Multiple forms of rat-serum alpha 1-protease inhibitor. Involvement of sialic acid in the multiplicity of three original forms

The isolation and partial characterization of alpha 1-proteinase inhibitor from the serum of the ostrich (Struthio camelus)

1. Native and cleaved alpha 1-proteinase inhibitor was purified from ostrich serum using Sepharose-blue dextran chromatography, ammonium sulfate precipitation and ion exchange chromatography on DEAE-Toyopearl 650 M at pH 8.8 and 6.5. Ostrich alpha 1-PI displayed M(r) values of 68,100 using gradient PAGE and 66,200 using Ferguson plots. Isoelectric focusing of ostrich alpha 1-PI in the pH range 3-10 revealed pI values of 4.84 and 4.91, and in the pH range 4-6 the characteristic microheterogeneity observed for mammalian alpha 1-PIs was displayed. The presence of sialic acid, hexoses and hexosamines was detected using chemical methods, but were found in much lower quantities as compared to alpha 1-PIs of other species. Western blot analysis demonstrated a positive reaction between the native and cleaved ostrich alpha 1-PIs and the antibodies to the ostrich alpha 1-PIs raised in rabbits. No cross-reactivity was demonstrated by Western blot analysis between human alpha 1-PI and antibodies to ostrich alpha 1-PI. The inhibitory effect of alpha 1-PI on elastase and chymotrypsin was also investigated.

Caprine plasma proteinase inhibitors--I. Partial characterization

1. Using two-dimensional electrophoresis (IEF, pH 3.5-6.0 and PAGE, 11.5% T, pH 7.9) the caprine plasma proteinase inhibitors were classified into six distinct classes, designated PIA, PIB, PIC, PID, PIE and PIF. Differentiation of the six inhibitors was based on electrophoretic criteria, their abilities to inhibit bovine trypsin and chymotrypsin and their crossreactions with antisera to human alpha 1-antitrypsin and alpha 1-antichymotrypsin. 2. Polymorphic variants were identified for five of the protein systems (PIA, PIB, PIC, PID and PIE) and the electrophoretic data indicated that the variants were controlled by allelic genes. PIF proteins were poorly resolved and invariant. 3. Treatment of selected plasmas with neuraminidase demonstrated that the microheterogeneity observed in the PIA, PIB, PIC and PID proteins was attributable to sialic acid additions. 4. The inhibitory activities of all six caprine proteinase inhibitors were unaffected by chemical oxidation with chloramine-T.

Cloning, structure and expression of cDNA for mouse contrapsin and a related protein

A cDNA clone (lambda MC-2) for contrapsin, a serine-proteinase inhibitor, was isolated from a lambda ZAP mouse liver cDNA library. The 1.6 kb cDNA insert of lambda MC-2 contained an open reading frame that encodes a 418-residue polypeptide (46,970 Da), in which a signal peptide of 21 residues was identified by comparison with the N-terminal sequence of the purified protein. The predicted structure (MC-2) also contained other peptide sequences determined by Edman degradation. Four potential sites for N-linked glycosylation were found in the molecule, accounting for the difference in molecular mass between the predicted form and the purified protein (63 kDa). Further screening of the cDNA library with an EcoRI-EcoRI fragment (510 bp) of lambda MC-2 as a probe yielded another cDNA clone (lambda MC-7), which encodes a 418-residue polypeptide (MC-7) with a calculated mass of 47,010 Da. MC-2 showed 83% similarity at the amino acid level to MC-7, in contrast with 44% similarity to alpha 1-proteinase inhibitor. The possible reactive site (P1-P'1) for serine proteinase is suggested to be Lys-Ala for MC-2 and Ser-Arg for MC-7. Northern-blot analysis revealed that both MC-2 and MC-7 mRNAs have the same size of 1.8 kb and are markedly induced in response to acute inflammation. Construction of the expression plasmids pSVMC-2 and pSVMC-7 and their transfection into COS-1 cells demonstrated that pSVMC-2 directs the synthesis of a 63 kDa form whereas pSVMC-7 expresses a 56 kDa form. The difference in molecular mass between the two may be explained by the fact that the MC-7 sequence contains three potential sites for N-glycosylation, one site less than that of MC-2.

