Characterization of mucus glycoprotein fatty acyltransferase from gastric mucosa

Methionine aminopeptidase 2 and cancer

Methionine aminopeptidase (MetAP) is a bifunctional protein that plays a critical role in the regulation of post-translational processing and protein synthesis. In yeasts and humans, two proteins are known to possess MetAP activity, which are known as MetAP1 and MetAP2. MetAP2 has attracted much more attention than MetAP1 due to the discovery of MetAP2 as a target molecule of the anti-angiogenic compounds, fumallin and ovalicin. MetAP2 plays an important role in the development of different types of cancer. Recently, we observed a high expression of MetAP2 in human colorectal cancer tissues and colon cancer cell lines. In addition, pp60(c-src) expression was correlated with the expression of MetAP2 and N-myristoyltransferase. In this review, we discuss the recent developments of MetAP2 and its inhibitors. Future detailed studies related to MetAP2 and apoptosis will shed light on the involvement of this enzyme in the regulation of various apoptotic factors.

Ghrelin: structure and function

Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein-coupled receptor whose ligand has only been discovered recently. Using a reverse pharmacology paradigm with a stable cell line expressing GHS-R, we purified an endogenous ligand for GHS-R from rat stomach and named it "ghrelin," after a word root ("ghre") in Proto-Indo-European languages meaning "grow." Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a threonine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the bloodstream under fasting conditions, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining GH release and energy homeostasis in vertebrates.

Role of palmitoylation/depalmitoylation reactions in G-protein-coupled receptor function

G-protein-coupled receptors (GPCRs) constitute one of the largest protein families in the human genome. They are subject to numerous post-translational modifications, including palmitoylation. This review highlights the dynamic nature of palmitoylation and its role in GPCR expression and function. The palmitoylation of other proteins involved in GPCR signaling, such as G-proteins, regulators of G-protein signaling, and G-protein-coupled receptor kinases, is also discussed.

Purification and biochemical characterization of a protein-palmitoyl acyltransferase from human erythrocytes

Protein palmitoylation involves the post-translational attachment of palmitate in thioester linkage to cysteine residues of proteins. The labile nature of the thioester linkage makes possible the palmitoylation-depalmitoylation cycles that have emerged in recent times as additions to the repertoire of cellular control mechanisms. However, detailed understanding of these cycles has been limited by the lack of knowledge of the transferases and thioesterases likely to be involved. Here, we describe the purification of a protein-palmitoyl acyltransferase (PAT) from human erythrocytes. PAT behaved as a peripheral membrane protein and catalyzed the attachment of palmitate in thioester linkage to the beta-subunit of spectrin. On SDS-polyacrylamide gel electrophoresis, PAT appeared as a 70-kDa polypeptide. Antibody against this polypeptide could immunodeplete PAT activity from the crude extract, confirming the assignment of the 70-kDa polypeptide as PAT. PAT-mediated spectrin palmitoylation could be inhibited by nonradioactive palmitoyl-, myristoyl-, or stearoyl-CoA. The apparent Km for palmitoyl-CoA was 16 microM.

Palmitoylation of brain capillary proteins

Palmitoylation is a reversible posttranslational modification which is involved in the regulation of several membrane proteins such as beta 2-adrenergic receptor, p21ras and trimeric G-protein alpha-subunits. This covalent modification could be involved in the regulation of the numerous membrane proteins present in the blood-brain barrier capillaries. The palmitoylation activity present in brain capillaries was characterized using [3H]palmitate labeling followed by chloroform methanol precipitation. Palmitate solubilizing agents such as detergents and bovine serum albumin (BSA), were used for optimizing activity. Some palmitoylated substrates were identified using [3H]palmitate labeling followed by immunoprecipitation with specific antibodies. Two optimal palmitate solubilization conditions were found, one involves cell permeabilization (Triton X-100) and the other represents a more physiological condition where membrane integrity is conserved (BSA). Sensitivity to the cysteine modifier N-ethylmaleimide and to hydrolysis, using hydroxylamine or alkaline methanolysis, indicated that palmitic acid was bound to the proteins by a thioester bond. Maximal palmitate incorporation was reached after 30 or 60 min of incubation in the presence of Triton or BSA, respectively. Depalmitoylation was observed in the presence of BSA, but not with detergents. The palmitoylation reaction was optimal at pH 8 or 9 in the presence of Triton or BSA, respectively, but palmitoylated substrates were detectable over a wide range of pH values. In the presence of Triton X-100, the addition of ATP, CoA and Mg2+ to the incubation medium increased palmitoylation by up to 80-fold. Two palmitoylated substrates were identified, a 42 kDa G-protein alpha subunit and p21ras. The study shows that the utilization of palmitate solubilizing agents is essential to measure in vitro palmitoylation in brain capillaries. Several palmitoylated proteins are present in the blood-brain barrier including five major substrates of 12, 21, 35, 42 and 55 kDa. It is suggested that palmitoylation could play a crucial role in the regulation of brain capillary function, since the two substrates identified in this study are known to be involved in signal transduction, vesicular transport and cell differentiation.

