The kinetics in vivo of the synthesis of albumin-like protein and albumin in rats

Accumulation of newly synthesized serum proteins by cadmium in cultured rat liver parenchymal cells

Effects of cadmium on secretion of the two major serum proteins, albumin and transferrin were studied in primary cultured rat liver parenchymal cells. Exposure to 7 microM cadmium for 8 hr did not inhibit synthesis of both albumin and transferrin whereas secretions of the two proteins were effectively depressed. In the cells exposed to cadmium serum type albumin and proalbumin were accumulated. Cadmium may affect intracellular processing of secretory proteins and also retard the discharge of these proteins resulting in inhibition of protein secretion from the liver parenchymal cells.

Albumin secreted by rat liver bypasses Golgi apparatus cisternae

Albumin was isolated immunologically from various subcellular fractions from livers of adult male rats receiving an intraperitoneal injection of [3H]leucine to investigate the kinetics and pathway of subcellular transfer of newly synthesized albumin during secretion. At appropriate time intervals, livers were excised and fractionated into endoplasmic reticulum and Golgi apparatus. Golgi apparatus were further subfractionated into cisternae and secretory vesicles. In endoplasmic reticulum fractions labeled albumin appeared within 7.5 min of injection of isotope, followed by a rapid decline in specific activity. Albumin in Golgi apparatus was labeled and concentrated in secretory vesicles over 25 min. The radioactivity in albumin per mg total protein was highest in secretory vesicles and insignificant in the cisternal fraction. Labeled albumin was present in serum by 30 min and radioactivity in serum albumin reached a plateau within 60-90 min after injection of isotope. Results provide evidence for the migration of albumin from its site of synthesis on endoplasmic reticulum membrane-bound polyribosomes to its site of secretion into the circulation via the Golgi apparatus. The pathway of albumin transport to secretory vesicles is suggested to involve peripheral elements of the Golgi apparatus. Secretory vesicle formation and maturation required 20 to 30 min for completion, via a mechanism whereby the inner spaces of the central saccules may be bypassed.

Human albumin synthesis via an albumin precursor in liver tissue slices

A percursor of plasma albumin on man was identified in liver obtained from cadaver renal transplant donors. After 30 incubation with 14C-(U) leucine, most of the labeled immunoreactive albumin was identified as proalbumin, as was evident from its characteristic elution profile on DEAE cellulose 52. After an additional 30 min incubation with unlabeled leucine (60 min total), no precursor form could be identified and the label coeluted with serum albumin. These data strongly suggest that human albumin is initially synthesized in a precursor form and is subsequently transformed into serum albumin.

Ultrastructural immunocytochemistry of nascent albumin topology: proposed cytosolic folding and membrane transit of the protein

The relationship of nascent albumin and hepatocyte organelles was studied with the immunoperoxidase reaction in rats given various drugs to alter cellular albumin content. colchicine was used to increase intracellular albumin. Cycloheximide inhibited synthesis but allowed nascent albumin to remain with its ribosome of origin. Puromycin also inhibited synthesis but released albumin from its ribosome. There was no difference in the appearance of attached ribosomes in hepatocytes from saline-injected rats and those given colchicine or cycloheximide. In these cases, membranes of the endoplasmic reticulum were consistently decorated with ribosomes positive for the presence of albumin antigenicity on their cytosolic surface. The cisternal and cytosolic compartments were negative. The situation after puromycin was different. Here the membranes appeared to be denuded of ribosomes and reaction product, indicative of albumin, was present only on the lumenal surface. To determine whether puromycin had caused the release of ribosomes, sections from puromycin-treated cells were stained nonspecifically with uranyl acetate. This showed that the normal amount of ribosomes was still bound but that they could not be seen when a probe specific only for albumin was used. It appears that nascent albumin can associate with its ribosome within the cytosol. Also, apparently after albumin passes through the membrane of the rough endoplasmic reticulum, it remains attached to its lumenal surface. A model incorporating cytosolic folding of albumin followed by its entropic membrane transit is presented.

N-terminal amino acid sequence of proalbumin from inbred buffalo rats

The sequence of radioactively labelled amino acids at the N-terminus of proalbumin was determined by automated Edman-degradation. [3H] Valine, [3H]phenylalanine or [14C]arginine was incorporated into protein in vivo for a time period of 10 min after injection. Since albumin remains unlabelled during this time period (Urban et al., 1976), separation of proalbumin and albumin was not required for this work. Hence, compared to previous methods, a shorter purification procedure could be used which increased the yield of anti-albumin-precipitable protein and reduced the risk of proteolysis. Microsomes were prepared from livers removed 10 min after injection of the radioactively labelled amino acids. A buffer extract of the acetone-dried powder from these microsomes was chromatographed on DEAE-cellulose. All protein obtained after chromatography which could be precipitated with antiserum to serum albumin was isolated by immunoprecipitation and subsequent separation of the antigen-antibody complex. The sequence of radioactive amino acids in this antigen preparation suggests that about 20-25% of proalbumin possessed at the N-terminus the pentapeptide sequence X-Val-Phe-Arg-Arg- whereas 75-80% contained the hexapeptide sequence Arg-X-Val-Phe-Arg-Arg-.

The synthesis and secretion of serum albumin in Morris hepatomas 5123tc and 9121

In vivo, Morris hepatomas 5123tc and 9121 do not secrete serum proteins into the bloodstream. However, they synthesize albumin, at a reduced rate, if compared with liver. The produced albumin accumulates, leading to an enlargement of the Golgi apparatus and the appearance of intracellular vesicles. The proportion and the absolute amount of albumin found within the hepatoma cells is increased compared with liver. As described recently for liver in vivo and for hepatocyte suspensions, albumin is synthesized also in hepatoma cells via a precursor protein. Hepatoma cells in culture secrete albumin into the medium. However, albumin accumulates in the cells with time in culture, indicating again a limited secretory capacity of the hepatoma cell.

Studies on the pre-alpha-lipoprotein of human serum. I. characterization of an albumin-containing lipoprotein

In this study an albumin-containing lipoprotein is isolated by column affinity chromatography and electrophoresis. Its electrophoretic mobility is faster than that of albumin in agarose gel. Immunologically it reacts with anti-albumin and anti-apolipoprotein-A-I. No reaction is obtained against antibodies to any of the other previously known lipoportein families: Apo-AII, Apo-B, Lp(a), Apo-C, LpX, Apo-D or Apo-E ('the arginine rich peptide'). It differs from purified albumin in having higher amounts of glycine, serine and glutamic acid. Moreover, the molecular weight is estimated to 80,000 as compared to 67,000 for albumin.