hydroxylase in rat liver may not be relevant to the situation in human liver.     

Albumin stimulates the release of arachidonic acid from phosphatidylcholine in hamster lungs

¹⁴C-Arachidonic acid injected into the pulmonary circulation of isolated hamster lungs was effectively incorporated into lung lipids. Once retained the radiolabel was relatively stable but the release of radioactivity increased up to 10-fold when bovine serum albumin (1 %) was added to the perfusate. This efflux of radioactivity was not blocked by quinacrine, a phospholipase A₂ inhibitor. In albumin experiments the released ¹⁴C-araehidonate griginated mainly from the phospholipid fraction in which phosphatidylcholine was the main source of the released radioactivity.Pulmonary infusion of albumin had no significant effect on the amount of ¹⁴C-arachidonic acid in the neutral lipid or free fatty acid fractions of perfused lungs. In experiments with albumin about 80 % of the released radioactivity co-chromatographed with unlabelled arachidonic acid whereas in the absence of albumin only about 20 % of the released radioactivity was unmetabolized arachidonic acid. This study indicates that albumin stimulates the release of arachidonic acid from isolated hamster lungs and that the release is increased mainly from the phosphatidyl choline fraction.     

Quantitative relationships between the ingestion of protein-rich material and ovarian development in the Australian sheep blowfly,Lucilia cuprina (Wied.)

The mature ovaries accounted for about 25% of dry weight and 30% of the protein content of wild-type (anautogenous) females of L. cuprina which had fed ad lib. on liver. The protein content of gravid females was 50% greater than at emergence. Protein ingested during the adult stage, therefore, plays an important role in ovarian maturation. The protein content of the ovaries of females fed measured amounts of liver exudate was from 37 to 52% of the amount of protein ingested.

Only limited ovarian development occurred in females fed only protein (high MW fraction of liver exudate or bovine serum albumin). The presence, in addition, of low MW components (low MW fraction of liver exudate or a salt mixture) was necessary for ovarian maturation. Quantitative feeding showed that the high and low MW fractions of liver exudate were, respectively, superior to bovine serum albumin and the salt mixture, which was based on the cation analysis of the low MW fraction of the exudate.     

Conversion of proalbumin into serum albumin in the secretory vesicles of rat liver

The conversion site of proalbumin into serum albumin was investigated in the subcellular fractions of rat liver labeled with [³H] leucine in vivo. In the cisternae-rich fraction of the Golgi complex as well as in the microsomal fraction most of the labeled albumin was detected as proalbumin, while in the secretory vesicles, which were obtained in increased amount by oral administration of ethanol, more than 70% of the labeled albumin was found as serum type, indicating that conversion of proalbumin into serum albumin occurs within the secretory vesicles in rat liver. Little accumulation of albumin was observed in colchicine-treated rats.     

Effects of cyclic AMP on the kinetics of serum protein synthesis in cultured mouse hepatoma cells

The mouse hepatoma cell line, Hepa, was cultured in the presence of either 1 mM N⁶, O^2′-dibutyryl cyclic AMP (Bt₂cAMP), 0.5 mM 3-isobutyl-1-methylxanthine, or 1 μg/ml cholera toxin. The synthesis and secretion of albumin, α-fetoprotein, and transferrin were elevated above controls by 24 h reaching two- to fourfold stimulations within 72 h. These effects were reversible and were specific for the serum proteins. The stimulation of serum protein synthesis was accompanied by a decrease in the rate of cell proliferation. Protein synthetic parameters were analyzed in Hepa cells 72 h after exposure to N⁶,O^2′-dibutyryl cyclic AMP. The cellular rate of albumin synthesis was increased fourfold and the relative rate of albumin synthesis was increased approximately threefold. N⁶,O^2′-Dibutyryl cyclic AMP did not affect the size distribution of either total Hepa polyribosomes or of albumin-synthesizing polyribosomes. The elongation rate on total mRNA and on albumin mRNA was decreased by approximately 40%. These results indicate that the rate of initiation of total Hepa mRNA and of albumin mRNA also decreased by 40%. The nonspecific nature of the N⁶,O^2′-dibutyryl cyclic AMP effect on Hepa protein synthetic parameters must be due to an alteration in the level of a common substrate, perhaps ATP. The specific threefold increase in the relative rate of albumin synthesis with no alteration in polyribosome sizes requires a threefold increase in the relative amount of functional albumin mRNA in Hepa cells. This prediction was confirmed by cell-free translation of Hepa polyribosomes.     

