Tris reversibly inhibits secretion of albumin and alpha-1-antitrypsin at different sites.

We have investigated the effect of the weak base Tris on the processing and secretion of albumin and alpha-1 antitrypsin by hepatocytes in culture. We show that the secretion of both proteins is 90% inhibited by 30 mM Tris. The post-synthetic processing of both proteins is inhibited to the same extent. These effects are completely reversible. Cell fractionation indicates that albumin accumulates in the Golgi, whereas alpha-1 antitrypsin fails to leave the endoplasmic reticulum.

Potassium depletion inhibits the intracellular transport of secretory proteins between the endoplasmic reticulum and the Golgi complex.

In this paper we show that hepatocytes that have been depleted of K+ secrete albumin, alpha-1-anti-trypsin and transferrin at a slower rate than cells to which K+ has been returned. K+ depletion has no effect on the intracellular nucleotide pools, and we provide evidence that the inhibitions of secretion caused by depletion of K+ and depletion of ATP are independent. Studies of the processing of alpha-1-anti-trypsin show that K+ depletion inhibits the formation of the mature form of the protein, but that immature forms are never secreted. In cells to which K+ was returned, secretion of the mature form was restored. This implies that transport is blocked at a point before the proteins reach the processing enzymes. Proteins delayed by K+ depletion are not removed from the secretory pathway, but are free to mix with protein synthesized subsequently. These data are supported by subcellular fractionation experiments, which show that the secretory proteins are delayed before reaching the Golgi complex, and by immunoelectron microscopic studies. These show that in K+-deficient cells the morphology of both the endoplasmic reticulum and the Golgi complex is normal. The secretory proteins are trapped in smooth vesicles that contain reaction product when incubated for glucose-6-phosphatase, a marker for the endoplasmic reticulum.

The processing and secretion of rat serum albumin by oocytes from Xenopus laevis.

Microinjection of rat liver mRNA into Xenopus oocytes led to the synthesis of intracellular proalbumin and the secretion of mature albumin into the incubation medium. The ionophore monensin abolished the secretion of albumin but not the processing of the precursor. A variety of protease inhibitors were added to the incubation medium but there was no detectable inhibition of proalbumin cleavage.

Secretion and biosynthesis of N-acetyl-beta-D-hexosaminidases by human placental tissues in vitro.

Chorionic villi selectively secrete the B isoenzyme and, to a lesser extent, the A isoenzyme of lysosomal beta-hexosaminidase when incubated in vitro, suggesting that this tissue could contribute to maternal serum beta-hexosaminidase B and A activities in vivo. The results make it unlikely, however, that the placenta is the source of the I2 isoenzyme found in maternal serum. Samples of amnion and term chorion laeve secreted predominantly the B isoenzyme whereas first-trimester chorion laeve secreted the A, B and I2 isoenzymes. Amnion and chorion laeve may, therefore, be a source of the A, B and I2 activities found in amniotic fluid. Chorionic villi supported the cycloheximide-sensitive incorporation of [3H]leucine into immunoreactive beta-hexosaminidase thereby providing direct evidence for enzyme biosynthesis. Newly synthesized beta-hexosaminidase was detectable in the medium only after extended incubation and the secretion of beta-hexosaminidase activity continued at normal rates for several hours in the absence of protein synthesis. These results indicate that enzyme synthesis is not a primary factor in the regulation of enzyme secretion.

Subcellular localisation of inositol lipid kinases in rat liver.

The subcellular distribution of the enzymes which phosphorylate phosphatidylinositol sequentially to form phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was investigated in rat liver. We demonstrate that whilst phosphatidylinositol kinase is present in Golgi, lysosomes and plasma membranes, the kinase that forms phosphatidylinositol 4,5-bisphosphate is localised predominantly at the plasma membrane. The role of the inositol lipid kinases in cell function is discussed.

Human Z alpha 1-antitrypsin accumulates intracellularly and stimulates lysosomal activity when synthesised in the Xenopus oocyte.

Microinjection of human liver mRNA from a patient homozygous for alpha 1-antitrypsin deficiency (PiZZ) into Xenopus oocytes led to a 2--10-fold increase in lysosomal activity. Stimulation of lysosomal activity was not observed when mRNA from a normal human liver (alpha 1-antitrypsin PiMM), or water was injected into the oocyte. This lysosomal activity was oocyte derived and was not due to translation products of the human liver mRNA. Thus a protein that accumulates intracellularly in the secretory pathway is capable of stimulating lysosomal activity.