Evaluation of assays for detecting alpha-1-protease inhibitor during purification from rat serum

We purified the R1 alpha-1-protease inhibitor from rat serum and developed a convenient assay for its detection during purification procedures. Purification was accomplished by desalting, DEAE-Sephacel, zinc chelate, and reactive green-agarose columns. The resultant antiprotease had a molecular weight of 54,000 and inhibited elastase, chymotrypsin, and trypsin. By isoelectric focusing, five bands were produced with pI values from 4.3 to 4.7. Functional assays utilizing protease substrates imbedded in agarose plates were evaluated for the ability to distinguish the R1 alpha-1-protease inhibitor from the other serum antiproteases eluted in column chromatography fractions. This technique of screening for anti-protease activity was compared to conventional spectrophotometric methods and was found to correlate well when quantifying inhibition of elastase and chymotrypsin, but not trypsin. The presence of alpha-1-protease inhibitor was most reliably detected by testing for anti-elastase activity. Technician time and expense were saved by employing protease substrate plates to test chromatogrpahy fractions. This technique may facilitate purification of other protease inhibitors.

Purification, characterization, and acute phase response of plasma alpha-1-antiproteinase in the hamster, Mesacricetus auratus

1. alpha-1-Antiproteinase (also called alpha-1-antitrypsin or alpha-1-proteinase inhibitor) with a molecular mass of 60 kDa was purified to apparent homogeneity from hamster plasma. 2. It inhibited elastase, chymotrypsin and trypsin, but did not significantly affect pancreatic kallikrein, plasma kallikrein or plasmin. 3. It has the same N-terminal heptapeptide sequence as that of rat alpha-1-antiproteinase. 4. Its plasma level decreased after injection of bacterial lipopolysaccharide.

Mammalian alpha 1-antitrypsins: comparative biochemistry and genetics of the major plasma serpin

1. Human alpha 1-antitrypsin (alpha 1 AT) has been extensively characterized and reviewed. It is the archetypal member of the superfamily of serine proteinase inhibitors, the serpins. As human alpha 1-antitrypsin exhibits a relatively high concentration in plasma and is usually the highest concentration serpin, it can be referred to as the major plasma serpin. 2. alpha 1-Antitrypsin from species other than man has been characterized for two major reasons: (1) for use in a model animal system to assist with the study of the human alpha 1 AT deficiency disease; and (2) to find polymorphism for use in gene mapping and linkage studies or for parentage analysis. 3. The diverse range of reasons for studying alpha 1AT has yielded a vast array of literature that is often not well cross-referenced. 4. The characteristic features of alpha 1AT in all species examined to date will be presented with a view to examining which features are important structurally and functionally from an evolutionary perspective. 5. In mouse, horse, rabbit and guinea pig, multigene families which appear to have arisen from alpha 1AT have been found. The functional and evolutionary implications of these paralogous genes will also be discussed.

Intracellular processing of complement pro-C3 and proalbumin is inhibited by rat alpha 1-protease inhibitor variant (Met352----Arg) in transfected cells

Complement C3, when its cDNA was transfected into COS-1 cells, was synthesized as a precursor, pro-C3, which was intracellularly processed into the alpha and beta subunits, although not completely. A cDNA for rat alpha 1-protease inhibitor (alpha 1-PI) was mutated in vitro to encode its variant with the modified active site (Met352----Arg). In cells co-transfected with the mutant alpha 1-PI cDNA and the C3 cDNA, pro-C3 expressed was secreted without being processed into the subunits. Co-transfection of the mutant alpha 1-PI cDNA and the albumin cDNA also resulted in the inhibition of intracellular conversion of proalbumin into serum-type albumin. No inhibition of the processing of each preform was observed in cells co-transfected with the normal alpha 1-PI cDNA. Taken together, the results indicate that the alpha 1-PI variant (Met352----Arg) expressed inhibits specifically an intracellular enzyme which is involved in the proteolytic processing of both pro-C3 and proalbumin.

Primary structure of human placental 5'-nucleotidase and identification of the glycolipid anchor in the mature form

A cDNA was cloned coding for human placental 5'-nucleotidase. The 3547-bp cDNA contains an open reading frame that encodes a 574-residue polypeptide with calculated size of 63 375 Da. The NH2-terminal 26 residues comprise a signal peptide, which is followed by the NH2-terminal sequence of the purified protein. four potential N-linked glycosylation sites are found in the molecule, accounting for a larger mass of the mature form (71 kDa). The predicted structure contains a hydrophobic amino acid sequence at the COOH terminus, a possible signal for the post-translational modification by glycophospholipid. To confirm this possibility, we tried to isolate and characterize the membrane-anchoring domain of 5'-nucleotidase. BrCN-cleaved fragments of the protein were extracted with hexane and subjected to HPLC, resulting in purification of a single component of 2.3 kDa. Chemical analyses revealed that the purified fragment contains the tetradecapeptide Lys-Val-Ile-Tyr-Pro-Ala-Val-Glu-Gly-Arg-Ile-Lys-Phe-Ser, ethanolamine, glucosamine, mannose, inositol, palmitic acid, and stearic acid. The peptide sequence determined is identified at positions 510-523 in the primary structure deduced from the cDNA sequence, which predicts a further extension to position 548, containing the hydrophobic amino acid sequence. Thus, it is concluded that the mature 5'-nucleotidase lacks the predicted COOH-terminal peptide extension (524-548), which has been replaced by the glycophospholipid functioning as the membrane anchor of 5'-nucleotidase.

