Characterization of human foetal intestinal alkaline phosphatase. Comparison with the isoenzymes from the adult intestine and human tumour cell lines

Aptameric Probe Specifically Binding Protein Heterodimer Rather Than Monomers

Dimerization of proteins occurs frequently and plays integral roles in biological processes. However, no single molecular probe is available for in situ detection of protein dimers on cells and tissues because of the difficulty of isolating complete protein dimers for probe preparation and screening, which has greatly hampered the biomedical study of protein dimers. Herein, a G-rich DNA aptamer (termed BG2) that only binds alkaline phosphatase (AP) heterodimers rather than monomers is reported. This aptamer is generated by the cell-SELEX (systematic evolution of ligands by exponential enrichment) technique and proves to fold into a duplex stabilized antiparallel G-quadruplex structure. Using BG2 as molecular probe, AP heterodimers are found to be expressed on several kinds of cancer cells. As an affinity ligand, BG2 could isolate AP heterodimers from cell lysate. BG2 is also demonstrated to be applicable for tumor imaging in mice xenografted with cells highly expressing AP heterodimers. AP isozymes are found in several tissues and blood throughout the body, but the function and tissue distribution of AP heterodimers are totally unknown; therefore, BG2 could serve as a molecular probe to uncover the mystery of AP heterodimers. The generation of aptameric probes by cell-SELEX will open up a new situation for the study of protein dimers.

Alkaline phosphatase retained in HepG2 hepatocarcinoma cells vs. alkaline phosphatase released to culture medium: difference of aberrant glycosylation

Liver tissue is the source of 90% of serum alkaline phosphatase (AP). The serum levels and structures of tumor marker proteins change under many disease conditions as well as cancer. The study was aimed at determining the type of alkaline phosphatase (AP) present in HepG2 hepatocellular carcinoma cell line. Alkaline phosphatase rich extracts of healthy human liver, HepG2 hepatocarcinoma cells, as well as the condition medium of HepG2 cells were prepared by extraction with 40% n-butanol and 30-50% acetone precipitation, and subjected to various chromatographic procedures. Lectin affinity chromatography of the samples with concanavalin A-Sepharose 4B showed considerable differences in the elution patterns. Non-denaturing polyacrylamide gel electrophoresis of the culture medium yielded a relatively slow migrating band of activity that coincided with none of the three bands of activity produced by the normal liver extract, nor with the bands of the cell pellet extract. Inhibition patterns were established by measuring the enzyme activities in the presence of varying concentrations of L-phenylalanine, L-leucine, L-homoarginine, and levamisole. The APs from the cell line were neuraminidase sensitive. According to the results the main AP produced and released to the medium by HepG2 cell line is an aberrantly glycosylated tissue non-specific AP. In addition, the differences between the cell-pellet AP and the culture medium AP seemed to stem from different sugar moieties in their structures.

Characterization of structural and catalytic differences in rat intestinal alkaline phosphatase isozymes

To understand the differences between the rat intestinal alkaline phosphatase isozymes rIAP-I and rIAP-II, we constructed structural models based on the previously determined crystal structure for human placental alkaline phosphatase (hPLAP). Our models of rIAP-I and rIAP-II displayed a typical alpha/beta topology, but the crown domain of rIAP-I contained an additional beta-sheet, while the embracing arm region of rIAP-II lacked the alpha-helix, when each model was compared to hPLAP. The representations of surface potential in the rIAPs were predominantly positive at the base of the active site. The coordinated metal at the active site was predicted to be a zinc triad in rIAP-I, whereas the typical combination of two zinc atoms and one magnesium atom was proposed for rIAP-II. Using metal-depleted extracts from rat duodenum or jejunum and hPLAP, we performed enzyme assays under restricted metal conditions. With the duodenal and jejunal extract, but not with hPLAP, enzyme activity was restored by the addition of zinc, whereas in nonchelated extracts, the addition of zinc inhibited duodenal IAP and hPLAP, but not jejunal IAP. Western blotting revealed that nearly all of the rIAP in the jejunum extracts was rIAP-I, whereas in duodenum the percentage of rIAP-I (55%) correlated with the degree of AP activation (60% relative to that seen with jejunal extracts). These data are consistent with the presence of a triad of zinc atoms at the active site of rIAP-I, but not rIAP-II or hPLAP. Although no differences in amino acid alignment in the vicinity of metal-binding site 3 were predicted between the rIAPs and hPLAP, the His153 residue of both rIAPs was closer to the metal position than that in hPLAP. Between the rIAPs, a difference was observed at amino acid position 317 that is indirectly related to the coordination of the metal at metal-binding site 3 and water molecules. These findings suggest that the side-chain position of His153, and the alignment of Q317, might be the major determinants for activation of the zinc triad in rIAP-I.