Activation of apoalkaline phosphatase by serum albumin with Zn2+ in rat hepatoma cells.

Reuber rat hepatoma cells (R-Y121B) cultured at 0.5% serum accumulated apoalkaline phosphatase in intact cells. When R-Y121B cells were cultured in the presence of bovine serum albumin, alkaline phosphatase activity increased in the cells, and the associated increase in enzyme activity differed amongst bovine serum albumin preparations. The treatment of bovine serum albumin with activated charcoal not only enhanced the effect of serum albumin on alkaline phosphatase activity, but also cancelled the differences due to different preparations of serum albumin. In contrast, no effect from serum albumin was observed in the increase of alkaline phosphatase activity in R-Y121B cell homogenates incubated at 37 degrees C. The activated-charcoal treatment of bovine serum albumin increased the amount of Zn2+ bound to the protein. When R-Y121B cells were cultured with bovine serum albumin, the concentration of Zn2+ in the cytosol fraction slightly increased. However, the effect of serum albumin on Zn2+ concentration in the cytosol fractions was independent of charcoal treatment. It was concluded that serum albumin with Zn2+ induces the activation of apoalkaline phosphatase due to Zn2+ binding.     

THE SECRETORY PATHWAYS OF RAT SERUM GLYCOPROTEINS AND ALBUMIN : Localization of Newly Formed Proteins within the Endoplasmic Reticulum

These studies compare the secretory pathways of newly formed rat serum glycoproteins and albumin by studying their submicrosomal localization at early times after the beginning of their synthesis and also by determining the submicrosomal site of incorporation of N-acetylglucosamine, mannose, galactose, and leucine into protein. N-acetylglucosamine, mannose, and galactose were only incorporated in vitro into proteins from membrane-attached polysomes and not into proteins from free polysomes. Mannose incorporation occurred in the rough endoplasmic reticulum, was stimulated by puromycin but not by cycloheximide, and 90% of the mannose-labeled protein was bound to the membranes. Galactose incorporation, by contrast, occurred in the smooth microsome fraction and 89% of the radioactive protein was in the cisternae. Albumin was mostly recovered (98%) in the cisternae, with negligible amounts in the membranes. To determine whether the radio-active sugars were being incorporated into serum proteins or into membrane protein, the solubilized in vivo-labeled proteins were treated with specific antisera to rat serum proteins or to albumin. Immunoelectrophoresis of the ¹⁴C-labeled leucine membrane and cisternal proteins showed that the membranes contained radioactive serum glycoprotein but no albumin, while the cisternal fraction contained all of the radioactive albumin and some glycoproteins. The results indicate that newly formed serum glycoproteins remain attached to the membranes of the rough endoplasmic reticulum after they are released from the membrane-attached polysomes, while albumin passes directly into the cisternae.     

Variations in serum protein fractions following a continuous long term intake of eugynon and lyndiol by iranian women

Serum total protein, albumin and globulin fractions were estimated in 150 subjects in the reproductive stage of life, using lyndiol or eugynon for 1–72 months and results were compared with those obtained from 32 healthy subjects, of the same social class, who had never used oral contraceptives. There was no statistically significant variations observed in serum total protein and γ globulin. Serum albumin decreased in 100% of the subjects, whereas α₁, α₂ and β globulins increased in over 95% of the individuals.     

Production of recombinant human serum albumin from Saccharomyces cerevisiae

Human serum albumin has been constitutively expressed in a Saccharomyces cerevisiae brewing yeast. After cell growth and disruption the product was associated with the insoluble fraction and represented approximately 1% of total cell protein. After the cell debris was extensively washed, the albumin was solubilized with 8 M urea and 28 mM 2-mercaptoethanol in 50 mM sodium carbonate buffer, pH 10. The denatured albumin was refolded by dialysis and further purified by anion exchange and gel filtration chromatography. Losses of renatured material could be reduced, or higher protein concentrations used during refolding, if the denatured product was purified by cation-exchange chromatography in urea prior to refolding. Apart from an additional N-terminal N-acetyl methionine, the refolded product proved identical to human serum albumin derived from plasma when compared by a variety of physical, chemical, and biological analytical methods.     