Brefeldin A arrests the intracellular transport of viral envelope proteins in primary cultured rat hepatocytes and HepG2 cells

We have studied the effect of brefeldin A (BFA) on the intracellular transport of the envelope proteins of vesicular stomatitis virus (VSV) and sindbis virus in primary cultured rat hepatocytes. BFA (2.5 micrograms/ml) inhibited not only the secretion of plasma proteins into the medium, but also the assembly of both G protein of VSV and E1 and E2 proteins (envelope proteins) of sindbis virus into respective virions. Concomitantly, both the acquisition of endo-beta-N-acetylglucosaminidase H resistance by the G protein and the proteolytic conversion of PE2 to E2 were found to be inhibited in the BFA-treated cells, suggesting that the intracellular transport of the envelope proteins was arrested in the endoplasmic reticulum. Such inhibitory effects of the drug were variable depending upon the culture conditions of the hepatocytes. In the 1-day-cultured cells, even in the presence of the drug, newly synthesized envelope proteins were assembled into the virions after a 3 h chase period, at the same time as secretion of plasma proteins into the medium resumes. In contrast, in 4-day-cultured hepatocytes, BFA continuously blocked the entry of the envelope proteins into the virions and the release of plasma proteins into the medium for at least 5 h. BFA also completely inhibited the exocytotic pathway in HepG2 cells. These results indicate that the duration time of the effect of BFA is different from one cell to another and may change depending upon the culture conditions of the cells.

Brefeldin A inhibits the targeting of cathepsin D and cathepsin H to lysosomes in rat hepatocytes

Effect of brefeldin A on the transport of lysosomal acid hydrolases (cathepsins D and H) was investigated in primary cultured rat hepatocytes. Both cathepsins were synthesized as proenzymes and progressively converted to mature enzymes in the control cells. However, BFA strongly inhibited the appearance of the mature enzymes in the cells in a dose dependent manner, suggesting that transport of newly synthesized lysosomal enzymes from the endoplasmic reticulum to lysosomes is blocked by the drug. The inhibitory effect by brefeldin A was reversible. Upon recovery from brefeldin A-intoxication, procathepsin D was effectively targeted into lysosomes, whereas a substantial amount of procathepsin H was found to be missorted, resulting in its secretion into the culture medium.

Proline-rich protein is a glycoprotein and an acute phase reactant

Proline-rich protein (PRP) is a plasma protein associated with lipoproteins. In an attempt to clarify the biological significance of this protein, we isolated and characterized it and studied the biological role in plasma. PRP was isolated by immunosorber column chromatography and by gel filtration and ion-exchange chromatography. The molecular weight determined by gel filtration chromatography was 352,000, that is, about 5-times larger than the weight determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (73,800), indicating pentamer formation. About 10 or 11 isoproteins (pI 5.89-6.55) were observed by isoelectric focusing gel electrophoresis. PRP contained fucose, mannose, galactose, glucosamine and sialic acid accounting for 8.0% of the dry weight. PRP also had a hydrophilic property, as determined by charge shift electrophoresis. Levels of this protein in the human serum related to triacylglycerol-rich lipoproteins. The concentration of PRP correlated to the erythrocyte sedimentation rate (ESR), the C-reactive protein (CRP) and alpha 1- and alpha 2-globulin. Sera from patients with infection and inflammation showed significantly higher PRP levels than those noted in controls. Levels of PRP rose in parallel with ESR and CRP levels following acute myocardial infarction, and the maximal level was noted on the 7th postinfarction day. The PRP levels were elevated during the active phase of pneumonia, followed normalization. These data suggest that PRP is an acute phase reactant and may be important in the metabolism of triacylglycerol-rich lipoproteins.

