# Alkaline Phosphatase: Function, Mechanism, and Assay Methods ## Introduction to Alkaline Phosphatase ### Definition and Nomenclature Alkaline phosphatase (ALP; EC 3.1.3.1) is a ubiquitous, membrane-bound metalloenzyme that catalyzes the hydrolysis of phosphate monoesters under alkaline pH conditions (optimal pH 8–10). The enzyme removes a phosphate group from a wide range of substrates, including nucleotides, proteins, and small organic phosphates, releasing inorganic phosphate and the corresponding alcohol or phenol. The name "alkaline" distinguishes it from acid phosphatase (EC 3.1.3.2), which operates optimally at acidic pH (around pH 4–5) and resides primarily in lysosomes. The systematic name for the enzyme is phosphate-monoester phosphohydrolase (alkaline optimum). In clinical and research contexts, "ALP" is the standard abbreviation, though "AP" is occasionally encountered in older literature. The enzyme is encoded by four distinct gene loci in humans: *ALPL* (tissue-nonspecific), *ALPI* (intestinal), *ALPP* (placental), and *ALPPL2* (germ cell/placental-like). Each gene produces a polypeptide that dimerizes to form the catalytically active enzyme. ### Biological Significance Alkaline phosphatase is not a single enzyme but a family of isozymes with overlapping yet distinct physiological roles. Its most well-characterized functions include: - **Bone mineralization**: TNAP hydrolyzes inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal formation, thereby promoting mineral deposition in osteoblasts and chondrocytes. - **Dephosphorylation of signaling molecules**: ALP can remove phosphate groups from nucleotides, such as ATP and ADP, and from phosphorylated proteins, thereby modulating extracellular signaling cascades. - **Intestinal lipid absorption**: Intestinal ALP participates in the dephosphorylation of lipopolysaccharides and nucleotides in the gut lumen, contributing to the regulation of the gut microbiome and inflammatory responses. - **Placental function**: Placental ALP is highly expressed at the maternal-fetal interface and is thought to facilitate nutrient transport and immune modulation during pregnancy. In molecular [cell signaling](/blog/guides/cell-signaling), ALP is particularly relevant because it acts on extracellular phosphorylated metabolites and proteins, effectively terminating or modifying signals initiated by kinases. For example, ALP-mediated dephosphorylation of extracellular ATP generates adenosine, a ligand for purinergic receptors, thereby linking [nucleotide metabolism](/knowledge/molecular-biology/nucleotide-metabolism) to [Signal Transduction](/knowledge/molecular-biology/signal-transduction) pathways. This places ALP at a critical juncture between metabolic flux and receptor-mediated signaling. ## Types and Isoforms of Alkaline Phosphatase ### Tissue-Specific Isoforms Humans express four ALP isozymes, each encoded by a separate gene and exhibiting tissue-restricted expression patterns: | Isozyme | Gene | Chromosomal Location | Primary Tissues | Notable Features | |---------|------|---------------------|-----------------|------------------| | Intestinal ALP | *ALPI* | 2q37.1 | Small intestine, liver (fetal) | Heat-stable; resistant to urea denaturation | | Placental ALP | *ALPP* | 2q37.1 | Placenta (syncytiotrophoblast) | Extremely heat-stable; polymorphic variants | | Germ cell ALP | *ALPPL2* | 2q37.1 | Testis, thymus, some cancers | Similar to placental ALP; expressed in seminomas | | Tissue-nonspecific ALP (TNAP) | *ALPL* | 1p36.12 | Bone, liver, kidney, brain | Heat-labile; subject to [alternative splicing](/blog/guides/alternative-splicing) | The three tissue-specific genes (*ALPI*, *ALPP*, *ALPPL2*) are clustered on chromosome 2q37.1 and share approximately 90% sequence identity at the nucleotide level, suggesting a common evolutionary origin through [gene duplication](/knowledge/molecular-biology/gene-duplication). In contrast, *ALPL* resides on chromosome 1 and shares only about 50% identity with the tissue-specific isoforms. ### Tissue-Nonspecific Alkaline Phosphatase (TNAP) TNAP is the most widely distributed isoform and is expressed in bone (osteoblasts), liver (hepatocytes and biliary epithelium), kidney (proximal tubules), and brain (neurons and endothelial cells). The *ALPL* gene undergoes [alternative splicing](/blog/guides/alternative-splicing), producing at least three mRNA variants that differ in their 5' untranslated regions but encode the same protein. However, post-translational modifications—particularly glycosylation—differ among tissues, giving rise to bone, liver, and