Biology of colon cancer: an overview

The altered growth characteristics of neoplastic cells have recently been associated with changes in membrane glycoproteins present on the cell surface. Since the carbohydrate moieties of surface membrane glycoproteins are asymmetrically located on the external cell surface, these glycoconjugates are likely candidates for providing cell surfaces with many of their biological properties. Using specific external cell surface labeling techniques, we have broadened our investigation of tumor cell surface glycoconjugates to include studies on cultured human epithelial cells from fetal intestine and from colonic carcinoma. We have isolated by affinity chromatography and gel filtration and integral membrane glycoprotein, termed Galactoprotein I, from a cultured human colonic adenocarcinoma cell line, which appears to be identical in many respects to CEA. Further examination of cell surface glycoproteins regarding quantitative and qualitative alterations and topographical redistribution should provide an insight into the biological aspects of tumor development.

Glycoprotein alterations in human colonic adenocarcinoma

A marked diminution in membrane glycoproteins containing blood group A activity was observed in colonic cancer tissues. This change was associated with a reduction of the enzyme responsible for its biosynthesis and a decreased concentration of N-acetylgalactosamine in the cancer tissues. Glycosidase activities were unchanged. In addition to the changes associated with blood group A, we also found a decrease in sugar content, alterations in other antigens, and changes in the levels of several glycosyltransferases in cancerous tissues.

How Gymnodinium breve red tide toxin(s) produces repetitive firing in squid axons

Partially purified toxin(s), GbTX, extracted from Gymnodinium breve red tide organisms elicits a spontaneous train of action potentials in the squid giant axon. The spikes have a shape similar to that in the normal seawater control except for an increase in the rate of recovery from the afterhyperpolarization. With this more rapid recovery, the membrane potential overshoots the resting potential and threshold, triggers another spike, and thus produces repetitive firing. Voltage-clamp studies revealed that the toxin has no effect on the normal sodium or potassium conductance changes produced by step depolarization. However, consistent with the faster recovery after an action potential, GbTX speeds recovery of the "shut-off" currents to their steady-state values after a depolarization. The most likely mechanism by which the toxin accelerates recovery after an action potential (leading to repetitive firing) is the induction of a small additional inward current which was found to be reduced by prehyperpolarization. This toxin-induced current which speeds recovery is blocked by tetrodotoxin and hence presumably flows through the sodium channel.

Alterations in enterkinase, trypsin, and alkaline phosphatase in response to variation in dietary protein content in the rat

In rats fed equal amounts of isocaloric high-protein (HPR) and low-protein (LPR) diets, studies were performed on the mucosal activities of enterokinase and alkaline phosphatase and on the activities of these enzymes and of trypsin in washings obtained from the contents of two standard 5 cm. segments of duodenum, located proximal (Segment I) and distal (Segment II) to the main pancreatic duct. Mean mucosal weights and tissue protein per segment were 1.3-fold higher in rats fed HPR diets. In Segment I, but not Segment II, mucosal activities per segment were higher in HPR for enterkinase (threefold) and alkaline phosphatase (twofold). In luminal washings trypsin did not differ between the two groups; in HPR luminal levels of enterkinase were significantly higher in Segment II and those of alkaline phosphatase were similarly elevated in Segment I. Irrespective of diet the major activity of both enzymes was in mucosal fractions. Studies of total activity of each enzyme showed that the enzymes behave rather similarly, with the major differences between the dietary groups discernible in Segment I. These data stress the importance of dietary protein content in intestinal enzyme adaptation and reveal regional variation in the responses of the different enzymes.

A simple quantitative method to determine short chain fatty acid levels in biological fluids

A simple method has been developed for quantitative acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocaproic, and caproic acids in intestinal fluids, feces and blood. The method utilizes extraction with ether and gas chromatography. It is accurate over a wide range of SCFA concentrations and appears to be applicable to any biological fluid.

Relationships between mucosal hydrolysis and transport of two phenylalanine dipeptides