Hepatoma-associated alterations of serum alpha 1-antitrypsin in hereditary hepatitis LEC rats as a new animal model of liver disease

1. The LEC rats, a novel animal model of hepatitis and liver tumor, were found to have charge variants of serum alpha 1-antitrypsin (alpha 1AT) at the stage of liver tumor as judged by isoelectric focusing. 2. Treatment of the sera with neuraminidase showed that acidic variants of alpha 1AT in LEC rats seemed to be highly sialylated forms. 3. The concentration and enzymatic activity of serum alpha 1AT in LEC rats were not affected before the onset of hepatitis and even during the development of fulminant hepatitis and hepatocarcinogenesis. This indicates that hereditary hepatitis and subsequent liver tumor in LEC rats do not appear to be associated with alpha 1AT deficiency.

Purification and characterization of a major glycoprotein in rat hepatoma plasma membranes. One of the membrane proteins released by phosphatidylinositol-specific phospholipase C

A major glycoprotein of rat hepatoma plasma membranes was selectively released as a soluble form by incubating the membrane with phosphatidylinositol-specific phospholipase C. The soluble form corresponding to the glycoprotein was also prepared by butan-1-ol extraction of microsomal membranes at pH 5.5, whereas extraction at pH 8.5 yielded an electrophoretically different form with a hydrophobic nature. The soluble glycoprotein extracted at pH 5.5 was purified by sequential chromatography on concanavalin A-Sepharose, Sephacryl S-300 and anti-(alkaline phosphatase) IgG-Sepharose, the last step being used to remove a contaminating alkaline phosphatase. The glycoprotein thus purified was a single protein with Mr 130,000 in SDS/polyacrylamide-gel electrophoresis, although it behaved as a dimer in gel filtration on Sephacryl S-300. The glycoprotein was analysed for amino acid and carbohydrate composition. The composition of the carbohydrate moiety, which amounted to 64% by weight, suggested that the glycoprotein contained much larger numbers of N-linked oligosaccharide chains than those with O-linkage. It was confirmed that the purified glycoprotein was immunologically identical not only with that released by the phospholipase C but also with the hydrophobic form extracted with butan-1-ol at pH 8.5. The results indicate that the glycoprotein of rat hepatoma plasma membranes, which has an unusually high content of carbohydrate, is another membrane protein released by phosphatidylinositol-specific phospholipase C, as documented for alkaline phosphatase, acetylcholinesterase and Thy-1 antigen.

Effects of weakly basic amines on proteolytic processing and terminal glycosylation of secretory proteins in cultured rat hepatocytes

We examined the effects of weakly basic amines on the secretion and post-translational modifications of secretory proteins in cultured rat hepatocytes. Weakly basic amines such as methylamine, chloroquine and NH4Cl strongly inhibited not only protein secretion, but also the proteolytic conversion of a proform of complement C3, allowing the precursor to be released into the medium. The amines, however, had no effect on the proteolytic conversion of prohaptoglobin into its subunits. Since available evidence indicates that the conversion of pro-C3 occurs at the Golgi complex while that of prohaptoglobin takes place in the endoplasmic reticulum, it is most likely that the weak bases specifically affect the proteolytic event occurring at the Golgi complex. Electron microscopic observations confirmed that the amines caused morphological changes of the Golgi complex, consisting of dilated cisternae and swollen vacuoles. When the glycosylation of alpha 1-protease inhibitor and haptoglobin was examined, it was found that the amines caused a marked accumulation in the cells of both glycoproteins corresponding to the mature secreted forms. Neuraminidase digestion demonstrated that the glycoproteins accumulating in response to the amines had acquired terminal sialic acid. The results indicate that the amines do not significantly affect terminal glycosylation, in contrast with their definite effect on proteolytic processing, despite the fact that both modifications take place in the Golgi complex.

Structural analysis of the asparagine-linked oligosaccharides of rat haptoglobin metabolically labeled in a hepatocyte culture system

We analyzed the asparagine-linked oligosaccharide chains of rat haptoglobin which were synthesized and secreted by hepatocytes in primary culture. When the cells were incubated with either [3H]mannose, [3H]galactose, or [3H]fucose, all the radioactive precursors were incorporated into the beta subunit of haptoglobin. [3H]Mannose-labeled haptoglobin was purified from the culture medium by immunoaffinity chromatography, and [3H]oligosaccharides were prepared by strong alkali-borohydride treatment. The oligosaccharides obtained were analyzed by anion-exchange high-performance liquid chromatography, concanavalin-A--Sepharose chromatography and Bio-Gel P-4 chromatography before and after sequential exoglycosidase digestions. The oligosaccharides labeled with [3H]fucose or [3H]galactose were also characterized by the above methods. The results indicate that rat haptoglobin contains two complex-type oligosaccharide chains in each beta subunit; one with a possible structure of ( +/- NeuAc----Gal beta----GlcNAc beta----)3(Man alpha----)2 Man beta----GlcNAc----( +/- Fuc alpha----)GlcNAc and the other with ( +/- NeuAc----Gal beta----GlcNAc beta----Man alpha----)2 Man beta----GlcNAc----( +/- Fuc alpha----)GlcNAc.

