Rat ileal alkaline phosphatase activity and secretion is stimulated by alterations in calcium metabolism

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.

Reduced alkaline phosphatase activity in diabetic rat bone: a re-evaluation

We found previously that human bone alkaline phosphatase (AP) was glycated by aseptic incubation with glucose, and partially broken down by reactive oxygen species. In this study, we examined whether selective in vivo glycation of AP molecules occurred in bone tissue, using experimental diabetic rats induced by streptozotocin and spontaneously diabetic rats. Additionally, the effects of hyperlipidemia on bone AP activity were examined. Serum AP activity was significantly elevated after incipient onset of diabetes, and the increased activity originated from the intestinal isozyme. High levels of intestinal AP activity were also observed in rats with hyperlipidemia induced by feeding high-fat or high-fructose chow, but the AP activity in bone tissues was maintained at a constant level. AP activity in bone was reduced after the onset of diabetes. The resulting bone AP molecule bound to an aminophenylboronic acid column, which had affinity for glycated proteins, and contained smaller molecular sizes than the native bone AP. These results suggest that elevated levels of serum AP activity originated from the intestinal isozyme accompanied with hyperlipidemia induced by diabetes. In contrast, the reduced serum levels of AP activity in diabetic rats might be dependent on inactivation of bone AP, which was glycated, followed by partial breakdown of bone AP molecules, possibly due to reactive oxygen species.

Liver-like alkaline phosphatase in the tissue-unspecific type enzyme found in rabbit organs

Rabbit liver and kidney tissues are known to produce an intestinal-like alkaline phosphatase (IAP-like enzyme) as a dominant isozyme, with a minor isozyme of tissue-unspecific type (UAP), unlike humans and other mammalians. We investigated immunohistochemically and biochemically these unique isozymes in the rabbit liver and bone, and compared them with the human isozyme. In rabbit liver, UAP was found to be localized only in the apical part of the membrane of cells lining the bile duct, whereas IAP-like enzyme was found in the sinusoidal membrane of hepatocytes. Rabbit liver UAP was separated from IAP-like enzyme by DEAE-cellulose column chromatography. Rabbit bone tissue contained only one UAP isozyme. The two UAPs were biochemically and physicochemically compared with human liver AP. Both UAPs reacted with an anti-human liver AP monoclonal antibody, not with an anti-human bone AP monoclonal antibody, indicating that both enzymes have the same antigenicity as human liver AP. Rabbit liver and bone UAPs had similar N-linked sugar-chain heterogeneities to the respective human enzymes. In addition, rabbit bone AP also had an O-linked sugar chain, as did human bone AP, unlike rabbit and human liver APs.

Intestinal calcium transfer and alkaline phosphatase activity in relation with vitamin D and glucide diet

For four weeks after weaning, rats were fed either on a diet without any calcium utilization factors (-D) or on the same diet with cholecalciferol (+D) or sorbitol (S). In the -D group, blood calcium levels decreased whilst alkaline phosphatase activities in blood and bone were increased. For +D and S groups, these parameters were normal. Using everted or in situ ligatured loops, calcium transfer from a CaCl2 + 45Ca solution was measured in the duodenum, the jejunum and in the ileum. Alkaline phosphatase activity from these regions was also measured. For the three diets and for all regions of the intestine, there was a good correlation between calcium transfer and phosphatase activity. These values were higher in the duodenum than in the ileum or jejunum, and also higher in the ileum in the +D group than in the -D and S groups although this was not significant. These low levels in the S group which were, sometimes, even lower than those seen in the -D group contrasted with blood and bone levels of alkaline phosphatase, which were normal for the S and +D groups. There was also a discrepancy between the low values found for both phosphatase activity and calcium transfer in rats S in the experiments where the calcium transfer assay was carried out in calcium solution and those found in experiments were both calcium and carbohydrate were present. In the latter, enhanced levels of intestinal phosphatase activity were observed, as well as a marked, albeit delayed, increase in intestinal calcium transfer. Onset latency and rapid offset are reminiscent of induction of bacterial enzymes by carbohydrates.

Ultrastructural and functional abnormalities of intestinal and renal epithelium in the SHR

Intestinal calcium transport, renal tubular calcium reabsorption, and plasma 1.25 (OH)2 vitamin D3 (calcitriol) levels have all been reported to be diminished in the spontaneously hypertensive rat (SHR) compared with its genetic control the Wistar Kyoto rat (WKY). In the present study, absorptive duodenal and renal tubular epithelia of 12- to 14-week-old male SHR and WKY were examined by electron microscopy to determine whether such disturbances could be related to structural abnormalities. Patchy loss of microvilli in both duodenal and proximal tubular epithelia was observed in the SHR, whereas brush border membrane was entirely normal in the WKY. Irregular spaces were observed between the basal aspects of SHR intestinal epithelial cells and their basement membrane. In addition, the average height of duodenal and renal microvilli was reduced in the SHR. Two specific markers of the brush border membrane, alkaline phosphatase and villin, as well as the cytoplasmic vitamin-D dependent calcium-binding proteins, CaBP9K and CaBP28K were determined. Duodenal alkaline phosphatase activity was reduced in the SHR, compared with the WKY: 0.145 +/- 0.002 vs. 0.186 +/- 0.002 IE/min.microns 3 x 10(3) brush border, mean +/- SEM, N = 10 pairs, P less than 0.001. However, duodenal villin expression was not different from that of the WKY. Duodenal CaBP9K and renal CaBP28K content was diminished in the SHR: 21.0 +/- 0.80 vs. 29.9 +/- 2.19 micrograms/mg protein, N = 6 pairs, P less than 0.01 for duodenum, and 4.47 +/- 0.39 vs. 7.67 +/- 0.54 micrograms/mg protein, N = 6 pairs, P less than 0.001 for kidney. These data showing structural and functional abnormalities of intestinal and kidney cells in the SHR appear to reflect a disorder of transporting epithelia which may be either intrinsic or related to reduced circulating calcitriol.