Xenopus oocytes can synthesise but do not secrete the Z variant of human alpha 1-antitrypsin.

Human liver mRNA was prepared from a patient homozygous for alpha 1-antitrypsin deficiency (PiZZ) and from a normal subject (PiMM). Both liver RNAs were microinjected into Xenopus oocytes and alpha 1-antitrypsin identified by immunoprecipitation. The normal M variant of alpha 1-antitrypsin is synthesised and secreted by Xenopus oocytes, the abnormal Z protein is not secreted and an intracellular form accumulates in the oocytes. In the presence of tunicamycin an unglycosylated form of M alpha 1-antitrypsin appears in the incubation medium but no corresponding unglycosylated version of the Z protein is secreted.

Isolation of highly purified Golgi membranes from rat liver. Use of cycloheximide in vivo to remove Golgi contents.

Following administration of cycloheximide to rats in order to deplete the liver of secretory products, Golgi membranes have been isolated largely free of internal contents. These membranes have a high specific activity of galactosyltransferase (400 times that of the homogenate) and are thought to be derived from the trans Golgi. Their phospholipid and polypeptide composition resembles that of Golgi membranes prepared by other procedures but their triacylglycerol and cholesterol contents are greatly reduced. These results conflict with previous reports that trans Golgi membranes are rich in cholesterol.

Analysis and purification of the blood-sinusoidal domain of rat liver plasma membrane.

Methods are described for the analysis and purification of the blood-sinusoidal domains of rat liver plasma membranes using a combination of sucrose and Ficoll density gradient centrifugation. Use has been made of 125I-labelled wheat-germ agglutinin and hormone-stimulated adenylate cyclase to identify the blood sinusoidal fraction, which may be resolved from Golgi and endoplasmic reticulum markers on Ficoll gradients.

Cleavage of human placental lactogen precursor by an enzyme from microbial membranes.

When membranes from Escherichia coli MRE 600 are added to an in vitro translation system, they are able to cleave correctly prehuman placental lactogen to yield the mature hormone. The protein was identified by SDS/polyacrylamide-gel electrophoresis and by determining its amino acid sequence. These studies were aided by the development of a new method for the separation of human placental lactogen from its precursor.

Calcium-dependent Golgi-vesicle fusion and cathepsin B in the conversion of proalbumin into albumin in rat liver.

1. An enzyme from rat liver that converts proalbumin into albumin is described. Partial purification, inhibitor studies and the conditions for maximum activity suggest that the enzyme is cathepsin B. 2. A membrane-bound enzyme, located mainly in lysosomes, also converts proalbumin into albumin. This appears to be a membrane-bound form of cathepsin B. 3. Isolated Golgi vesicles, incubated under conditions suitable for cathepsin B, convert endogenous proalbumin into albumin. 4. This conversion in Golgi vesicles has an absolute requirement for Ca2+ at micromolar concentrations. Mg2+ does not affect or substitute for Ca2+. Both the proalbumin and the albumin formed from it are intravesicular. 5. Converting activity is enhanced by pretreatment with the known chemical fusogen, poly(ethyleneglycol). 6. Vesicles preincubated at pH above 7 in the presence of dithiothreitol show a marked fall in converting activity. This can be partially restored by incubation with native vesicles. These results suggest that vesicle fusion is a requirement for conversion of proalbumin into albumin.

Evidence for the coupling of biosynthesis and secretion of serum albumin in the rat. The effect of colchicine on albumin production.

1. By using isotopic-dilution techniques it was found that colchicine causes a slight increase in the proalbumin content of liver, from 0.63+/-0.06 to 0.83+/-0.10mg/g of liver, but has no effect on albumin content (0.50+/-0.05mg/g of liver). All the proalbumin and 67% of the albumin is found in vesicles from which they are liberated by detergents. 2. Colchicine inhibits secretion of albumin, decreases the rate of conversion of proalbumin into albumin and decreases the rate of incorporation of l-[1-(14)C]leucine into proalbumin. 3. Balance studies in vivo show that all the (14)C appearing in serum albumin can be accounted for by the flow of (14)C through the proalbumin, in the presence or absence of colchicine. 4. When cycloheximide is given to the rats, 2min after [(14)C]leucine, further synthesis of protein stops. The label in proalbumin disappears and the proalbumin content of the liver falls, so as to account for the albumin appearing in the plasma. This occurs both in the presence and in the absence of colchicine. By contrast, there is little change in liver albumin. Studies with isolated perfused livers are in agreement with the above. Lumicolchicine has no effect on any of these systems at doses at which colchicine exerts its action. 5. These results suggest that biosynthesis and conversion of proalbumin into albumin, and secretion of serum albumin are controlled at each step.