Selective preparation and characterization of membranous and soluble forms of alkaline phosphatase from rat tissues. A comparison with the serum enzyme

We developed a method for selective preparation of two forms of alkaline phosphatase from rat tissues. The enzyme was extracted by n-butanol treatment at pH 5.5 and pH 8.5 as soluble and aggregated (membranous) forms, respectively. The soluble form prepared from liver was found to be identical with the serum enzyme. Complete solubilization of the membrane-bound enzyme without detergents had a great advantage in its purification. Rat hepatoma AH-130 cells enriched in alkaline phosphatase were first used for purification of the liver-type enzyme. The hepatoma enzyme, purified by chromatographies on concanavalin-A-Sepharose, Sephacryl S-300 and hydroxyapatite was used for production of antibodies specific for the liver-type isozyme. An immunoaffinity column, prepared with anti-(hepatoma-enzyme) IgG was utilized for the enzyme purification from other tissues including the membranous form. Analyses of amino acid composition of the purified enzymes revealed that all the liver-type enzymes from hepatoma, liver, kidney and serum had the same composition, whereas the intestinal type consisted of the composition distinctly different from that in the liver type. In addition, there was no significant difference in amino acid composition between the soluble and membranous forms, suggesting a possible involvement in the membranous form of a hydrophobic component other than its polypeptide domain. The present method for selective preparation of the soluble and membranous forms of alkaline phosphatase will be useful for a further investigation on the interaction of the enzyme with membranes.

Different effects of the glucosidase inhibitors 1-deoxynojirimycin, N-methyl-1-deoxynojirimycin and castanospermine on the glycosylation of rat alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein

The glucosidase inhibitors 1-deoxynojirimycin, N-methyl-1-deoxynojirimycin and castanospermine were used to inhibit oligosaccharide processing in primary cultures of rat hepatocytes. Their effect on the glycosylation of alpha 1-proteinase inhibitor (alpha 1PI) and alpha 1-acid glycoprotein (alpha 1AGP) was studied. Of the three glucosidase inhibitors examined, 1-deoxynojirimycin inhibited not only oligosaccharide trimming but also glycosylation de novo of newly synthesized proteins, resulting in the formation of alpha 1PI with two and three (normally carrying three) and alpha 1AGP with two to five (normally carrying six) oligosaccharide side chains. In the presence of the glucosidase inhibitors, glucosylated high-mannose-type oligosaccharides accumulated. Whereas most of the endoglucosaminidase-H-sensitive oligosaccharides formed in the presence of 1-deoxynojirimycin contained only one glucose residue, N-methyl-1-deoxynojirimycin and castanospermine led mainly to the formation of oligosaccharides with three glucose residues. None of the three glucosidase inhibitors completely prevented the formation of complex-type oligosaccharides. Thus, in their presence, alpha 1PI and alpha 1AGP with a mixture of both high-mannose and complex-type oligosaccharides were secreted.

Deglycosylation of alpha 1-proteinase inhibitor by endo-beta-N-acetylglucosaminidase F

The glycosidase endo-beta-N-acetylglucosaminidase F (endo F) from Flavobacterium meningosepticum was used for the deglycosylation of rat alpha 1-proteinase inhibitor (alpha 1 PI). alpha 1 PI containing three oligosaccharide side chains of the complex type was isolated from rat serum or from the medium of rat hepatocyte primary cultures. High-mannose-type alpha 1 PI or hybrid-type alpha 1 PI was isolated from the media of hepatocytes treated with 1-deoxymannojirimycin or swainsonine, respectively. The susceptibility of complex-type alpha 1 PI to endo F was studied in the presence of various detergents. 3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate and octyl glucopyranoside turned out to be most effective. In the absence of detergents, digestion of alpha 1 PI with high concentrations of endo F and/or long times of incubation led to the formation of alpha 1 PI with one and two oligosaccharide side chains. In the presence of 0.5% octyl glucopyranoside, the major cleavage products were unglycosylated alpha 1 PI and alpha 1 PI carrying one carbohydrate side chain. In contrast to the complex-type alpha 1 PI, the high-mannose type can be totally deglycosylated by endo F even in the absence of detergents. The susceptibility of the hybrid-type alpha 1 PI to endo F is between that of the complex and the high-mannose types.