kidney isoforms that can be distinguished by electrophoretic mobility and heat stability. Bone TNAP is anchored to the outer leaflet of the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. This membrane localization is critical for its role in mineralization, as it positions the enzyme at the site of hydroxyapatite deposition. Liver TNAP is also GPI-anchored but is released into the circulation by the action of phospholipase C, which cleaves the GPI anchor. This release accounts for the measurable serum ALP activity used in clinical diagnostics. ## Molecular Structure and Active Site ### Dimeric Assembly All ALP isozymes function as homodimers, with each monomer contributing to the formation of two active sites located at the dimer interface. The monomeric molecular weight ranges from 55 to 70 kDa depending on the isoform and glycosylation state. The dimer is stabilized by extensive hydrophobic interactions and a network of hydrogen bonds across the interface. Dimerization is essential for catalytic activity; monomeric ALP is enzymatically inert. The three-dimensional structure of bacterial ALP (from *Escherichia coli*) has been solved at high resolution and serves as the archetype for the mammalian enzymes. Each monomer folds into an α/β structure with a central β-sheet flanked by α-helices. The active site is a deep cleft formed by residues from both subunits, explaining why dimerization is required for catalysis. Mammalian ALPs share the same overall fold but contain additional loops and glycosylation sites that contribute to their thermal stability and tissue-specific properties. ### Metal Ion Coordination Each active site coordinates three metal ions: two zinc ions (Zn1 and Zn2) and one magnesium ion (Mg). These ions are essential for catalysis and are coordinated by conserved histidine, aspartate, and asparagine residues: - **Zn1** is coordinated by His331, His412, and Asp51 (numbering based on *E. coli* ALP; equivalent residues in human TNAP are His320, His432, and Asp42). - **Zn2** is coordinated by His370, Asp369, and the same Asp51 that bridges to Zn1. - **Mg** is coordinated by Asp153, Glu322, and a water molecule, and it sits adjacent to Zn2. The two zinc ions are positioned approximately 4 Å apart, creating a bimetallic center that polarizes the phosphate ester bond and stabilizes the transition state. The magnesium ion does not participate directly in bond cleavage but stabilizes the enzyme's active conformation and enhances catalytic efficiency. Removal of Mg²⁺ reduces activity by approximately 80%, while removal of Zn²⁺ abolishes activity entirely. The metal ions are bound with high affinity (Kd in the nanomolar to micromolar range for Zn²⁺) but can be displaced by chelating agents such as EDTA, which irreversibly inactivates the enzyme. This property is exploited in assay protocols that use EDTA as a stop reagent. ## Catalytic Mechanism of Alkaline Phosphatase ### Phosphoryl Transfer Steps ALP catalyzes the hydrolysis of phosphate monoesters (R-O-PO₃²⁻) via a two-step mechanism involving a covalent phosphoserine intermediate. The reaction proceeds as follows: 1. **Substrate binding**: The phosphate ester binds to the active site, with the phosphate group coordinating to both zinc ions. The Zn1 ion interacts with two of the phosphate oxygens, while Zn2 interacts with the third oxygen and the leaving group oxygen. 2. **Nucleophilic attack**: The hydroxyl group of Ser102 (in *E. coli*; Ser92 in human TNAP) is activated by Zn1, which lowers its pKa from approximately 13 to approximately 7. The activated serine oxygen attacks the phosphorus atom, forming a trigonal bipyramidal transition state. 3. **Phosphoserine intermediate formation**: The bond between the phosphate and the leaving group (R-O) is cleaved, and the leaving group is protonated by a water molecule coordinated to Zn2. This produces a covalent phosphoserine intermediate and releases the alcohol or phenol product. 4. **Hydrolysis of the intermediate**: A water molecule, activated by Zn2 and the magnesium ion, attacks the phosphorus atom of the phosphoserine intermediate. This displaces the serine oxygen and releases inorganic phosphate (Pi). 