Placental and intestinal alkaline phosphatases are receptors for Aeromonas sobria hemolysin

The Aeromonas sobria hemolysin causes diarrhea following infection by this enteropathogenic bacterium. We previously identified the putative receptor for A. sobria hemolysin as a p66 protein on Intestine 407 cells (Microb. Pathog. 27 (1999) 215-221). Here, we have partially purified and obtained a peptide mass fingerprint of p66 which revealed its identity with placental alkaline phosphatase (PLAP). Recombinant PLAP expressed in 293T cells was also found to bind to hemolysin and the binding was found not to be dependent on the N-linked glycosylation of PLAP. By immunohistochemical analysis, PLAP expression was detected in human intestinal mucosa, the target tissue in disease. In addition to PLAP, hemolysin also binds to intestinal alkaline phosphatase (IAP), an enzyme that is also abundantly expressed in intestine. Thus, both PLAP and IAP are very likely involved in the pathogenesis of diarrhea caused by this bacterial toxin.

Structural evidence of functional divergence in human alkaline phosphatases

The evolution of the alkaline phosphatase (AP) gene family has lead to the existence in humans of one tissue-nonspecific (TNAP) and three tissue-specific isozymes, i.e. intestinal (IAP), germ cell (GCAP), and placental AP (PLAP). To define the structural differences between these isozymes, we have built models of the TNAP, IAP, and GCAP molecules based on the 1.8-structure of PLAP(1) and have performed a comparative structural analysis. We have examined the monomer-monomer interface as this area is crucial for protein stability and enzymatic activity. We found that the interface allows the formation of heterodimers among IAP, GCAP, and PLAP but not between TNAP with any of the three tissue-specific isozymes. Secondly, the active site cleft was mapped into three regions, i.e. the active site itself, the roof of the cleft, and the floor of the cleft. This analysis led to a structural fingerprint of the active site of each AP isozyme that suggests a diversification in substrate specificity for this isozyme family.

Ontogenic and longitudinal activity of Na(+)-nucleoside transporters in the human intestine

The objectives of our study were to identify the types of nucleoside transporters present in the human fetal small intestine and to characterize their developmental activity, longitudinal distribution, and transport kinetics compared with those present in the adult intestine. Nucleoside uptake by intestinal brush-border membrane vesicles was measured by an inhibitor-stop rapid filtration technique. Only the purine-specific (N1; hCNT2) and the pyrimidine-specific (N2; hCNT1) Na(+)-dependent nucleoside transporters were found to be present on the brush-border membranes of the enterocytes along the entire length of the fetal and adult small intestines. The activity of these transporters was higher in the proximal than in the distal small intestine. Both the N1 and N2 transporters found in the fetal intestine shared similar kinetic properties (Michaelis-Menten constant and Na(+)-nucleoside stoichiometry) to those in the adult intestine. During the period of rapid morphogenesis (11-15 wk gestation), no temporal differences were apparent in the activity of the N1 and N2 transporters in the fetal small intestine. These findings have implications for the absorption of drugs from the amniotic fluid by the fetus after maternal drug administration of nucleoside drugs such as the antivirals zidovudine and didanosine.

The two isozymes of rat intestinal alkaline phosphatase are products of two distinct genes

Rat intestinal alkaline phosphatases (IAP-I and -II) differ in primary structure, substrate specificity, tissue localization, and response to fat feeding. This study identifies two distinct genes ( approximately 5-6 kb) corresponding to each isozyme and containing 11 exons of nearly identical size. The exon-intron junctions are identical with those found in IAP genes from other species. The 1.7 and 1.2 bp of 5' flanking regions isolated from each gene, respectively, contain Sp1 and gut-enriched Kruppel-like factor (GKLF) binding sites, but otherwise show little identity. There is a potential CAAT-box 14 bp 5' to the transcriptional start site, 36 bp upstream from IAP-I, and a TATA-box 31 bp 5' to the transcriptional start site, 55 bp upstream from IAP-II. Transfection of these promoter regions (linked to luciferase as a reporter gene) into a kidney cell line, COS-7, produced the differential response to oleic acid expected from in vivo studies, i.e., threefold increase using the 5' flanking region of IAP-II, but not IAP-I. This response was not reproduced by 5,8,11,14-eicosatetraynoic acid (ETYA) or clofibrate, suggesting that peroxisome proliferator response elements are not involved. Isolation of the IAP-II gene will allow determination of the sequences responsible for dietary fat response in the enterocyte.