Secretion of high-mannose-type alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein by primary cultures of rat hepatocytes in the presence of the mannosidase I inhibitor 1-deoxymannojirimycin

Two different forms of alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein were found in primary cultures of rat hepatocytes. After a 2.5-h labeling period with [35S]methionine the high-mannose-type precursor of alpha 1-proteinase inhibitor (Mr 49000) and alpha 1-acid glycoprotein (Mr 39 000) and the mature-complex-type alpha 1-proteinase inhibitor (Mr 54 000) and alpha 1-acid glycoprotein (Mr 43 000-60 000) could be immunoprecipitated from the cells, but only the complex-type forms of the two glycoproteins were secreted into the hepatocyte media. When hepatocytes were incubated with the mannosidase I inhibitor 1-deoxymannojirimycin at a concentration of 4 mM, the 49 000-Mr form of alpha 1-proteinase inhibitor and the 39 000-Mr form of alpha 1-acid glycoprotein could be detected in the cells as well as in their media. Neither the secretion of alpha 1-proteinase inhibitor nor that of alpha 1-acid glycoprotein was impaired by 1-deoxymannojirimycin. While alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein, secreted by control cells, were resistant to endoglucosaminidase H, alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein, secreted by hepatocytes treated with 4 mM 1-deoxymannojirimycin, could be deglycosylated by endoglucosaminidase H. When the [3H]mannose-labeled oligosaccharides of alpha 1-proteinase inhibitor, secreted by 1-deoxymannojirimycin-treated hepatocytes, were cleaved off by endoglucosaminidase H and analyzed by Bio-Gel P-4 chromatography, they eluted at the position of Man9GlcNAc, indicating that mannosidase I had been efficiently inhibited. 1-Deoxymannojirimycin did not inhibit the synthesis or the cotranslational N-glycosylation of alpha 1-proteinase inhibitor or alpha 1-acid glycoprotein.

Proteolytic cleavages in alpha 1-antitrypsin and microheterogeneity

Antitrypsin was resolved into two pools by ion-exchange chromatography. Pool 2 contained three anodal isoinhibitors and an N-terminal sequence identical with the one found by others. Pool 1 contained, in addition to the anodal ones two cathodal isoinhibitors as well. The sequencing data of Pool 1 indicate that the cathodal proteins are formed from the anodals by a cleavage of the Gly5-Asp6 bond in the molecule.

Electrophoretic characterization of hepatic alkaline phosphatase released by phosphatidylinositol-specific phospholipase C. A comparison with liver membrane and serum-soluble forms

Alkaline phosphatase was solubilized from plasma membrane of rat liver with butanol-ol, bile acids or sodium deoxycholate, and electrophoretically compared with a soluble form in serum which was derived from the liver. The three enzyme preparations from the plasma membrane migrated at the same position on polyacrylamide-gel electrophoresis in the presence of either Triton X-100 or sodium dodecyl sulphate. The mobility of them, however, was distinctly different from that of the serum-soluble form of the liver-derived alkaline phosphatase. On the other hand, phosphatidylinositol-specific phospholipase C isolated from Bacillus cereus was used to release alkaline phosphatase from plasma membrane. The released alkaline phosphatase was demonstrated to have the same mobility as the serum-soluble form on polyacrylamide-gel electrophoresis in the presence or absence of detergents. The phospholipase C also converted the butan-1-ol-extracted membrane form into the serum-soluble form. The results suggest that release of alkaline phosphatase from the liver into serum is not simply caused by a detergent effect of bile salts, but involves an enzymic hydrolysis of phosphatidylinositol, with which alkaline phosphatase may strongly interact in the membrane.

Proteinase inhibitors in rat serum. Purification and partial characterization of three functionally distinct trypsin inhibitors

Three different serine proteinase inhibitors were isolated from rat serum and purified to apparent homogeneity. One of the inhibitors appears to be homologous to alpha 1-proteinase inhibitor isolated from man and other species, but the other two, designated rat proteinase inhibitor I and rat proteinase inhibitor II, seem to have no human counterpart. alpha 1-Proteinase inhibitor (Mr 55000) inhibits trypsin, chymotrypsin and elastase, the three serine proteinases tested. Rat proteinase inhibitor I (Mr 66000) is active towards trypsin and chymotrypsin, but is inactive towards elastase. Rat proteinase inhibitor II (Mr 65000) is an effective inhibitor of trypsin only. Their contributions to the trypsin-inhibitory capacity of rat serum are about 68, 14 and 18% for alpha 1-proteinase inhibitor, rat proteinase inhibitor I and rat proteinase inhibitor II respectively.