Apical release of base-labile fatty acyl groups commensurate with stimulation of glycoprotein sialosyl Lewis(a) secretion in colorectal carcinoma cells

The rate of polarized secretion of a putative adhesion ligand, sialosyl Lewis(a) (19-9), by SW1116 colorectal carcinoma cells is stimulated at least 20-fold after pre-incubation with, and the incorporation of, retinoic acid (RA). In order to investigate the possible involvement of fatty acylation in the export of the epitope, purified ligands from carcinoma-cell membranes, membrane subfractions and media were analyzed during RA-induced secretion. Incorporation of radioactivity from (3H)palmitate into membrane subfractions and purified sialosyl Lewis(a) antigenic molecular species of M(r) > 150,000 (SiaLeams) was stimulated by RA treatment. Most of the intracellular lipid radioactivity which bound to solid-phase 19-9 antibody behaved chromatographically, either like ganglioside or like NH2 OH-labile acyl groups, but most of the (3H) bound to SiaLeams of post-incubation media behaved like base-labile fatty acyl groups, or free fatty acid. Release of base-labile lipid radioactivity after 3 hr (associated with antigen) was almost exclusively into the apical media of membrane inserts. Gas-liquid chromatography/mass spec. analyses of purified Sialeams revealed the presence of palmitate (16:0), as well as stearate (18:0) and oleate (18:1) fatty acyl groups. Our results suggest that fatty acylation of SiaLeams may be co-ordinated with alterations in glycosylation and participate in directing these molecules to the apical surface. Lipid analyses were consistent with ganglioside chaperonage of SiaLeams to the apical surface, where N-fatty-acylated gangliosides remain for the most part integrated into the bilayer, but some oxyester or thioester bonds may be cleaved to permit release of SiaLeams to the apical medium.

Repression of the Lewis fucosyl transferase by retinoic acid increases apical sialosyl Lewis(a) secretion in colorectal carcinoma cultures

The rate of polarised secretion of sialosyl Lewis(a)(19-9) molecular species (SiaLeams) by SW1116 colorectal carcinoma cells is stimulated at least ninefold by the presence of 3 microM retinoic acid (RA). In order to investigate the intracellular origins of this augmentation, carcinoma cell membranes, membrane subfractions, and media were studied to determine alterations in sialosyl Lewis(a) levels, oligosaccharide composition, and core structures accompanying the capacity to increase export of this epitope. We observed a nine- to twentyfold increase in sialosyl Lewis(a) epitope levels in a light membrane subfraction from RA-treated cells. Antigenic molecules of < 200,000 M(r) on acrylamide gradient gels were concentrated in two doublets in the apparent M(r) range 106,000-152,000 on Western blots. Carbohydrate analyses of oligosaccharides from SiaLeams of membrane subfractions and apical media indicated much higher fucose/mannose, fucose/sialic, fucose/sialosyl Lewis(a), fucose/total CHO, and (3H) fucose incorporation in control samples than RA samples. Western blots of samples from membrane subfractions and media indicated that, in contrast to the effect of RA on the sialosyl Lewis(a) epitope, RA treatment did not augment cysteine-rich, PDTRP, blood group H-2, blood group A, and EGF receptor-like region epitopes in the media. In addition, Northern blots using the Lewis fucosyl transferase (FTIII) cDNA showed a dramatic diminution of mRNA encoding FTIII but apparently unaltered levels of sialyl transferase (ST4) mRNA. Since subterminal fucosylation of lactosyl termini blocks terminal sialylation, we conclude that one mechanism of sialosyl Lewis(a) induction in this culture system is the lower expression of the Lewis fucosyl transferase mRNA. Therefore less subterminal fucosylation of GlcNAc permits the prior sialylation of terminal Gal beta 1-3 moieties at oligosaccharide termini destined for export from the Golgi.