Biosynthesis of serum albumin in rat liver. Isolation and probable structure of 'proalbumin' from rat liver.

1. Two methods are described for the preparation of 'proalbumin' in good yields from rat liver. 2. One of the methods does not depend on the use of specific antisera. 3. The product from both methods is identical as judged by electrophoresis on polyacrylamide gel, isoelectric focusing on pH gradients, ion-exchange chromatography and quantitative immunoelectrophoresis. The protein also appears to be radiochemically pure by these criteria. 4. The protein is free from serum albumin as judged by isoelectric focusing and co-chromatography on ion-exchange columns. It is judged to be free from other proteins by these same criteria and by specific precipitation with antibody. 5. It is converted into serum albumin by limited tryptic hydrolysis. The albumin so produced has the same N-terminal (glutamic acid) and C-terminal (alanine) amino acids as reported for rat serum albumin. 6. A hexapeptide is liberated from the N-terminal end of 'proalbumin' simultaneously. It contains three arginine, one phenylalanine, one valine and one glycine residues.

Biosynthesis of serum albumin in rat liver. Evidence for the existence of 'proalbumin'.

1. A protein(s) of rat liver (precipitated from soluble extracts of the microsomal fraction by anti-albumin) yields albumin after limited hydrolysis by trypsin. 2. Evidence that the product of limited tryptic hydrolysis is albumin, is based upon ion-exchange chromatography, electrofocusing and peptide ;mapping'. 3. The albumin ;precursor' is recognized by anti-albumin and is apparently not distinguished from albumin by anti-albumin. 4. A small peptide is liberated from the presumptive albumin precursor during limited tryptic hydrolysis. This peptide is labelled by arginine, but not by leucine, lysine or methionine. 5. These results support our previous suggestion based on kinetic evidence that the albumin-like protein(s), in the anti-albumin precipitate from rat liver, is an albumin precursor.

Intracellular distribution of serum albumin and its possible precursors in rat liver.

1. The fractionation of intracellular albumin labelled with radioactive l-leucine was studied in rat liver by means of isoelectric focusing. 2. Isoelectric fractionation was compared with ion-exchange chromatography for purification of radioactive intracellular albumin obtained by antibody precipitation. Similar results were obtained with both methods of separation. Purified albumin contains only a minor amount of the radioactivity. The remainder is associated with albumin-like protein(s). 3. The albumin-like protein has the properties of a precursor of plasma albumin. 4. The distribution and turnover of radioactive albumin in rough and smooth microsomal fractions and in a Golgi-rich fraction were studied. 5. It is concluded that newly synthesized albumin, as such, appears only momentarily if at all in any intracellular structure before its appearance in the plasma. 6. It is also concluded that the rate-limiting step in the secretion of plasma albumin is the conversion of precursor(s) into albumin. We can find no evidence to suggest that there is any significant transport of albumin, as such, during the course of secretion.

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.

The separation of intracellular serum albumin from rat liver.

1. Antibody precipitation of serum albumin from rat liver extracts yields impure preparations of the protein. 2. When rat liver is labelled with l-[1-(14)C]leucine, antibody precipitation of albumin leads to material that is contaminated with a protein or proteins of very high specific radioactivity. Only 10-25% of the radioactivity of the antibody precipitate is associated with serum albumin. 3. A chromatographic procedure is described that can be used to separate radiochemically pure serum albumin from antibody precipitates obtained from extracts of rat liver. 4. Extracellular albumin secreted by liver slices yields a precipitate with antibody which contains much less radioactive impurity. About 70-90% of the radioactivity is associated with serum albumin. Serum albumin separated by antibody precipitation from rat serum labelled in vivo was not contaminated with the radiochemical impurities associated with intracellular albumin. 5. A simple method is described of obtaining the content of serum albumin in rat liver extracts by the technique of isotope dilution and ion-exchange chromatography.