Disparate effects of monensin and colchicine on intracellular processing of secretory proteins in cultured rat hepatocytes

We have studied the biosynthesis and intracellular processing of three major secretory proteins, albumin, alpha 1-protease inhibitor and alpha 2u-globulin, in cultured rat hepatocytes. The effect of secretion-blocking agents, monensin, a monovalent ionophore, and the microtubule-affecting agents colchicine and taxol was determined. In the control cells, alpha 1-protease inhibitor, a glycoprotein, was first synthesized as an endoglycosidase-H-sensitive form with Mr 51 000, and then processed to two endoglycosidase-H-resistant forms having Mr 51 000 and 56 000, the latter of which was secreted into the medium. Initially synthesized proalbumin was converted with chase to serum-type albumin, while no pro-type precursor was identified for alpha 2u-globulin. In the cells treated with colchicine or taxol, in which secretion was greatly inhibited, the fully processed alpha 1-protease inhibitor and albumin accumulated and were finally secreted into the medium. In the monensin-treated cells, however, most of the newly synthesized alpha 1-protease inhibitor and albumin were not processed to the final mature forms, resulting in accumulation of two 51 000-Mr forms and proalbumin, respectively. Moreover in treated cells, proalbumin and the endoglycosidase-H-resistant alpha 1-protease inhibitor were finally secreted into the medium. Such an effect was not caused by NH4Cl which also inhibited the secretion and is known to exert the similar effect as monensin on the receptor-mediated endocytosis pathway. Based on these results, the use of monensin may prove valuable for more detailed analysis of intracellular processing of various proteins.

Biosynthesis, intracellular processing and secretion of haptoglobin in cultured rat hepatocytes

Biosynthesis, intracellular processing and secretion of the hetero-tetrameric (alpha 2 beta 2) glycoprotein, haptoglobin, were studied in primary cultured rat hepatocytes. The results obtained from pulse-chase experiments demonstrated that haptoglobin was initially synthesized as a larger precursor (pro-form), a single polypeptide chain comprising both the alpha- and beta-subunits, and immediately cleaved into subunits during intracellular transport, although about 8% of the newly synthesized haptoglobin was secreted as a pro-form. Monensin which impedes the secretory process at the Golgi complex blocked the complete glycosylation of beta-subunit but rather accelerated the conversion of the pro-form to subunits. These results indicate that the proteolytic processing of the haptoglobin precursor takes place at an early stage before the Golgi complex of the intracellular transport.

The biosynthesis of acute-phase proteins in primary cultures of rat hepatocytes

The biosynthesis and secretion of alpha 2-macroglobulin, transferrin, alpha 1-acid glycoprotein and alpha 1-proteinase inhibitor were studied in rat hepatocyte primary cultures. After labeling with [35S]methionine, two forms, which can be separated electrophoretically differing by molecular weight, were found for each of the four glycoproteins. The following molecular weights were estimated for the intracellular precursors and the secreted forms: alpha 2-macroglobulin, 176 000 and 182 000; transferrin, 84 000 and 86 000; alpha 1-acid glycoprotein, 39 000 and 43 000-60 000; alpha 1-proteinase inhibitor, 49 000 and 54 000. Carbohydrate moieties could be removed from intracellular forms by treatment with endoglucosaminidase H indicating that their oligosaccharide chains were of the high-mannose type. The extracellular forms were sensitive to sialidase. They incorporated [3H]galactose and [3H]fucose showing that their oligosaccharide chains were of the complex type. Pulse-chase experiments revealed a precursor-product relationship for the high-mannose and the complex type glycoproteins. In the hepatocyte medium newly synthesized albumin was detected after 30 min and newly synthesized glycoproteins after 60 min. Unglycosylated alpha 2-macroglobulin (162 000), transferrin (79 000), alpha 1-acid glycoprotein (23 000), and alpha 1-proteinase inhibitor (41 000) were found in the cells as well as in the medium, when the transfer of oligosaccharide chains onto the polypeptide chains was blocked by tunicamycin. Tunicamycin led to a marked reduction of the secretion of alpha 2-macroglobulin, alpha 1-acid glycoprotein and alpha 1-proteinase inhibitor, whereas the secretion of transferrin was less affected.