The role of airway mucus in pulmonary toxicology

Airway mucus is a complex airway secretion whose primary function as part of the mucociliary transport mechanism is to to serve as renewable and transportable barrier against inhaled particulates and toxic agents. The rheologic properties necessary for this function are imparted by glycoproteins, or mucins. Some respiratory disease states, e.g., asthma, cystic fibrosis, and bronchitis, are characterized by quantitative and qualitative changes in mucus biosynthesis that contribute to pulmonary pathology. Similar alterations in various aspects of mucin biochemistry and biophysics, leading to mucus hypersecretion and altered mucus rheology, result from inhalation of certain air pollutants, such as ozone, sulfur dioxide, nitrogen dioxide, and cigarette smoke. The consequences of these pollutant-induced alterations in mucus biology are discussed in the context of pulmonary pathophysiology and toxicology.

Enzymatic sulfation of gastrin in rat gastric mucosa

An enzyme which catalyzes the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to gastrin (G17) was identified in rat gastric mucosal cells. The enzyme activity was detected in the 105,000xg supernatant fraction. Formation of gastrin sulfate was shown by using 125I-gastrin and non-radioactive PAPS. The product was sensitive to acid hydrolysis, arylsulfatase treatment and removed by gastrin antibody, but not changed by treatments with chondro-4-sulfatase and chondro-6-sulfatase. The product had a molecular weight of 2050 daltons, close to the molecular weight of G17 sulfate, and, therefore, indicating the sulfated product is not APS derived from the degradation of PAPS. The enzyme activity showed a Km value of 5 microM for PAPS and a pH optimum of 6.0. The activity was not detected in the liver preparation.

Agonist-enhanced palmitoylation of platelet proteins

When washed human platelets were incubated with [3H]palmitic acid, radioactivity was incorporated into a major 38 kDa doublet and several minor proteins that were resolved on polyacrylamide gels. The radioactivity associated with the proteins remained after extractions with organic solvents, but it was lost after hydroxylamine treatment or mild alkali methanolysis. The products of these reactions were analyzed by thin-layer chromatography and HPLC. They were identified as palmitohydroxamate and methyl palmitate, respectively, indicating that the palmitic acid was covalently linked to the proteins via oxygenester or thioester bonds. In resting platelets, radioactivity was detected in the 38 kDa proteins 2 min after the addition of [3H]palmitic acid. A plateau was reached between 5 and 11 min, at which time radioactivity was also detected in a 23 kDa protein. Thrombin elicited faster and greater incorporation of label into both proteins. Phorbol 12-myristate 13-acetate (PMA) led to a similar, but slower increase of radioactivity in the 38 kDa proteins, while collagen and A23187 were less effective. Enhanced palmitoylation may be closely linked to platelet activation, as suggested by the following observations: (1) in thrombin- or PMA-activated platelets, the time-course of aggregation correlated with the time-course of enhanced palmitoylation of the 38 kDa proteins; (2) in platelets activated by various concentrations of thrombin with or without prostacyclin, aggregation was correlated with the enhanced incorporation of radioactivity into the 38 kDa proteins.

Protein acylation in Tetrahymena

Examination of exhaustively delipidated Tetrahymena mimbres cells by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of several protein bands containing covalently linked fatty acids. Palmitic (16:0) and stearic (18:0) acids together accounted for approximately 90% of the protein-linked acyl chains, with myristic acid (14:0) comprising most of the remainder. Each of these three fatty acids was present mainly in alkali-stable linkage, indicating that unlike most other systems examined, fatty acids are attached to proteins of Tetrahymena principally by amide bonds. Smaller proportions of the acyl chains were susceptible to release by hydroxylaminolysis or by alkaline hydrolysis as would be expected from an ester linkage. The protein-bound acyl chains accounted for 0.3% of the cells' total fatty acids. They closely resembled in composition the highly saturated free fatty acid pool but not the vast pool of glycerolipid-associated fatty acids, which were mainly unsaturated. Cells subjected to thermal stress by rapid chilling from 39 to 15 degrees C responded by sharply increasing the ratio of palmitate to stearate in covalent association with proteins.