Interactions of ganglioside GM1 with human and fetal calf sera. Formation of ganglioside-serum albumin complexes

The interactions of ganglioside G_M1 with human and fetal calf sera were studied, the following main results being obtained: (a) G_M1, upon incubation with both sera gave origin to two G_M1-protein complexes, which also occurred after interaction of G_M1 with the albumin fractions prepared from the same sera. Instead no complex formation occurred using the albumin-free fractions. Therefore G_M1 appeared to specifically bind serum albumin and to form G_M1-albumin complexes. (b) G_M1 binding to serum albumin started at ganglioside concentrations surely micellar (above 10^?6 M), was time and concentration dependent, and resulted in a relevant degree of G_M1 complexation (up to 80% of total G_M1 in human serum and up to 18% in fetal calf serum). (c) the binding kinetics appeared, in both serum and the correspondent albumin fraction, to be biphasic: in the first phase, occurring till about 2 · 10^?4 M G_M1, the ratio between bound and total G_M1 increased linearly with increasing G_M1 concentration; in the second phase, occurring above 2 · 10^?4 M, the ratio remained practically constant. After these findings it should be expected that G_M1, when present in serum containing systems, forms complexes with albumin. This should be appropriately considered when studying the effects of exogeneous G_M1 in in vivo and in vitro (tissue cultures) systems.     

Strategy combining separation of isotope-labeled unfolded proteins and matrix-assisted laser desorption/ionization mass spectrometry analysis enables quantification of a wide range of serum proteins

A novel strategy for the quantitative profiling of serum proteome is described. It includes an ammonium sulfate depletion of the serum, an affordable stable isotope labeling chemistry for samples with a large amount of protein, separation of the unfolded proteins, and relative quantification by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). Labeling of unfolded proteins was performed using normal (D₀) acrylamide and deuterated (D₃) acrylamide. The workflow for separating the unfolded proteins includes whole gel elution and ion exchange liquid chromatography, and it combines electrophoretic separation based on the protein molecular weight followed by chromatographic separation in the presence of 8 M urea based on protein charge. This was followed by trypsinolysis and MALDI MS analysis, leading to the quantification of a large number of serum proteins, including those with an abundance of 10^-5 less than albumin. This robust and inexpensive workflow is suitable for the quantitative profiling of protein changes in serum associated with preanalytical variables.     

Proteolysis of canine apolipoprotein by acid proteases in canin liver lysosomes

Canine liver lysosomes were purified by sucrose discontinuous density gradient centrifugation and then ruptured by sonicaton to obtain the soluble fraction. This soluble lysosomal fraction, which contained a 25-fold increase in acid phosphatase activity per mg of total protein when compared with the original homogenate, was incubated with a subfraction (1.110 < d < 1.210 g/cm³, HDL₃) of canine high density lipoproteins (HDL) at pH 3.8. HDL₃ proteolysis by lysosomal proteases, measured as the release of peptides and amino acids by the ninhydrin reaction, followed hyperbolic curves with straight lines (r = 0.99) obtained on Lineweaver-Burk plots. K_m calculated from the Lineweaver-Burk plot was 635 μg of HDL₃ protein per 0.5 ml of incubation mixture. Optimum HDL₃ proteolysis was observed from pH 3.8 to 4.5. Incubation with the other subcellular organelle fractions did not result in HDL₃ proteolysis. To evaluate the effects of enzyme inhibitors, iodoacetate, p-chloromercuribenzoate (both specific for the endopeptidase, cathepsin B (EC 3.4.22.1)) and pepstatin (specific for the endopeptidase, cathepsin D (EC 3.4.23.5)) were tested. Iodoacetate and p-chloromercuribenzoate inhibited HDL₃ proteolysis 100% and bovine serum albumin proteolysis 65%. Pepstatin inhibited HDL₃ proteolysis 45% and bovine serum albumin proteolysis 70%. The in vitro data presented support the hypothesis that hepatic lysosomes play an important role in HDL₃ catabolism in the dog. Furthermore, results obtained from enzyme inhibition studies suggest that a specific lysosomal endopeptidase, cathepsin B, may play the key role in HDL₃ proteolysis.     