In order to investigate the source of free amino acids found in the gut lumen during absorption of dipeptides, as well as evaluating the role of brush border peptidases in the mucosal hydrolysis of dipeptides during absorption, rates of dipeptide disappearance and appearance of hydrolytic products were measured during perfusion of rat jejunum and ileum in vivo with buffered and unbuffered 10 mM solutions of glycl-L-phenylalanine (Gly-Phe) and L-phenylalanyl-glycine (Phe-Gly). Mucosal brush border peptidase activity was then measured in the perfused segments in vitro at luminal pH and at two substrate concentrations. In addition cytosol peptidase activity in the perfused segments was measured at pH 7-4 and at 10 mM substrate concentrations. In the jejunum, there was a relationship between rates of free phenylalanine appearance in vivo (Phe-Gly greater than Gly-Phe) and rates of brush border (Phe-Gly greater than Gly-Phe) rather than cytosol (Gly-Phe greater than Phe-Gly) peptidase activities. No constant relationship between free phenylalanine appearance and hydrolysis of the dipeptides by either brush border or cytosol peptidases was observed in the ileal studies. These findings suggest that, in the jejunum, hydrolytic products originate from the surface of the cell whereas, in the ileum, hydrolytic products originate from both the intracellular compartment as well as from the surface of the mucosal cell. In the jejunum, in vitro rates of brush border hydrolysis of Gly-Phe were always less than in vivo disappearance rates, whereas rates of Phe-Gly brush border hydrolysis always exceeded luminal disappearance rates. These data imply that Gly-Phe is predominantly transported intact and hydrolysed by cytosol peptidases, In contrast, brush border peptidases play an importnat role in the mucosal hydrolysis of Phe-Gly.

Hydrolysis of peptides within lumen of small intestine

The quantitative significance of intraluminal peptide hydrolases in the terminal stages of peptide digestion has been investigated, and the precise origins of these enzymes have been determined. Intestinal contents and mucosae were obtained from rats anethetized with ether. Experiments carried out on pancreaticobiliary secretions and germfree rats show that pancreatic and bacterial enzymes do not contribute significantly toward the luminal digestion of dipeptides. Chemical assay data, thermostability studies, and examination of electrophoretic mobilities of luminal peptide hydrolases indicate that jejunal enzymes originate predominantly from the cytoplasm of intestinal mucosal cells, whereas the brush border of muosal cells is a major source of the enzymes in the ileum. With glycl-L-phenylalanine and L-phenylalanyl-glycine as substrates, jejunal luminal activity was less than 2.6% of mucosal activity. Brush-border peptide hydrolase activity in ileal contents, however, was 11.9% and 40.7% of mucosal brush-border activity for the two substrates. Luminal enzymes thus play an insignifcant role in the terminal digestion of peptides in the jejunum, but have a much more important role in the ileal digestion of peptides.

Some comments on gamma-ray implant dosimetry

Dosimetry calculations were made on the basis of recent photon interaction data and compared with existing sources. The build-up effect in muscle, photon attenuation in the source, filtration medium, and muscle, and 12 principal photon lines were incorporated in the calculations, and radium seeds of two standard dimensions were used. The results were within 1% agreement with previous findings for a 1-mCi radium source but showed an apparent discrepancy for a 13.3-mCi radium source filtered through a platinum tube, due to photon attenuation and scatter. These results may be extrapolated to fit circumstances such as drug-impregnated tissue.

Isolation and characterization of three unusual membrane glycopeptides present in rat intestinal endoplasmic reticula

Three glycopeptides have been isolated from the mucosal homogenates of the rat small intestine without using proteolysis. These glycopeptides appear to be localized exclusively in the membranes of the endoplasmic reticula. Although they have similar molecular weights of about 2550 and have similar amino acid compositions, they differ in the carbohydrate constituents. The major glycopeptide has 2 mol glucose per polypeptide chain while the two other glycopeptides contain 1 mol fucose, mannose and galactose with either 1 or 2 mol glucose. No hexosamine or sialic acid was detected in any of the glycopeptides. An unusual physical property was found associated with these glycopeptides. Below pH 6.5 they formed a precipitate which prevented them from diffusing through a dialysis membrane and allowed them to be rapidly purified following solubilization from the membrane. These glycopeptides appear to represent a new group of heretofore uncharacterized membrane constitutents which may play a role in some function specific for the endoplasmic reticula.

Rat intestinal brush border membrane peptidases. II. Enzymatic properties, immunochemistry, and interactions with lectins of two different forms of the enzyme