Molecular cloning and expression of a cDNA encoding the membrane-associated rat intestinal alkaline phosphatase

Rat intestinal alkaline phosphatase (IAP) has been purified and proteolytic fragments sequenced. A cDNA library was constructed from duodenal poly(A) + RNA and screened for IAP positive clones by a full-length cDNA clone-encoding human IAP. A full length rat IAP clone (2237 bp) was isolated and sequenced, revealing a predicted primary sequence of 519 amino acids (61.974 kDa) with an additional signal peptide of 20 amino acids. 80% of amino acids from residues 1-474 were identical when compared with the human IAP, but there was only 31% identity in the COOH-terminal 45 amino acids. The homology diverges just before the putative binding site for the phosphatidylinositol-glycan (PI-glycan) anchor. The resulting peptide in rat AP contains five hydrophilic amino acids not present in the primary structure of human IAP. Binding of a synthetic 48-mer encoding a portion of this unique and divergent region (residues 476-491) was compared with that of the full-length clone on Northern blots of rat intestinal RNA. Two mRNAs, 3.0 and 2.7 kb, were detected by both probes, confirming earlier results, but the 48-mer bound preferentially to the 3.0 kb mRNA. The protein product of the full-length cDNA in a cell-free system was 62 kDa, corresponding with the smaller of the two IAP proteins produced by rat duodenal RNA. The cDNA transfected into COS-1 cells produced a membrane-bound IAP that was released by phosphatidylinositol-specific phospholipase (PI-PLC). These data provide definitive evidence that IAP is anchored by PI-glycan and conclusively demonstrate that the unique COOH-terminal structure encoded by this rat mRNA supports the addition of a PI-glycan anchor.

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.

Translocation of intestinal alkaline phosphatase in streptozotocin-induced diabetic rats

1. We determined the organ of origin and possible mechanism of translocation into the circulation of alkaline phosphatase (ALPase) in the diabetic rat. 2. Experimental diabetes was induced by injection of streptozotocin, resulting in a 8.2-fold elevation in serum ALPase activity. In this case, the major ALPase isozyme detected in serum was intestinal ALPase. 3. In in vitro experimental systems, ALPase was readily released from the duodenal plasma membrane by bacterial phosphatidylinositol-specific-phospholipase C (PI-PLase C) but little if any was released from the ileal membrane. 4. Serum and ileal ALPases were identical in terms of molecular size, whereas duodenal ALPase clearly differed from the serum enzyme. 5. Based on an investigation of the sugar moiety, more of the fraction having higher concanavalin A affinity was found in serum ALPase than with in the case of either of the intestinal ALPases. Serum and intestinal ALPases also differed slightly regarding isoelectric points. 6. Consequently, these data suggest that the serum ALPase of the diabetic rat is derived from ileal ALPase, and it is unlikely that the appearance of ALPase in the circulation is simply the result of solubilization by the action of PI-PLase C or phospholipase D.

Relationship between the uptake of calcium or phosphorus and alkaline phosphatase activity induced by certain modulators in rat organs

The effects of phorbol ester or calmodulin on the calcium and phosphorus uptakes by rat tissues and their relationship to the alkaline phosphatase activity (ALP) were investigated in vivo. In rat tissues, ALP activity and calcium uptake in the duodenum and liver were clearly induced by phorbol ester treatment, whereas in the calvarium and ileum they were decreased. Phosphorus uptake was increased by the administration of phorbol ester only in the calvarium. In rats pretreated with an injection of indomethacin as an inhibitor of prostaglandin-synthesizing enzyme, the selective uptake of calcium by phorbol ester was eliminated in the duodenum and liver, as was the ALP activity. In contrast, rats showed a marked increase in ALP activity in the ileum after calmodulin treatment. Moreover, the increased uptake of calcium after calmodulin treatment was clearly seen in the ileum, calvarium, and kidney, and an increased uptake in phosphorus was seen in the duodenum, ileum, and calvarium, but not in kidney. Furthermore, prior injection of W-7 or calmidazolium as an antagonist of calmodulin, reduced the increased ALP activities and the uptake of calcium in all organs tested, but did not reduce the increased phosphorus uptake by the calvarium. Consequently, it is suggested that calcium uptake under the above conditions correlated well with changes in the ALP activity; however, phosphorus uptake seemed to be less in accord with ALP activity. The amount of tested other mineral metabolic markers suggests that the Ca uptake and ALP activity induced by certain effectors regulate 1,25(OH)2D3 level by the modulation of 25(OH)D3-1 alpha-hydroxylase activity.