The properties of a rat serum protein labelled by the injection of sodium selenite

Properties of a serum selenoprotein which was labelled after the injection of low levels of ⁷⁵SeO₃^2? to rats on a selenium-adequate diet were investigated. When serum was collected three hours after injection the label was incorporated preferentially into one major serum fraction as shown by polyacrylamide gel electrophoresis. Binding was strong, a demonstrated by the fact that very little label was removed by dialysis against 0.60 M NaCl or 0.50 M β-mercapto-ethanol; however, 80% of the ⁷⁵Se was removed by dialysis against 0.50 M NaOH.Molecular weight studies by sodium dodecyl sulfate polyacrylamide gel elecphoresis gave a subunit size of 49 000 under most conditions; when exposed to high concentrations of the detergent (2%) some ⁷⁵Se was associated with a protein having a molecular weight of 25 000. The native selenoprotein eluted before serum albumin on Sephadex G-150 indicating a molecular weight greater than 67 000. The selenoprotein did not coelute with glutathione peroxidase on DEAE-Sephadex A-50.The prior administration of actinomycin D and cycloheximide resulted in a reduction in incorporation of ²⁷Se into serum by 46% and 70%, respectively, which is consistent with the hypothesis that ⁷⁵Se is going into newly synthesized protein. The isoelectric point was determined to be 5.4; when heparin was present, the pI was lowered to 3.2. Treatment with 2 M NaCl did not dissociate the protein · heparin complex, while exposure to 0.25 M β-mercaptoethanol resulted in the dissociation of 60% of the complex.The fact that selenium is so tigthly associated with one serum protein when administered at levels that would be considered normal under most nutritional conditions suggests an important role for this protein, perhaps in the transport of this essential micronutrient throughout the body.     

Characterization of the stimulatory effects of PGF2α on the release of arachidonic acid

Fibroblasts derived from a rat carrageenin granuloma were cultured in the presence of radioactive arachidonic acid, palmitic acid and linoleic acid. More than 90% of each labeled fatty acid was incorporated into a phospholipid fraction by the cells in 18 hrs. Arachidonic acid was evenly incorporated into phosphatidylcholine and phosphatidylethanolamine, while both palmitic acid and linoleic acid were almost entirely incorporated into phosphatidylcholine. The position of phosphatidylcholine where the fatty acids were incorporated was different for each fatty acid. The ratio of the amount of fatty acid incorporated into the 2-position to the amount incorporated into the 1-position of phosphatidylcholine for each fatty acid was >90% for arachidonic acid, 2:1 for palmitic acid and 5:1 for linoleic acid. In the case of phosphatidylethanolamine, most arachidonic acid (>90%) was incorporated into the 2-position. PGF_2^α caused the stimulation of arachidonic acid release but not of palmitic acid and linoleic acid from pre-labeled fibroblasts.The serum in the medium was completely replaceable by bovine serum albumin. The effect of PGF_2^α increased with an increasing concentration of bovine serum albumin, suggesting that serum only acts as a ‘trap’ for released arachidonic acid. The effect of PGF_2^α was greater than bradykinin, and no synergistic effect was seen, although an additive effect was observed.The effect of PGF_2^α depended on the concentration of calcium ions under magnesium-supplemented conditions.     

Radioimmunoassay of arginine-rich apolipoprotein of rat serum.