The properties of two purified peptidases derived from the intestinal brush border membrane of the rat have been investigated. The pH optima, heat stabilities, substrate specificities, and metal ion requirements of the two enzymes and the effects of inhibitors on their activities were nearly identical. The isoenzymes catalyzed the hydrolysis of a wide range of peptides containing from 2 to 8 amino acid residues. The enzymes are aminopeptidases; no evidence for carboxypeptidase or endopeptidase activity was found. For hydrolysis, there appears to be an absolute requirement for an L-amino acid at the NH2-terminus of the peptide substrate. There was a similar but less absolute requirement for the penultimate NH2-terminal amino acid. Thus, although peptides of the type L-aminoacyl-L-proline, L-aminoacyl-L-prolyl-(L-amino acid)n, or L-aminoacyl-D-amino acid were not hydrolyzed, L-leucyl-beta-naphthylamide could be utilized as a substrate. The enzymes appeared to be metalloenzymes in that metal ion-chelating agents could inhibit their activities. Co2+ partially restored the activities lost by chelation. Immunodiffusion studies showed that the two enzymes were immunologically identical. The antipeptidase antisera were specific for the enzymes and did not react with other constituents of the intestinal cell. Both enzymes have binding sites for the lectin phytohemagglutinin which recognizes N-acetylgalactosamine residues located at or near the terminal positions of glycoprotein carbohydrate chains. Both the lectin and the antibodies inhibited enzyme activities, but the mechanisms of inhibition appeared to be different.

Rat intestinal brush border membrane peptidases. I. Solubilization, purification, and physicochemical properties of two different forms of the enzyme

Two brush border peptidases have been isolated from the particulate fraction of the rat intestinal mucosa and purified to homogeneity as judged by polyacrylamide gel electrophoresis, starch gel electrophoresis, isoelectric focusing, and double immunodiffusion. For convenience, the peptidases have been designated peptidase F (fast) and S (slow) on the basis of their anodic mobilities. The isoelectric point of peptidase F was 4.76 and of peptidase S, 5.10. Both enzymes are glycoproteins. The amino acid compositions of the two peptidases are similar. The same carbohydrates are found in both enzymes, but there are differences in the molar concentrations of individual sugars. Peptidase S has greater concentrations of mannose and galactose and of hexosamines than peptidase F, while sialic acid is slightly greater in peptidase F. Carbohydrate accounted for approximately 19% and 23% of the weight of peptidases F and S, respectively. Estimates of the molecular weights of both enzymes by gel filtration gave values of 280,000. Electrophoresis of the enzymes under denaturing conditions on sodium dodecyl sulfate polyacrylamide gels indicated that each enzyme is a dimer consisting of two subunits of equal molecular weight, 140,000.

Release of peptide hydrolases during incubation of intact intestinal segments in vitro

1. Rat intestinal segments have been incubated in isotonic saline in vitro and release of peptide hydrolase enzymes into the incubation media examined over a 90 min study period. 2. Chemical assay data, as well as analysis of electrophoretic mobilities of release enzymes on starch gel, indicate that peptide hydrolase enzymes in the incubation media originate predominantly from the cytoplasm of the mucosal cells. 3. Peptide hydrolases were released rapidly from intact intestinal segments. Release occurred from the start of the in vitro incubations and was not affected by temperature and shaking. 4. It is concluded that compared to the in vivo situation, cytoplasmic peptide hydrolases are released from intestinal tissue very rapidly in vitro. Caution is therefore required when comparing results of in vivo and in vitro peptide absorption experiments.

Oxalate, calcium uptake and ATPase activity of sarcoplasmic reticulum vesicles

Ca++-uptake and Mg++-Ca++-dependent ATPase activity of skeletal muscle sarcoplasmic reticulum vesicles were reciprocally affected by increasing the oxalate concentration from 0 to 4 mM. At 0-0.1 mM oxalate approximately 17% of the calcium was removed by the vesicles from the medium while the ATPase activity was maximal (approximately 0.66 mumoles Pi mg-1 protein min-1). Between 0.1 to 0.2 mM oxalate the ATPase activity was reduced to one-fifth but the uptake rose sharply and 100% of the 45Ca++ was removed from the medium. The uptake was maintained at this level at oxalate concentrations greater than 0.4 mM but the ATPase activity remained inhibited. The kinetics of Ca++-uptake and ATPase activity were also differentially affected by oxalate. In the presence of oxalate, ruthenium red had only a very slight inhibitory effect on the calcium uptake. Addition of 0.1 mM EGTA removed 80% of the Ca++ from preloaded vesicles within 10 min. The formation of insoluble Ca-oxalate salt on the surface of the vesicle is suggested by these results. Calculations based on the Ksp of the calcium oxalate salt are presented to show its formation and the possible speciation of a Ca-oxalate complex which may affect the Ca++-uptake and ATPase activity.