Lower alkaline phosphatase activity and occurrence of an abnormal hybrid intestinal/tissue non-specific isoform in Down's syndrome amniotic fluids

Quantitative and qualitative analysis of alkaline phosphatases (AP) was performed on amniotic fluid in 59 normal pregnancies and 14 Down's syndrome (DS) pregnancies at 16, 18 and 19 weeks of gestation. In DS cases, intestinal and placental isoenzyme levels were significantly reduced (P<0.001) and the AP electrophoretic pattern was seen to be modified on polyacrylamide gel electrophoresis. A unique component was detected. After extraction and purification of the abnormal isoenzyme, peptide fragments obtained after cyanogen bromide cleavage indicated a hybrid heterodimeric AP composed of intestinal and tissue non-specific subunits, as evaluated by SDS polyacrylamide gel electrophoresis.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Plasma intestinal alkaline phosphatase isoenzymes in neonates with bowel necrosis

AIM

To determine if the intestinal isoenzymes of alkaline phosphatase (ALP) are biochemical markers of bowel necrosis in neonates.

METHODS

Plasma ALP isoenzymes were measured in 22 babies with bowel necrosis, histologically confirmed, and in 22 matched controls. The isoenzymes were also measured in 16 infants with signs of necrotising enterocolitis, who recovered without histological confirmation of bowel necrosis. The isoenzymes were separated by polyacrylamide gel electrophoresis. Auxiliary tests for identification included neuraminidase digestion and treatment with monoclonal and polyclonal antiplacental antibodies.

RESULTS

Intestinal ALP was detected in 16 infants with bowel necrosis--13 had fetal intestinal ALP (FI-ALP) and three had adult intestinal ALP (AI-ALP). FI-ALP was detected in nine of the controls. In the babies with bowel necrosis intestinal ALP was found over all gestations, but in the controls only in those less than 34 weeks. The percentages of total ALP activity due to intestinal ALP were significantly higher in those with bowel necrosis compared with matched controls (p = 0.028). In babies of all gestations diagnostic sensitivity for the presence of intestinal ALP as a marker of bowel necrosis was 73% and diagnostic specificity 59%. In babies greater than 34 weeks' gestation, diagnostic sensitivity fell to 60% but the test became completely specific. In two babies FI-ALP increased from zero/trace to high activity coincident with the episode of bowel necrosis. In 16 babies with signs of necrotising enterocolitis but unconfirmed bowel necrosis FI-ALP was detected in four.

CONCLUSION

Intestinal ALP seems to be released into the circulation in some babies with bowel necrosis, but its detection does not have the diagnostic sensitivity and specificity to be a reliable biochemical marker of the condition.

Nuclear localization and characterization of alkaline phosphatase in neutrophils from normal controls and pregnant women

There were controversial data concerning localization of alkaline phosphatase (AP) in neutrophil nuclei under physiological conditions. In this context, the AP pattern has been determined on nuclei preparations from normal human neutrophils. Blood cells were isolated from 10 healthy adults and from 3 women in the third trimester of an uncomplicated pregnancy. Purity of nuclear suspension was checked by electron microscopy and assay of organelle marker enzymes. Electron microscope cytochemistry and immunocytochemistry studies were carried out on WBC. Enzyme characterization was performed by the usual biochemical procedures. AP was found in nuclear preparations from four of ten normal controls. When present, AP was detected in approximately two-thirds of the nuclei examined, representing an average of 20% of the total cell activity. Conversely, a large amount of nucleus-bound enzyme (55% of total AP activity) was recognized in all pregnant women samples. Biochemical and immunological characteristics clearly differentiate AP forms in the two groups of subjects. Normal controls have an heterogeneous enzyme pattern. AP positive preparations contain a mixture of isoenzymes: a prominent heat labile form and a relatively heat stable minor component. The heat stable fraction displays some properties similar to those previously described in leukocyte AP. Pregnant women express a unique very heat labile isoenzyme identical in its main characteristics to the early placental type.