Acylation in vitro of the myelin proteolipid protein and comparison with acylation in vivo: acylation of a cysteine occurs nonenzymatically

Characteristics of fatty acylation of myelin proteolipid protein (PLP) in vitro were compared with the corresponding process in vivo. Rapid and efficient separation of labelled PLP from other proteins and lipids was effected by extraction into chloroform/methanol/0.1 N HCl (10/10/1) and chromatography on Sephadex LH-60 in the same solvent. Covalent linkage of [3H]-palmitate to PLP was demonstrated by repetitive chromatography on LH-60, thin layer chromatography, and polyacrylamide gel electrophoresis. Reductive cleavage with sodium borohydride of PLP acylated in vitro or in vivo yielded [3H]-hexadecanol, identifying at least one of the acyl linkages as a thiolester bond. When PLP was acylated with acyl-CoA as the fatty acid donor, the reaction occurred non-enzymatically as supported by the following observations: 1) acylation activity increased with increasing pH above pH 7.5, 2) acylation activity was heat stable, 3) acylation activity was not removed from PLP during purification in organic solvents or in Triton X-100-containing buffers, and 4) acylation of tryptic fragments occurred in the absence of an exogenously added enzyme source. The relevance of in vitro fatty acylation of PLP to that in vivo was confirmed by comparison of proteolytically derived peptide maps that showed that likely the same domain of PLP was acylated in vitro and in vivo.

Incorporation of myristate and palmitate into the sheep reticulocyte transferrin receptor: evidence for identical sites of labeling

The ability of sheep reticulocytes and plasma membranes isolated from them to incorporate fatty acids into the transferrin receptor has been examined using both [3H]palmitate and [3H]myristate. Both fatty acids, when incorporated into the transferrin receptor, can be released by treating the protein with 1 M hydroxylamine at pH 7.0. After treatment of the 3H-acylated receptor with borohydride, an 3H-labeled alcohol is released, suggesting that the receptor-bound fatty acid is in thioester linkage. With both [3H]myristate and [3H]palmitate, Cleveland maps from immunoprecipitates of the transferrin receptor labeled in intact cells and isolated membranes show that identical peptides are labeled. No evidence was obtained for qualitatively different labeling with the two fatty acids. In intact reticulocytes, incorporation of [3H]palmitate into the transferrin receptor is approximately 3.5 times greater than the incorporation of [3H]myristate from equivalent concentrations of the labeled fatty acids. However, in isolated reticulocyte plasma membranes, there is much less difference between palmitate and myristate incorporation (with ATP) or between their acyl-CoA derivatives. The reason for the discrepancy between cells and membranes is unknown but may be due to the presence in intact cells of more than one enzyme for activating the fatty acids. Acylation of the receptor in isolated plasma membranes is fourfold greater with the CoA derivatives than with the free fatty acids. The fatty acid activating enzyme(s) as well as the acyltransferase(s) appear to be membrane bound in reticulocytes.

Co-translational processing and intracellular transport of rat salivary mucus glycoprotein

A preparation of peptidyl-tRNA from intact microsomes of mucin-synthesizing polysomes of sublingual salivary gland cells contained fatty-acylated galactosamine-free and galactosamine-enriched peptidyl-tRNA fractions, whereas trypsin-chymotrypsin treated microsomes yielded predominantly the acylated galactosamine-enriched peptidyl-tRNA complexes. Radioscanning and chemical analyses revealed that palmitate was substituted on all nascent peptides, except those shorter than 20 amino-acid residues. In contrast, the [35S]-methionine label was detected only on galactosamine-free peptides containing up to 70 amino acids. On SDS-polyacrylamide gel, the peptides released from galactosamine-enriched tRNA complexes separated into a multitude of bands ranging in size from 6000 to 60,000 dalton, whereas the total preparation afforded peptides ranging from 2000 to 60,000 dalton. Pulse-chase experiments, using radiolabelled methionine, palmitic acid and N-acetylgalactosamine, combined with chemical characterization of the radiolabelled fatty acids and carbohydrates from purified peptidyl-tRNA, confirmed that the N-terminal fatty acylation and the initial O-glycosylation with N-acetylgalactosamine are the co-translational processes taking place as soon as peptide is sufficiently large to be acylated, trimmed, and translocated to the luminal site of endoplasmic membrane.