Purification of detergent-solubilized form and membrane-binding domain of rat gamma-glutamyltransferase by immuno-affinity and hydrophobic chromatography

A new method to purify papain- or detergent-solubilized form (papain or detergent form) of gamma-glutamyltransferase from rat hepatomas as well as from rat kidney by immuno-affinity column chromatography is presented. The antibody-column was prepared by coupling the anti-kidney papain form antibody, which had been purified by using a kidney papain form-Sepharose column, to CNBr-activated Sepharose 4B. The enzyme bound to the antibody-column was eluted with 0.04 M NH4OH. By this method, detergent forms were purified 300 and 1600-fold in approx. 50% yields from rat kidney and rat ascites hepatoma AH 13, respectively, and the papain form was also purified 16 000-fold in a similar yield from primary hepatoma which has a very low activity of this enzyme. Preparations thus obtained apparently did not contain any peptide other than heavy and light subunit peptides of this enzyme on SDS-polyacrylamide gel electrophoresis. The detergent form of kidney enzyme was preferentially absorbed to a hydrophobic column of aminooctyl-Sepharose, while the papain form was not, suggesting that the detergent form might be adsorbed to the column through hydrophobic interaction of the membrane-binding domain. The domain peptide was also purified by the hydrophobic column after release from the detergent form by papain treatment. The molecular weight of the peptide was estimated to be about 16 000 on SDS-polyacrylamide gel electrophoresis. On double immunodiffusion, the domain peptide reacted with anti-detergent form antibody but not with anti-papain form antibody. The domain-specific antibody was also purified from the anti-detergent form antibody.

N-terminal amino acid sequences of precursor and mature forms of alpha-1-antitrypsin

alpha-1-Antitrypsin is found in hepatocytes as a high-mannose glycoprotein (Mr 49 000), extracellularly as a complex-type glycoprotein (Mr 54 000). Deglycosylation of both forms with peptide: N-glycosidase led to proteins of identical app. Mr (41 000). The sequence of 26 N-terminal amino acids of rat alpha 1-antitrypsin was determined. A high content of polar amino acids was found. The partially characterized presequence of in vitro synthesized alpha 1-antitrypsin showed a cluster of hydrophobic amino acids. A pre-peptide of 24 amino acids is proposed. There is no evidence for the existence of a propeptide.

Biosynthesis and secretion of alpha 1-antitrypsin in primary cultures of rat hepatocytes. Characterization of differently glycosylated intracellular and extracellular forms

The biosynthesis and secretion of alpha 1-antitrypsin was studied in rat hepatocyte primary cultures. After labeling with [35S]methionine an alpha 1-antitrypsin with an apparent molecular weight of 49000 estimated by sodium dodecyl sulfate/polyacrylamide slab gel electrophoresis was immunoprecipitated from the cell homogenate. This intracellular form of alpha 1-antitrypsin could be deglycosylated by endoglycosidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. Pulse-chase experiments showed that about 30 min after its synthesis the transformation of the 49000-Mr alpha 1-antitrypsin to a protein with an apparent molecular weight of 54000 began. Only this 54000-Mr protein was secreted by the hepatocytes. The 54000-Mr alpha 1-antitrypsin was not sensitive to endoglycosidase H, but sensitive to neuraminidase, and it incorporated [3H]galactose and [3H]fucose indicating that its oligosaccharide chains were of the complex type. In the presence of tunicamycin, which blocks the formation of N-asparagine-linked oligosaccharide chains, an unglycosylated alpha 1-antitrypsin with an apparent molecular weight of 41000 was found in the cells as well as in the medium. However, tunicamycin decreased the secretion of alpha 1-antitrypsin by 60-70%, whereas the secretion of albumin remained unaffected. In the presence of colchicine the secretion of both alpha 1-antitrypsin and albumin was impaired. The results demonstrate the importance of glycosylation for the secretion of alpha 1-antitrypsin.

A simple one-step purification of human alpha 1-proteinase inhibitor by immunoadsorbent column chromatography

A one-step purification of human alpha 1-proteinase inhibitor was described using the rabbit anti-alpha 1-proteinase inhibitor antibody coupled to CNBr-activated Sepharose 4B. The elution of alpha 1-proteinase inhibitor from the immunoadsorbent column using 0.1 M Na2CO3/0.5 M NaCl solution gave an 85% yield. The properties of eluted alpha 1-proteinase inhibitor were identical with that of alpha 1-proteinase inhibitor that was purified by the conventional method. In addition, the specific activity of purified alpha 1-proteinase inhibitor was more than 93% of that of the theoretical value.