Purification and partial characterization of intestinal-like alkaline phosphatase in rabbit kidney

Two types of alkaline phosphatase (AP) isozymes in rabbit kidney, a major intestinal-like type and a minor tissue-unspecific type, have been identified. The former enzyme was purified from rabbit kidney by immunoaffinity chromatography using monoclonal anti-human intestinal AP antibody. The purified enzyme yielded a single protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the apparent molecular size of its monomer subunit was found to be 72,000. Three amino acid residues within the first 16 N-terminal amino acid residues were different in purified AP and human intestinal AP. Although the rabbit enzyme possessed some peptide bands identical to those of human adult intestinal AP after Staphylococcus aureus V8 protease digestion, the enzyme did not react with monoclonal antibody against human adult intestinal AP alone, whereas it did react with monoclonal antibody against both human adult and fetal intestinal APs. The affinity of the enzyme for concanavalin A was identical to that of the fetal intestinal AP, but different from that of the adult enzyme. These results indicate that the antigenicity and certain properties of purified rabbit AP are more like those of human fetal intestinal AP or Kasahara isozyme, so-called intestinal-like AP, than like human adult intestinal AP.

Developmental changes in the antigenicity and sugar-chain heterogeneity of rabbit alkaline phosphatases

1. Rabbit alkaline phosphatases (APs) clearly fused with the anti-human AP antibodies. In particular, fetal liver and kidney APs reacted slightly less with the anti-intestinal AP antibody as did adult enzymes, suggesting that intestinal AP-like isozyme is expressed at earlier stages of gestation in rabbit liver and kidney. 2. Immunohistochemical data indicated that intestinal AP-like isozyme in the kidney was mainly localized in the distal convoluted tubules and slightly in the proximal straight tubules, whereas liver/bone/kidney AP-like enzyme was found more in the glomeruli and interstitial capillary walls as a major component. 3. The sugar-chain heterogeneity of adult and fetal rabbit APs displayed organ-specificity as did of rat and human APs. Moreover, in fetal development, the expression of high-mannose type or hybrid type sugar chains precedes the expression of complex type sugar chains in fetal development.

Segment-specific localization of intestinal-type alkaline phosphatase in human kidney

Specific monoclonal antibodies raised against human intestinal and human tissue-unspecific alkaline phosphatase (AP) were developed and were used to study the expression of these two isoenzymes in human renal tissue and their release into urine. Approximately 25% of the total AP content of renal tissue at the transition between cortex and medulla was of the intestinal type; the remainder was of the tissue-unspecific type (liver, bone, kidney AP). Immunoperoxidase staining using specific monoclonal antibodies against liver and intestinal AP revealed that the tissue-unspecific AP isoenzyme is present through-out the different segments of the proximal tubule, whereas intestinal-type AP is found exclusively in tubuloepithelial cells of the S3-segment of the proximal tubule. The intestinal-type enzyme obtained from the kidney had a similar heat stability and Km value, and similar immunologic and inhibitory (L-p-bromotetramisole; L-phenylalanine) characteristics compared to adult intestinal and fetal intestinal AP. Its electrophoretic mobility in agarose gel was intermediate between that of adult intestinal and fetal intestinal AP; after neuraminidase treatment it became indistinguishable from the adult intestinal isoenzyme. The intestinal-type AP found in the urine was not sensitive to neuraminidase and had a molecular weight significantly lower than the urinary tissue-unspecific AP isoenzyme. In conclusion, intestinal AP in the kidney is a specific marker for the brush border of the S3 segment of the proximal tubule, and this finding opens new perspectives in the cell biology of this particular part of the nephron.

Evidence for the expression of a primitive intestinal-like alkaline phosphatase in the intestinal 407 cell line

Intestinal-like alkaline phosphatase was found to be expressed in the intestinal 407 cell line. This enzyme was identified by use of monoclonal antibodies specific for human placental (H7 and HPMS-1) and intestinal alkaline phosphatase (2HIMS-1 and 2HIMS-3) separately. Purification of this isozyme by use of two different monoclonal antibody immunoaffinity chromatographies demonstrates a single protein band on SDS-polyacrylamide gel electrophoresis indicating that this enzyme is not formed as a heterodimer. The apparent monomer subunit molecular weight and the dimer molecular weight of this isozyme were determined to 70000 and 160000, respectively. The enzyme is a homodimer according to molecular weight determinations. Furthermore, this isozyme is neuraminidase sensitive and comparatively heat stable, properties also characteristic for the placental enzyme. Our data suggest that the intestinal-like alkaline phosphatase in the intestinal 407 cell line displays properties intermediate of the intestinal and placental isozymes which may reflect the existence and reexpression of a new primitive isozyme.