Short chain fatty acids in rats with jejunal blind loops. I. Analysis of SCFA in small intestine, cecum, feces, and plasma

The luminal and plasma levels of short chain fatty acids (SCFA), products of bacterial fermentation, were measured in rats with surgically produced, self-filling blind loops located in the proximal small intestine. High levels of acetic, propionic, and butyric acids were detected in the blind loop segment and in the distal small bowel, regions which in normal and sham-operated rats contain no SCFA. Isobutyric, isovaleric, and valeric acids were also present. Feeding or fasting made little difference in the amount or composition of luminal SCFA. Although the amount of SCFA in each animal varied, the ratio of these acids was relatively constant. The ceca of the blind loop rats had relatively less acetic acid (48% of total SCFA) than did normal rats (64%) and proportionately more isobutryic, isovaleric, and valeric acids. The concentrations of SCFA increased in the feces of blind loop rats. The acetic acid concentration was 50% higher in blind loop rat feces; propionic, isobutryic, and isovaleric acids were elevated to a greater extent. The total output of most of the SCFA was also of acetic acid (137 +/- 32 mug per ml), the rest being isovaleric (5.2 +/- 2.6 mug per ml) and isobutyric (1.4 +/- 0.7 mug per ml) acids. Blind loop animals had nearly twice the concentration of acetic acid in the plasma (240 +/- 29 mug per ml) as normal animals, while the other acids were unchanged. The present study suggests that endogenous substances may be important substrates for the production of SCFA in the intestinal lumen. The high levels of SCFA in the small intestine and in feces and the substantial increase in the concentration of acetic acid in thvergrowth syndrome if the same relationships were found in man.

A study of intraluminal peptide hydrolase activity in the rat

1. Uncentrifuged and centrifuged rat intestinal contents were assayed for peptide hydrolase activity with glycyl-L-phenylalanine (Gly-Phe) and L-phenylalanyl-glycine (Phe-Gly) as substrates in the absence and presence of the intestinal cytosol peptide hydrolase inhibitor p-hydroxymercuribenzoate. 2. Jejunal contents hydrolysed Gly-Phe faster than Phe-Gly. Conversely, ileal contents hydrolysed Phe-Gly faster than Gly-Phe. 3. p-Hydroxymercuribenzoate markedly inhibited jejunal peptide hydrolase activity. There was ten times as much PHMB-resistant activity towards both dipeptides in ileal contents as in jejunal contents. 4. Most of the luminal enzyme activity was present in the supernatants after centrifugation, indicating the luminal enzymes exist in the soluble form. Although the presence of soluble bacterial enzymes cannot be excluded, peptide hydrolase enzymes in jejunal contents have the characteristics of mucosal cytosol enzymes whereas enzymes in ileal contents have the characteristics of mucosal bruch border as well as cytosol enzymes.

Glycoprotein metabolism in inflammatory and neoplastic diseases of the human colon

Carbohydrate compositions of the membrane and cytoplasmic fractions of human normal and cancerous colonic mucosa were compared in patients with blood groups O and B. The total sugar content in both fractions was reduced in the cancer tissues to about one-third of that in the normal colonic mucosa. The sugars that are associated with mucinous glycoproteins such as fucose and N-acetylgalactosamine were reduced significantly, while sugars that are primarily associated with "serum-type" glycoproteins were relatively unchanged or reduced to a lesser extent. The activities of glycoprotein:glycosyltransferases were variable, some showing so significant change, others beinb significnatly reduced in cancerous tissues. A polypeptidyl:N-acetylgalactosaminyltransferase (an enzyme that catalyzes the transfer of the first sugar to hydroxyamino acids of the protein core of mucinous glycoproteins), a sialyltransferase (involved in the addition of sialic acid to mucinous glycoproteins), and a galactoxyltransferase (thought to be responsible for blood group B antigenicity) were reduced in the cancerous colonic tissue. In contrast, the activities of these glycosyltransferases were unchanged in the colonic mucosa of patients with granulomatosis or ulcerative colitis. Glycosidase activities in the normal, cancerous, and inflammatory tissues were the same. These results suggest that in colonic cancer tissues the synthesis of one type of oligosaccharide chain may be greatly affected, while another family of oligosaccharides may remain relatively unaffected.

Studies on the binding of amylopectin sulfate with gastric mucin

Amylopectin sulfate, a sulfated polysaccharide that has an antipeptic property, was examined for its ability to bind gastric mucins. After chemically cross-linking the amylopectin sulfate into an insoluble gel, its binding with mucins isolated from antral and fundic mucosa of canine stomachs was studied with chromatography. A component present in both mucin fractions bound to the amylopectin sulfate gel below pH 4.5. This binding was reversible, and the complex dissociated above pH 5. Similar binding properties were found with soluble amylopectin sulfate. The component of the mucine which bound to amylopectin sulfate differed from the one which did not bind in its electrophoretic mobility and in its higher proportion of basic amino acids and a lower hexosamine, serine, and threonine content. This study suggests that amylopectin sulfate may bind to gastric mucins only under conditions of low pH.