5. **Product release**: Inorganic phosphate dissociates from the active site, regenerating the free enzyme for another catalytic cycle. The overall reaction can be written as: R-O-PO₃²⁻ + H₂O → R-OH + HPO₄²⁻ The rate-limiting step is the hydrolysis of the phosphoserine intermediate (step 4), which proceeds with a rate constant of approximately 50–100 s⁻¹ under optimal conditions. The enzyme exhibits a broad substrate specificity, accepting any phosphate monoester, but shows a slight preference for substrates with bulky leaving groups, such as p-nitrophenyl phosphate (pNPP). ### Role of Metal Ions in Catalysis The metal ions contribute to catalysis through several distinct mechanisms: - **Substrate positioning**: The zinc ions coordinate the phosphate oxygens, anchoring the substrate in the correct orientation for nucleophilic attack. - **Charge neutralization**: The positive charges of Zn²⁺ neutralize the negative charges on the phosphate group, reducing electrostatic repulsion and stabilizing the transition state. - **Activation of the nucleophile**: Zn1 deprotonates the serine hydroxyl, generating a potent alkoxide nucleophile. - **Activation of water**: Zn2 and Mg coordinate and deprotonate a water molecule, generating hydroxide for the second step of the reaction. - **Leaving group stabilization**: Zn2 coordinates the leaving group oxygen, stabilizing the developing negative charge during bond cleavage. The bimetallic center thus functions as a "charge relay" system, analogous to the catalytic machinery of other metalloenzymes such as phosphodiesterases and nucleases. The magnesium ion, while not essential, increases the rate of the phosphoserine hydrolysis step by approximately 10-fold, likely by orienting the attacking water molecule and stabilizing the pentacoordinate transition state. ## Physiological Functions and Signaling Roles ### Bone Mineralization The best-characterized physiological function of ALP is its role in bone mineralization, mediated by the TNAP isoform expressed on the surface of osteoblasts and chondrocytes. During bone formation, osteoblasts secrete collagen type I and non-collagenous proteins into the extracellular matrix, creating a scaffold for mineral deposition. However, mineralization cannot proceed spontaneously because the matrix contains high concentrations of inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal growth. TNAP hydrolyzes PPi to inorganic phosphate (Pi), thereby: - Removing the inhibitory PPi from the mineralization front. - Increasing local Pi concentration, which drives hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) supersaturation and crystal formation. This dual action—removing an inhibitor while supplying a substrate—places TNAP at the center of the mineralization process. The importance of this function is underscored by the genetic disease hypophosphatasia, caused by loss-of-function mutations in *ALPL*. Affected individuals exhibit defective bone mineralization, leading to rickets in children and osteomalacia in adults, along with elevated plasma levels of PPi and phosphoethanolamine. ### Dephosphorylation of Signaling Molecules Beyond mineralization, ALP participates in extracellular signaling by dephosphorylating nucleotides and phosphorylated proteins. In the extracellular space, ATP and other nucleotides are released by cells as signaling molecules, acting on purinergic P2 receptors. ALP, along with ecto-nucleotidases such as CD39 and CD73, sequentially dephosphorylates ATP to ADP, AMP, and finally adenosine. Adenosine then acts on P1 receptors, which are G-protein-coupled receptors that modulate cAMP levels and [Second Messenger](/knowledge/molecular-biology/second-messenger) signaling. This pathway is particularly important in: - **Vascular biology**: ALP on endothelial cells regulates vascular tone by controlling adenosine availability. - **Immune modulation**: Intestinal ALP dephosphorylates bacterial lipopolysaccharide (LPS), reducing its endotoxic activity and dampening inflammatory responses in the gut. - **Neurotransmission**: TNAP in the brain dephosphorylates extracellular ATP and pyridoxal phosphate (PLP), the active form of vitamin B6. PLP dephosphorylation is required for the synthesis of neurotransmitters such as dopamine, serotonin, and GABA. ALP also dephosphorylates phosphorylated proteins in the extracellular environment, although the physiological relevance of this activity is less well established. Some studies suggest that ALP can dephosphorylate phospho-ERK and other signaling proteins released into the extracellular space, thereby modulating [Protein Kinase](/knowledge/molecular-biology/protein-kinase) signaling cascades. However, the specificity and in vivo significance of these reactions remain areas of active investigation. ## Clinical Significance and Diagnostic Use ### Liver Disease Serum ALP activity is one of the most commonly ordered liver function tests. In healthy adults, serum ALP originates primarily from liver (approximately 50%) and bone (approximately 50%), with minor contributions from intestine and placenta. The reference range is typically 44–147 IU/L, though this varies with age, sex, and laboratory. Elevated serum ALP is a hallmark of cholestatic liver disease, where bile flow is impaired. Conditions associated with elevated ALP include: - **Biliary obstruction**: Gallstones, pancreatic tumors, or strictures blocking the bile duct. - **Intrahepatic cholestasis**: Primary biliary cholangitis, primary sclerosing cholangitis, drug-induced liver injury. - **Infiltrative liver diseases**: Metastatic cancer, lymphoma, sarcoidosis. The mechanism of elevation is twofold: (1) increased synthesis of ALP by biliary epithelial cells in response to bile acid accumulation, and (2) release of membrane-bound ALP into the circulation by the detergent action of retained bile acids. In contrast, hepatocellular diseases such as viral hepatitis typically show only mild ALP elevation (less than 3-fold), with more pronounced increases in transaminases (ALT and AST). To distinguish hepatic from bone sources of elevated ALP, clinicians use: - **Gamma-glutamyl transferase (GGT)**: If GGT is also elevated, the ALP is likely hepatic in origin. - **ALP isoenzyme electrophoresis**: Separates bone, liver, and intestinal isoforms based on charge and heat stability. - **Heat inactivation**: Bone ALP is more heat-labile than liver ALP; heating serum to 56°C for 10 minutes inactivates bone ALP more rapidly. ### Bone Disorders Bone ALP is a sensitive marker of osteoblast activity and bone turnover. Elevated bone ALP is observed in: - **Paget's disease of bone**: Characterized by excessive osteoclast-mediated bone resorption followed by disorganized osteoblast-mediated bone formation. ALP can be markedly elevated, often 10–20 times the upper limit of normal. - **Osteomalacia and rickets**: Defective mineralization leads to increased osteoblast activity and elevated ALP. - **Primary hyperparathyroidism**: Parathyroid hormone stimulates osteoblasts indirectly, increasing bone turnover and ALP. - **Metastatic bone disease**: Particularly osteoblastic metastases from prostate, breast, and lung cancers. - **Fracture healing**: Transient elevation during the reparative phase. Decreased serum ALP is less common but clinically significant. The most important cause is hypophosphatasia, where *ALPL* mutations reduce enzyme activity. Other causes include hypothyroidism, malnutrition (zinc or magnesium deficiency), and certain medications (e.g., bisphosphonates, which suppress bone turnover). ## Assay Methods and Protocols ### Colorimetric Assay with pNPP The standard method for measuring ALP activity uses p-nitrophenyl phosphate (pNPP) as substrate. ALP hydrolyzes pNPP to p-nitrophenol (pNP) and inorganic phosphate. Under alkaline conditions (pH 10–11), pNP is ionized to the yellow p-nitrophenolate anion, which absorbs light at 405 nm. The rate of absorbance increase is directly proportional to enzyme activity. **Standard protocol**: 1. **Prepare reaction buffer**: 100 mM diethanolamine (DEA) buffer, pH 9.8, containing 0.5 mM MgCl₂. DEA is preferred over glycine because it acts as a phosphate acceptor, enhancing enzyme activity by approximately 3-fold. 2. **Prepare substrate solution**: Dissolve pNPP in buffer to a final concentration of 10 mM (saturating, approximately 5 × Km). 3. **Set up reaction**: In a cuvette or microplate well, combine 180 µL of buffer/substrate solution and 20 µL of sample (serum, plasma, or cell lysate). Mix gently. 4. **Measure absorbance**: Record absorbance at 405 nm immediately, then at 1-minute intervals for 5–10 minutes at 37°C. The reaction should be linear during this period. 5. **Calculate activity**: Use the molar extinction coefficient of p-nitrophenolate at 405 nm (ε = 18,800 M⁻¹ cm⁻¹ at pH 9.8) to convert the rate of absorbance change (ΔA/min) to enzyme units. **Calculation**: Activity (U/L) = (ΔA/min × V_total × 1000) / (ε × d × V_sample) where V_total is the total reaction volume (mL), V_sample is the sample volume (mL), d is the path length (cm), and 1000 converts liters to milliliters. One unit (U) of ALP activity is defined as the amount of enzyme that hydrolyzes 1 µmol of pNPP per minute under the specified conditions. ### Kinetic Parameters and Units The kinetic parameters of ALP vary with isoform and assay conditions. For human liver ALP with pNPP as substrate: - **Km**: 0.5–1.5 mM (in DEA buffer) - **Vmax**: 100–300 U/mg protein - **kcat**: 50–150 s⁻¹ - **Optimal pH**: 9.5–10.5 (in DEA buffer) - **Optimal temperature**: 37°C (standard clinical assay temperature) The