A double-antibody radioimmunoassay was developed for quantification of rat arginine-rich apolipoprotein in sodium decyl sulfate. Arginine-rich protein, labeled with 125I by the chloramine-T method, had the same chromatographic characteristics on Sephadex G-200 as unlabeled arginine-rich protein and up to 70% of 125I-labeled arginine-rich protein was precipitated by antisera to arginine-rich protein in rabbits. The assay is sensitive at the level of 1-10 ng and has intraassay and interassay coefficients of variation of 5.4 and 6.8%, respectively. The specificity of the assay was established by competitive displacement of 125I-labeled arginine-rich protein from its antiserum by arginine-rich protein and lipoproteins containing this protein, but not by rat albumin or other purified apolipoproteins. Immunoreactivity of rat serum and lipoproteins was complete as demonstrated by comparison with their delipidated form. The accuracy of the immunoassay was further substantiated by comparison with the amount of arginine-rich protein in chromatographic fractions of total apoprotein of very low and high density lipoproteins, and by recovery experiments in ultracentrifugally separated fractions of serum. In contrast to an immunoassay reported previously for rat apo A-I, sodium decyl sulfate was not required for complete immunoreactivity of serum and lipoproteins. The inclusion of sodium decyl sulfate (9 mM final concentration) was necessary, however, for stability of labeled and unlabeled preparations of arginine-rich protein. Content (weight %, means values +/- S.D.), of immunoassayable arginine-rich protein in isolated lipoproteins was 15 +/- 1.5% in very density lipoproteins; 6.8% in low density lipoproteins (1.02 less than d less than 1.04 g/m); 7.1 +/- 0.3% in high density lipoproteins; and 4.8 +/- 0.5% in lymph chylomicrons. Concentration in whole serum was 18.1 +/- 1.4 and 20.4 +/- 2.3 mg/dl for male and female rats, respectively. Only about 55% of arginine-rich protein was recovered in the major lipoprotein classes and about 40% was in "lipoprotein-free" serum (d greater than 1.25 g/ml). Among the lipoproteins, the high density lipoprotein fraction contained twice the amount of arginine-rich protein recovered in very low or low density lipoproteins (26.6 vs. 13.5 and 13.4%, respectively). The significance of the large amount of arginine-rich protein in the 1.25 g/ml infranatant fraction is not apparent. Although repetitive centrifugation did not alter the amount recovered in this fraction, the possibility of an artifact induced by centrifugation and high salt concentration cannot be excluded.     

Immunological state of adult germfree miniature minnesota pigs

Fundamental hematological and immunological data were obtained on sexually mature germfree miniature pigs fed, after the milk diet period, with cereal-type diet sterilized by γ-radiation, and were compared with data of control conventional animals. Germfree adult pigs had a lower count of peripheral blood leukocytes with a lower percentage of neutrophil granulocytes and without any younger forms, a lower total serum protein level with a negligible amount of γ and α₂ globulin fractions and a higher serum albumin and β globulin level. In the mesenteric lymph nodes and in spleen, surface IgA-bearing cells predominated over surface IgG-bearing cells. Also a large amount of IgA-containing cells was found in the intestinal lamina propria, where the IgG cells were present in a negligible amount. IgM cells were the most frequent surface isotype in peripheral blood. The count of blood T lymphocytes was more than doubled.     

The Lack of Selective Reincorporation into Tissue Protein of the Amino Acids Derived from the Catabolism of Serum Protein

Homologous S³⁵-labeled albumin, gamma globulin, and alpha-beta globulin were transfused into rabbits and the specific activities of the electrophoretic fractions of the sera of the recipients were determined at various time intervals up to 12 days after injection. Detectable reincorporation into a fraction other than that transfused was found only in the gamma globulin fraction after albumin injection. This activity rose between 2 and 12 days and reached a level of 2 to 3 per cent of the extrapolated zero time activity of the albumin fraction. When homologous serum protein doubly labeled with I¹³¹ and S³⁵ was transfused into mice, marked drops in the ratios of I¹³¹ to S³⁵ in the serum and tissue proteins were observed between 1 and 48 hours after injection. On the basis of a determination of the absolute and relative amounts of I¹³¹ and S³⁵ found in the various tissue and serum proteins, the amount of reincorporation of S³⁵ into each protein was calculated. The relative amounts of reincorporation of S³⁵ among the various tissues were remarkably similar to the relative amounts of incorporation of S³⁵ after the injection of labeled free amino acids. It is concluded that serum protein does not form a major direct source of amino acids to the tissues but feeds them indirectly through the extracellular pool.     