Placental alkaline phosphatase as a tumor marker for primary intracranial germinoma

A sensitive enzyme-linked immunosorbent assay (ELISA) was used in a retrospective study of placental alkaline phosphatase (PLAP) levels in serum, cerebrospinal fluid (CSF), and intratumoral cyst fluid in primary intracranial germinoma. The ELISA showed no cross-reactivity with intestinal alkaline phosphatase except in very high concentrations, after samples had been heat-treated. Three patients with germinoma were studied for serum PLAP levels and in all the levels were elevated (3.78, 0.52, and 2.11 IU/liter). Two of the germinoma patients were studied for PLAP levels in the CSF, and both had elevated levels (0.83 and 9.83 IU/liter). The intratumoral cyst fluid in one case of germinoma was tested for PLAP and the level was found to be very high (603 IU/liter). These PLAP levels decreased concomitantly with the reduction in tumor size during irradiation. Serum PLAP levels were measured in 40 control adult male individuals and in the CSF of 20 nonpregnant patients with subarachnoid hemorrhage. The upper normal limits were 0.20 and 0.11 IU/liter in the serum and the CSF, respectively. All PLAP levels measured in the serum of patients with various brain tumors were 0.18 IU/liter or less. This study strongly suggests that PLAP is a clinically useful tumor marker for primary intracranial germinoma.

A monoclonal antibody capture assay for intestinal alkaline phosphatase and the measurement of this isoenzyme in pregnancy

Intestinal ALP was purified from meconium to a specific activity of 1,100 U/mg protein and used as antigen in the preparation of 7 monoclonal antibodies. Two of these antibodies were specific for intestinal ALP and reacted with different epitopes. Both bound adult intestinal ALP better than fetal intestinal ALP. One of these antibodies was used to establish a capture assay for intestinal ALP in human serum over the range 0.5-16 U/l. Reference ranges of serum intestinal ALP concentrations were established in relation to blood groups. Measurement of intestinal ALP in the serum of pregnant women showed no correlation with pre-term fetal passage of meconium.

Homodimer and heterodimer forms of adult rat intestinal alkaline phosphatase

Three forms of alkaline phosphatase have been isolated from different sections of the small intestine: F3 180 kDa from the duodenum; F2 150 kDa principally jejunal; F1 120 kDa the only ileal form. Their catalytic properties have been compared as well as the electrophoretic properties the dimer and monomer of their phosphorylated intermediates. Pi was a competitive inhibitor of F1 and F3, whereas glycerophosphate was competitive inhibitor only of F3. Pi was a non competitive inhibitor of F2 and of a mixture F1 plus F3. Heating the phosphorylated enzyme preparations led to their dissociation into the phosphorylated monomers: F1 and F3 appear to be homodimers 65 kDa and 90 kDa peptides respectively whilst F2 seems to be a dimer formed from one of each monomer. F1 was phosphorylated faster but less intensively than F3. F2 was strongly phosphorylated over a long time-course and its 65 kDa monomer fraction was phosphorylated more strongly for longer than that from F1.

Comparison of a tumour-derived form of intestinal alkaline phosphatase with foetal and adult intestinal alkaline phosphatases

An intestinal alkaline phosphatase-like (Kasahara) isoenzyme has been isolated from the serum of a patient with lung cancer and compared with foetal intestinal alkaline phosphatase from the serum of a premature infant and with adult intestinal phosphatase isolated from serum in the same way. Although the ligand-binding sites of the three enzymes were indistinguishable, the foetal intestinal and Kasahara isoenzymes differed slightly from the adult isoenzyme in heat stability and markedly in electrophoretic mobility and neuraminidase-sensitivity, while themselves being similar in these respects. Neither the Kasahara isoenzyme nor foetal phosphatase reacted with anti-placental phosphatase monoclonal antibodies. These results suggest that the Kasahara isoenzyme corresponds to the reappearance of foetal intestinal alkaline phosphatase, rather than to modification of the adult intestinal isoenzyme.