choice of buffer significantly affects kinetic parameters. Tris and glycine buffers are less activating than DEA and yield lower apparent Vmax values. This is because DEA and other amino alcohols act as phosphate acceptors, facilitating the release of inorganic phosphate from the active site and increasing turnover. For research applications, ALP activity can also be measured using: - **Fluorogenic substrates**: 4-methylumbelliferyl phosphate (4-MUP) produces a fluorescent product (excitation 360 nm, emission 450 nm), offering higher sensitivity than colorimetric assays. - **Chemiluminescent substrates**: CSPD or CDP-Star, used in highly sensitive immunoassays. - **Electrophoretic zymography**: Separates ALP isoenzymes on native gels and detects activity by incubating the gel with a chromogenic substrate. ## Common Pitfalls and Troubleshooting ### Sample Stability ALP activity in serum is stable for up to 7 days at 4°C and for several months at −20°C. However, several factors can compromise stability: - **Hemolysis**: Red blood cells contain acid phosphatase, which can interfere with the assay at alkaline pH. More importantly, hemolysis releases intracellular phosphate, which inhibits ALP. Avoid hemolyzed samples. - **EDTA contamination**: EDTA chelates the essential Zn²⁺ and Mg²⁺ ions, irreversibly inactivating ALP. Samples collected in EDTA tubes are unsuitable for ALP measurement. - **Prolonged storage at room temperature**: ALP activity gradually decreases at 25°C, with approximately 10% loss per day. Process samples within 4 hours of collection. - **Freeze-thaw cycles**: Repeated freezing and thawing can denature the enzyme. Aliquot samples before freezing. ### Inhibitors and Interference Several substances interfere with ALP measurement: - **Phosphate**: Inorganic phosphate is a competitive inhibitor (Ki ≈ 1 mM). Serum phosphate levels (0.8–1.5 mM) can inhibit ALP by 10–20%, which is why DEA buffer is used—it sequesters phosphate and minimizes inhibition. - **Theophylline and caffeine**: These compounds inhibit ALP by competing with the substrate. Patients on theophylline may show falsely low ALP. - **Bilirubin**: High bilirubin levels (>20 mg/dL) can interfere with absorbance readings at 405 nm, causing falsely elevated results. - **Lipemia**: Turbid samples scatter light, increasing absorbance and producing falsely elevated ALP values. Ultracentrifugation or lipid-clearing reagents can resolve this. - **Zinc or magnesium deficiency**: Low serum levels of these metals reduce ALP activity. This is a common cause of low ALP in malnourished patients. ### Isoform Interference In clinical samples, the presence of multiple ALP isoforms can complicate interpretation: - **Intestinal ALP**: In individuals with blood groups B or O who are secretors, intestinal ALP can appear in serum after a fatty meal, causing transient elevations. Fasting samples avoid this. - **Placental ALP**: In pregnant women, placental ALP contributes significantly to total serum ALP, with levels rising throughout gestation. This is physiological and not indicative of disease. - **Macro-ALP**: ALP bound to immunoglobulins forms high-molecular-weight complexes that are cleared slowly from circulation, causing persistently elevated ALP without underlying disease. This benign condition is identified by gel filtration or polyethylene glycol precipitation. For the [Phosphatase Test in Milk](/knowledge/molecular-biology/phosphatase-test-in-milk), a related application, the same pNPP-based assay is used to verify proper pasteurization. Raw milk contains endogenous ALP, which is heat-inactivated by pasteurization. Residual ALP activity above a threshold indicates inadequate pasteurization or post-pasteurization contamination. This test exploits the differential heat stability of bacterial and mammalian ALP: bacterial ALP is more heat-resistant, so its presence indicates contamination. ## Frequently Asked Questions ### What is alkaline phosphatase? Alkaline phosphatase (ALP) is a family of membrane-bound metalloenzymes that catalyze the hydrolysis of phosphate monoesters under alkaline conditions (pH 8–10). It requires zinc and magnesium ions for activity and functions as a homodimer. In humans, four genes encode distinct ALP isozymes with tissue-specific expression patterns. ### What are the main types of alkaline phosphatase? The four human ALP isozymes are: tissue-nonspecific ALP (TNAP, encoded by *ALPL*), intestinal ALP (*ALPI*), placental ALP (*ALPP*), and germ cell ALP (*ALPPL2*). TNAP is expressed in bone, liver, kidney, and brain; the other three are restricted to their