Cotranslational fatty acylation of mucus glycoprotein. Addition of palmitic acid to peptidyl-tRNA occurs prior to peptide chain completion and its release

1. The fatty acylation of mucus glycoprotein nascent peptides was investigated using [3H]palmitic acid and [35S]methionine-labeled peptidyl-tRNA of rat gastric mucous cells. 2. The mucus glycoprotein peptidyl-tRNA fraction was found to contain covalently bound palmitic acid in its complexes. 3. RNase digestion of the mucus glycoprotein peptidyl-tRNA released [3H]palmitic acid labeled peptides which, on SDS-polyacrylamide gel, separated into a multitude of bands ranging in size from 2000 to 60,000 Da. 4. The analyses of low molecular weight peptides revealed that palmitic acid was present in methionine-labeled peptides containing 30-43 amino acids and those of 18-25 amino acids or larger devoid of methionine, but was not identified in methionine-labeled peptides containing 10-15 amino acids. 5. The results indicate that the N-terminal fatty acylation of mucus glycoprotein nascent peptides is a cotranslational process which is occurring in an immediate vicinity of the signal peptide fragment.

Fatty acid acylation of mucin by gastric mucosa: effects of sofalcone and sucralfate

The effects of antiulcer drugs, sofalcone and sucralfate, on the activity of gastric mucosal mucus glycoprotein fatty acyltransferase were investigated. The acyltransferase enzyme, contained in the detergent extracts of the microsomal fraction of rat gastric mucosa, was incubated with the deacylated gastric mucin and palmitoyl-CoA substrates in the presence and absence of drugs, and the formed fatty acid acylated glycoprotein product was quantitated. In the absence of drugs, the enzymatic activity increased proportionally with increased concentrations of both substrates and of enzyme, and gave an apparent Km value of 5.6 X 10(-7) M. Introduction of sofalcone to the reaction mixtures led to an enhancement in the rate of mucus glycoprotein acylation. The rate of enhancement was proportional to sofalcone concentration up to 1.0 X 10(-5) M, with an apparent Km value of 3.7 X 10(-7) M. In contrast to sofalcone, the acyltransferase activity was inhibited by sucralfate. The rate of inhibition of mucus glycoprotein acylation by sucralfate was of the competitive type and at 1.0 X 10(-4) M reached a value of 25%. The apparent KI value calculated from the double-reciprocal plots for sucralfate was 9.1 X 10(-7) M. As the acylation of mucin with fatty acids plays an important role in the maintenance of gastric mucosal integrity, the results suggest that stimulation of the fatty acyltransferase enzyme by sofalcone may be one of the beneficial effects of this drug towards ulcer healing.

Cotranslational attachment of fatty acids to nascent peptides in gastric mucus glycoprotein

Using gastric mucous cells which are involved exclusively in the synthesis of secretory O-glycosidic glycoprotein (mucin), the relationship between protein core synthesis and its acylation with fatty acids was investigated. Labeling of the cells with [3H]palmitic acid and [35S]methionine followed by isolation of peptidyl-tRNA and release of nascent peptides, indicated that these peptides contain covalently bound fatty acids. The high performance thin layer chromatography, SDS-gel electrophoresis, and radioactivity scanning revealed that the preparation contained three fractions labeled with palmitate (Mr 15,000-3,600) and two (Mr 1,500 and less) without this label. Based on these data and the nascent peptides amino acid analysis, we conclude that the protein core of the O-glycosidic glycoprotein is acylated with fatty acids during translation, when the peptide chain is longer than 21 amino acid residues.