Partial sequencing of human adult, human fetal, and bovine intestinal alkaline phosphatases: comparison with the human placental and liver isozymes

Purification, molecular weights, amino acid compositions, and partial sequencing of intestinal alkaline phosphatases (EC 3.1.3.1) from human adult, human fetal, and bovine sources is reported. Additional sequence information is presented for the bovine liver isozyme. Comparisons are made of the partial primary structures of intestinal alkaline phosphatases with those of the isozymes from liver and placenta. Homologies among these isozymes provide structural data to corroborate some concepts of the etiology of these isozymes and refute others.

Alkaline phosphatase isozymes in cultured human cancer cells

Alkaline phosphatase, an ubiquitous enzyme is known to exist in several isozymic forms. At least three different isozymes have now been identified in humans. Alkaline phosphatase isozymes are among the substances synthesized ectopically by a variety of human tumors and many continuous cell lines derived from different cancers have retained the capacity to produce these membrane-located glycoproteins. This paper reviews the identification of alkaline phosphatase isozymes in cultured tumor cells and relates these findings with recent developments concerning these cell membrane located glycoproteins.

Pre- and postnatal development of differentiated functions in rat intestinal epithelial cells

A panel of monoclonal antibodies to intestinal cell surface components has been used to compare the expression of differentiation-specific antigens in the epithelial cells of fetal, suckling, and adult rat small intestine. Indirect immunofluorescence staining, and immunopurification of detergent-solubilized membrane proteins, followed by single- and two-dimensional slab gel electrophoretic analysis, have demonstrated that fetal intestinal cells (at day 21 of gestation) express most differentiation-specific markers typical of adult absorptive villus cells. A marked heterogeneity in antigen expression was observed among different villus cell populations in suckling rat intestine, and three cell surface components were identified which are exclusively present during this period of intestinal development. Striking changes in the patterns of antigen expression in crypt and villus cells, and variations in the apparent isoelectric points for most luminal membrane components, were associated with the maturation of the intestinal mucosa at weaning. These changes could not be prematurely induced by cortisone injection in newborn rats, suggesting that factors other than glucocorticoids are responsible for the postnatal development of the intestinal epithelium. These results suggest that basic differences in biological properties and regulatory mechanisms exist among intestinal epithelial cells at different stages of pre- and postnatal maturation.

Different genes code for alkaline phosphatases from human fetal and adult intestine

Alkaline phosphatases from human adult intestine and fetal intestine (meconium) were purified and compared. Electrophoresis in SDS showed one band of protein in the former. There were three bands of protein in the latter, all with essentially the same peptide map. Thus, two of the bands probably arose by proteolysis of the third, which was largest (Mr 73000). In gradient gels of polyacrylamide the alkaline phosphatase from fetal intestine showed only one band of protein coincident with the band of activity (Mr 151000). Radiolabeled mapping showed that the tryptic peptides of the alkaline phosphatase from fetal intestine were distinctly different from those of adult intestine and human liver, and placenta, indicating a gene distinct from the three that code for the enzyme in liver/kidney/bone, placenta, and adult intestine.

Size and stability to sodium dodecyl sulfate of alkaline phosphatases from their three established human genes

Subunit molecular weights of human alkaline phosphatases (orthophosphoric-monoester phosphohydrolases (alkaline optimum), EC 3.1.3.1) determined by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) were dependent upon acrylamide concentration, a reflection of their glycoprotein nature. Molecular weights at a concentration of 7% (w/w) or greater were 68300, 80800 and 79400 for the enzymes from placenta, liver and mucosa of small intestine, respectively. All enzymes were dimers, the respective native Mr values determined by gradient gel electrophoresis being 138000, 186000 and 180000. None of the molecular weights was altered by desialylation. Stability of the catalytic activity of the purified enzymes to SDS varied and was very dependent on pH. SDS at 1% (w/v) rapidly denatured both native and desialylated alkaline phosphatase from placenta at pH 7.5 but had little effect on these at pH 10.3. Compared with placenta, the native enzyme from liver had greater stability at pH 7.5 and both native and desialylated forms had lower stability at pH 10.3. The enzyme from intestinal mucosa was sharply different from the other two isoenzymes: SDS had little effect at pH 7.5 but very rapidly denatured the enzyme at pH 10.3. The size of alkaline phosphatases and their stability to SDS can be used to identify gene products and to recognize heterodimers formed between products of more than one gene.