Enantioselective gallopamil protein binding

The protein binding of the enantiomers of gallopamil has been investigated in solutions of human serum albumin, α₁-acid glycoprotein and serum. Over the range of concentrations attained after oral gallopamil administration, the binding of both enantiomers to albumin, α₁-acid glycoprotein, and serum proteins was independent of gallopamil concentration. The binding to both human serum albumin (40 g/liter) [range of fraction bound (f_b) R: 0.624 to 0.699; S: 0.502 to 0.605] and α₁-acid glycoprotein (0.5 g/liter) (range of f_b R: 0.530 to 0.718; S: 0.502 to 0.620) was stereoselective, favoring the (R)-enantiomer (predialysis gallopamil concentrations 2.5 to 10,000 ng/ml). When the enantiomers (predialysis gallopamil concentration 10 ng/ml) were studied separately in drug-free serum samples from six healthy volunteers the fraction of (S)-gallopamil bound (f_b: 0.943 ± 0.016) was lower (P < 0.05) than that of (R)-gallopamil (f_b: 0.960 ± 0.010). The serum protein binding of both (R)- and (S)-gallopamil was unaffected by their optical antipodes (f_b R: 0.963 ± 0.011; S: 0.948 ± 0.015) indicating that at therapeutic concentrations a protein binding enantiomer–enantiomer interaction does not occur. The protein binding of (R)- and (S)-gallopamil ex vivo 2 h after single dose oral administration of 50 mg pseudoracemic gallopamil (f_b R: 0.960 ± 0.010: predialysis [R] 6.9 to 35.3 ng/ml; S: 0.943 ± 0.016: predialysis [S] 9.5 to 30.7 ng/ml) was comparable to that observed in vitro in drug-free serum. Gallopamil metabolites formed during first-pass following oral administration, therefore, do not influence the protein binding of (R)- or (S)-gallopamil. © 1993 Wiley-Liss, Inc.     

The nature of the nutritional importance of serum albumin to Glossina morsitans

The nutritional significance of albumin protein and its constituent amino acids or associated impurities to Glossina morsitans was evaluated. Flies fed serum-free or albumin-free diets or diets containing delipidated serum or delipidated albumin failed to reproduce. The sizes of offspring produced by flies fed on diets containing different commercial albumins varied in proportion to the amount of bound lipid present in the albumin. FLies fed on albumin-containing diets supplemented with serum lipoproteins produced heavier offspring than flies fed on unsupplemented diets or on diets supplemented by other serum proteins. Delipidation of serum lipoproteins abolished this supplementary effect suggesting a possible similarity between lipoprotein-associated and albumin-bound lipid in terms of their importance to the nutrition and reproduction of tsetse. It is concluded that the observed nutritional importance of albumin to tsetse flies may derive from albumin-associated substances rather than albumin per se.     

Alterations in albumin secretion and total protein synthesis in livers of thyroidectomized rats.

Perfused rat livers and isolated rat hepatocytes exhibited a 50% decrease in the secretion of both albumin and total secretory proteins after thyroidectomy. In contrast, synthesis of non-secretory proteins was decreased by only 20% from the rates observed in liver preparations from euthyroid rats. These observations suggested a disproportionate effect of thyroidectomy on the synthesis of secretory proteins compared with non-secretory proteins. Disproportionate decreases in the synthesis of albumin in other endocrine-deficient states such as hypophysectomy and diabetes had previously been shown to be associated with decreases of similar magnitude in the relative abundance of albumin-mRNA sequences. In contrast, thyroidectomy did not affect the activity or amount of albumin mRNA in total liver poly(A)-containing RNA when assayed by cell-free translation and by hybridization with complementary DNA, respectively. Furthermore, labelling experiments in vivo demonstrated that albumin synthesis represented 12.9 +/- 0.5% and 12.4 +/- 0.4% of total protein synthesis in livers of thyroidectomized and euthyroid rats respectively. Therefore the fall in secretion of albumin and total secretory protein after thyroidectomy did not appear to be a reflection of disproportionate decreases in the synthesis of these proteins. Instead, defects in steps involved in the post-synthetic processing and secretion of albumin are suggested. A number of comparisons, including ribosome half-transit times, the size distributions of total and albumin-synthesizing polyribosomes, and the fraction of RNA present as inactive ribosomes, provided evidence that the overall decrease in protein synthesis after thyroidectomy was not due to generalized alterations in translational processes. Instead, the decrease in total protein synthesis appeared to reflect the RNA content of the liver, which fell in proportion to th decrease in protein synthesis.