Changes in mucus glycoprotein synthesized in rat gastric mucosa exposed to ethanol

The resistance to proteolysis by pepsin of gastric mucus glycoprotein synthesized by tissue culture in the presence and absence of 0.1 M ethanol was investigated. The glycoprotein product of ethanol-supplemented culture was found to contain 68% less associated lipids and 81% less covalently bound fatty acids, but exhibited unaltered content of carbohydrate and protein. The lipid and fatty acyl deficient glycoprotein was 5-times more rapidly and 2-3-times more extensively degraded by pepsin than the glycoprotein synthesized in the absence of ethanol. Following delipidation with organic solvents and deacylation with hydroxylamine both glycoproteins were digested at the same rate and degraded to the same extent. The lower content of fatty acyl residues markedly affected the overall pattern of the proteolytic fragments identified by SDS gel electrophoresis. The peptides corresponding to the acylated fragments of control were degraded and an increase in the amount of smaller peptides was observed. The in vitro assays of the fatty acyltransferase activity towards the substrates obtained from control and alcohol-containing cultures revealed that the enzyme activity was similar and increased proportionally with increased concentration of both glycoprotein substrates and enzyme. However, addition of 0.1 M ethanol to the assay tubes containing complete incubation mixture decreased the acylation of either glycoprotein by 40%. Based on the results presented here, and on previous studies of mucus glycoprotein synthesis in the presence of ethanol, we conclude that ethanol interferes with the process of acylation of mucus glycoprotein with fatty acids.

In vitro fatty acid acylation of mucus glycoprotein from sublingual salivary glands

The enzyme activity which catalyzes the transfer of palmitic acid from palmitoyl-coenzyme A to sublingual gland mucus glycoprotein has been demonstrated in the detergent extracts of the microsomal fraction of rat sublingual and parotid salivary glands. The acyltransferase activity of this fraction was similar in both types of glands. Further subcellular fractionation performed on sublingual glands revealed that the enzyme is associated with the Golgi-rich membrane fraction. Optimum enzymatic activity for fatty acylation of mucus glycoprotein was obtained using 0.5% Triton X-100, 2 mM dithiothreitol, 25 mM NaF, and 10 mM MgCl2 at a pH of 7.4. Higher concentrations of NaF, MgCl2 and dithiothreitol, however, were inhibitory. The apparent Km of the sublingual glands microsomal enzyme for mucus glycoprotein was 0.55 mg/ml and for palmitoyl-CoA, 3.5 X 10(-5) M. A 15% decrease in the acyltransferase activity was obtained with the reduced and alkylated mucus glycoprotein and it showed no activity towards the proteolytically degraded glycoprotein. The 14C-labeled product of the enzyme reaction gave in CsCl density gradient a band at the density of 1.49 in which the 14C label coincided with the glycoprotein. The 14C label in this glycoprotein was susceptible to deacylation with hydroxylamine, and the released labeled material was identified as palmitate.

Fatty acid acylation of rat brain myelin proteolipid protein in vitro: identification of the lipid donor

The immediate acyl chain donor for fatty acid esterification of proteolipid protein (PLP) was identified in an in vitro system. Rat brain total membranes, after removal of crude nuclear and mitochondrial fractions, were incubated with radioactive acyl donors, extracted with chloroform/methanol, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the presence of [3H]palmitic acid, CoA, ATP, and Mg2+, acylation of endogenous PLP occurred at a linear rate for at least 2 h. The radioactivity was associated with the protein via an ester linkage, mainly as palmitic acid. Omission of ATP, CoA, Mg2+, or all three reduced fatty acid incorporation into PLP to 44, 27, 8, and 4%, respectively, of the values in the complete system. Incubation of the membrane fraction with [3H]palmitoyl-CoA in the absence of CoA and ATP led to highly labeled PLP. These data demonstrate that activation of free fatty acid is required for acylation. Phospholipids and glycolipids were not able to acylate the PLP directly. Finally, when isolated myelin was incubated with [3H]palmitoyl-CoA in the absence of cofactors, only PLP was labeled, thus confirming the identity of palmitoyl-CoA as the direct acyl chain donor and suggesting that the acylating activity and the PLP pool available for acylation are both in the myelin.