Structural analysis of human adult and fetal alkaline phosphatases by cyanogen bromide peptide mapping

The adult and fetal forms of human intestinal alkaline phosphatase (ALPase; orthophosphoric-monoester phosphohydrolase, EC 3.1.3.1) are indistinguishable by a variety of analytical procedures. However, they differ electrophoretically and can be differentiated by binding studies with monoclonal antibodies. In this report, these two enzymes along with placental and liver ALPases are compared by the technique of CNBr peptide mapping, and the role of carbohydrate in generating these patterns is investigated. NaDodSO4/PAGE of CNBr digests of radiolabeled ALPases from fetal and adult intestine shows that these two isozymes share five of seven common-sized CNBr fragments. Placental ALPase shares only one common-sized fragment with either intestinal enzyme. Liver ALPase has no CNBr fragments in common with any of the others. These data indicate that fetal intestinal ALPase is not a heterodimer of one subunit each of intestinal ALPase and placental ALPase as has been postulated. CNBr digests of neuraminidase-treated enzymes reveal a change of mobility of only one CNBr band in each of fetal intestinal, placental, and liver ALPases, indicating the presence of sialic acid residues in these fragments. Periodic acid/Schiff reagent staining (specific for carbohydrate) of CNBr digests of fetal and adult intestinal ALPases reacts with only one band in each enzyme, which is the same band from the fetal enzyme shown to contain sialic acid. However, fetal and adult intestinal ALPases each contain at least one CNBr fragment of unique size that is apparently nonglycosylated.

Differentiation of human adult and fetal intestinal alkaline phosphatases with monoclonal antibodies

Two forms of intestinal alkaline phosphatase have been recognized in humans. They are very similar in a number of biochemical and immunologic characteristics, but the exact genetic relationship between them remains unclear. To further study this problem, six monoclonal antibodies and a polyclonal rabbit antiserum to human fetal intestinal alkaline phosphatase have been produced. All of the monoclonal antibodies and the rabbit antiserum crossreact with adult intestinal alkaline phosphatase and with the intestinal-like alkaline phosphatase found in D98/AH-2 human tissue-culture cells. Four of the monoclonal antibodies and the rabbit antiserum crossreact with placental alkaline phosphatase, while none of the antibodies or the antiserum recognize liver or kidney alkaline phosphatase. Four of the monoclonal antibodies can distinguish between adult and fetal intestinal alkaline phosphatase in electrophoretic titration-binding studies, with the relative binding of adult enzyme being significantly greater than that of the fetal enzyme in each case. One of these antibodies, which also reacts with placental alkaline phosphatase, can distinguish the type 3 allelic variant of the placental enzyme from types 1 and 2. This indicates that the antibody detects a structural difference in the protein moiety of one of the allelic forms of the enzyme. These data suggest that adult and fetal intestinal alkaline phosphatases represent structurally distinct proteins, either encoded for by different genes or produced by differential processing of a common precursor molecule determined by a single gene.

Quantitation of human intestinal and liver/bone alkaline phosphatase in serum by rocket electroimmunoassay

The separation and quantitation of the various human serum alkaline phosphatases has been largely by indirect and/or imprecise methods. A rocket electroimmunoassay method has been developed for the quantitation of the alkaline phosphatases, which uses enzymatic activity to detect the rocket and bromochloroindolylphosphate as substrate. The assay for intestinal phosphatase requires only one-dimensional electrophoresis, since the antibody to the intestinal enzyme does not cross-react with the other phosphatases. Both liver and bone enzymes give a line of identity when tested against antibody directed against the liver phosphatases, and these phosphatases require separation first by acrylamide gel electrophoresis before quantitation by crossed immunoelectrophoresis. Liver/bone enzyme levels in serum are easily detected within the linear range of the assay without concentration of the serum. The assay is 10-fold more sensitive than quantitation by acrylamide gel electrophoresis. The inability to detect circulating intestinal alkaline phosphatase in the serum of most fasting hospitalized patients has been documented by this sensitive assay, and the predominance of the liver isoenzyme has been confirmed. The assay should prove useful for determining the tissue of origin of serum alkaline phosphatases, and in providing quantitative data for physiological studies. It is not adaptable for automation and will not prove useful in the routine clinical laboratory.