namesake tissues. ### What is the function of alkaline phosphatase? ALP has multiple functions depending on tissue context. In bone, TNAP hydrolyzes inorganic pyrophosphate to promote mineralization. In the intestine, it detoxifies lipopolysaccharides and regulates the microbiome. In the liver, it is released into bile and serum. It also dephosphorylates extracellular nucleotides, generating adenosine and modulating purinergic signaling. ### How does alkaline phosphatase work mechanistically? ALP hydrolyzes phosphate esters via a two-step mechanism. First, the hydroxyl group of an active-site serine attacks the phosphate group, forming a covalent phosphoserine intermediate and releasing the alcohol product. Second, a water molecule hydrolyzes the phosphoserine intermediate, releasing inorganic phosphate. Two zinc ions coordinate the substrate and activate the nucleophile, while a magnesium ion stabilizes the transition state. ### What is the importance of alkaline phosphatase in clinical diagnosis? Serum ALP is a routine biomarker for cholestatic liver disease and bone disorders. Elevated levels indicate biliary obstruction, primary biliary cholangitis, Paget's disease, or metastatic bone disease. Decreased levels suggest hypophosphatasia, hypothyroidism, or zinc/magnesium deficiency. Isoenzyme analysis distinguishes hepatic from bone sources. ### What is the standard protocol for alkaline phosphatase assay? The standard assay uses p-nitrophenyl phosphate (pNPP) as substrate in diethanolamine buffer (pH 9.8) containing magnesium chloride. The reaction is monitored at 405 nm, where the product p-nitrophenolate absorbs. Activity is calculated from the linear rate of absorbance increase and expressed as international units per liter (U/L). ### What are common pitfalls in alkaline phosphatase measurement? Common pitfalls include using EDTA-anticoagulated samples (which inactivate the enzyme), hemolyzed samples (phosphate release inhibits activity), and lipemic samples (light scattering causes false elevation). Theophylline and high bilirubin also interfere. For clinical interpretation, remember that intestinal ALP rises after fatty meals and placental ALP increases during pregnancy. ## Key Takeaways - Alkaline phosphatase is a zinc- and magnesium-dependent homodimeric enzyme that hydrolyzes phosphate monoesters at alkaline pH, with four human isozymes encoded by *ALPL*, *ALPI*, *ALPP*, and *ALPPL2*. - The catalytic mechanism proceeds through a covalent phosphoserine intermediate, with two zinc ions coordinating the substrate and activating the nucleophile, and a magnesium ion stabilizing the transition state. - TNAP is essential for bone mineralization by hydrolyzing the mineralization inhibitor pyrophosphate, and its deficiency causes hypophosphatasia. - ALP participates in extracellular signaling by dephosphorylating nucleotides (ATP to adenosine) and modulating purinergic receptor signaling, linking metabolism to [Signal Transduction](/knowledge/molecular-biology/signal-transduction). - Serum ALP is a clinically valuable biomarker for cholestatic liver disease and bone disorders, with isoenzyme analysis and GGT measurement used to localize the source. - The pNPP colorimetric assay in diethanolamine buffer is the standard method for measuring ALP activity, with activity expressed in international units per liter. - Sample handling is critical: avoid EDTA, hemolysis, and prolonged storage; be aware of interferences from phosphate, bilirubin, and lipemia, as well as physiological sources such as pregnancy and postprandial intestinal ALP. ## Further Reading - Sharma U, Pal D, Prasad R. *Alkaline phosphatase: an overview*. Indian journal of clinical biochemistry : IJCB. 2014. [PubMed 24966474](https://doi.org/10.1007/s12291-013-0408-y) - Kaplan MM. *Alkaline phosphatase*. Gastroenterology. 1972. [PubMed 4551808](https://pubmed.ncbi.nlm.nih.gov/4551808/) - Kaplan MM. *Alkaline phosphatase*. The New England journal of medicine. 1972. [PubMed 4550137](https://doi.org/10.1056/NEJM197201272860407) - Posen S. *Alkaline phosphatase*. Annals of internal medicine. 1967. [PubMed 5339227](https://doi.org/10.7326/0003-4819-67-1-183) - Pinto CS et al. *Alkaline phosphatase and mortality in stroke patients: a systematic review*. Annals of translational medicine. 2023. [PubMed 38213797](https://doi.org/10.21037/atm-23-1627) - Bianchi ML, Vai S. *Alkaline Phosphatase Replacement Therapy*. Advances in experimental medicine and biology. 2019. [PubMed 31482501](https://doi.org/10.1007/978-981-13-7709-9_10)
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