Enzymic acylation of mucus glycoprotein with palmitic acid in rat submandibular salivary gland

The enzymic activity which catalyses transfer of palmitic acid from palmitoyl coenzyme A to mucus glycoprotein was found in Triton X-100 extracts of the microsomal fraction of rat submandibular and sublingual salivary glands. The acyltransferase activity of this fraction was 1.3-1.4 times greater in submandibular gland than in sublingual gland. Further subcellular fractionation of submandibular gland showed that the enzyme activity was associated with a Golgi-rich membrane fraction. Optimum enzyme activity for fatty acylation of mucus glycoprotein was at pH 7.4 using 0.5 per cent Triton X-100, 2 mM dithiothreitol 25 mM NaF and 10 mM MgCl2; higher concentrations were inhibitory. The apparent Km of the submandibular microsomal enzyme for mucus glycoprotein was 5.9 X 10(-7) M, and for palmitoyl-CoA, 3.3 X 10(-5) M. The 14C-labelled glycoprotein product of the reaction co-migrated on CsCl equilibrium, density-gradient centrifugation with submandibular mucus glycoprotein, and contained ester-bound palmitic acid. The fatty acyltransferase showed no activity with proteolytically-degraded glycoprotein; the acceptor capacity of reduced and S-carboxymethylated glycoprotein was only about 10 per cent lower than that of the intact mucus glycoprotein. This suggests that the acylation of salivary mucus glycoprotein with fatty acids occurs at its non-glycosylated, proteolysis-susceptible regions, and that the majority of these fatty acids are linked to the glycoprotein through hydroxyl esters.

Fatty acid acylation of salivary mucin in rat submandibular glands

The acylation of salivary mucin with fatty acids and its biosynthesis was investigated by incubating rat submandibular salivary gland cells with [3H]palmitic acid and [3H]proline. The elaborated extracellular and intracellular mucus glycoproteins following delipidation, Bio-Gel P-100 chromatography, and CsCl equilibrium density gradient centrifugation were analyzed for the distribution of the labeled tracers. Both preparations gave single bands at the CsCl density of 1.48, in which carbohydrate peaks coincided with that of the labels. The [3H]palmitic acid in these glycoproteins was susceptible to cleavage by alkali and hydroxylamine, thus indicating the ester nature of the bond. With both intracellular and extracellular glycoproteins deacylation caused the glycoproteins to band in the CsCl gradient at a density of 1.55. The incorporation of both markers into mucus glycoprotein increased steadily with time up to 4 h, at which time about 65% of [3H]palmitate and [3H]proline were found in the extracellular glycoprotein and 35% in the intracellular glycoprotein. The incorporation ratio of proline/palmitate, while showing an increase with incubation time in the extracellular glycoprotein, remained essentially unchanged with time in the intracellular glycoprotein and at 4 h reached respective values of 0.14 and 1.12. The fact that the proline/palmitate incorporation ratio in the intracellular glycoprotein at 1 h of incubation was 22 times higher than in the extracellular and 8 times higher after 4 h suggests that acylation occurs intracellularly and that fatty acids are added after apomucin polypeptide synthesis. As the incorporation of palmitate within the intracellular mucin was greater in the mucus glycoprotein subunit, it would appear that fatty acid acylation of mucin subunits preceeds their assembly into the mucus glycoprotein polymer.

Mucus glycoprotein fatty acyltransferase in patients with cystic fibrosis: effect on the glycoprotein viscosity

The presence of an acyltransferase activity which catalyzes the transfer of palmitic acid from palmitoyl coenzyme A to mucus glycoprotein has been demonstrated in the microsomal fraction of human rectal mucosa. The activity of this enzyme in the mucosa of patients with cystic fibrosis (CF) was found to be 3.5 times higher than that from normal individuals. The CF mucus glycoprotein in comparison to that of normal contained 1.3 times more associated lipids and 6 times more covalently bound fatty acids. The viscosity of the intact CF glycoprotein was 1.8 times higher than that of normal glycoprotein. Extraction of associated lipids led to 3-fold drop in the viscosity of CF glycoprotein and 5-fold drop in the case of normal glycoprotein. Further loss in the viscosity occurred following removal of the covalently bound fatty acids. The viscosity of such modified CF mucus glycoprotein was only about 10% higher than that of similarly treated normal glycoprotein.