Alkaline phosphatase, tissue-nonspecific isozyme (TNAP) is an enzyme that in humans is encoded by the ALPL gene. [5] [6]
Alkaline phosphatase, tissue-nonspecific isozyme (TNAP) is an enzyme that in humans is encoded by the ALPL gene. [5] [6]
There are at least four distinct but related alkaline phosphatases: intestinal, placental, placental-like, and liver/bone/kidney (tissue-nonspecific). The first three are located together on chromosome 2, whereas the tissue-nonspecific form is located on chromosome 1. The product of this gene is a membrane-bound glycosylated enzyme that is expressed in a variety of tissues and is, therefore, referred to as the tissue-nonspecific form of the enzyme. A proposed function of this form of the enzyme is in regulating matrix mineralization through its ability to degrade mineralization-inhibiting pyrophosphate. Mice that lack a functional form of this enzyme (gene knockout mice) show abnormal skeletal and dental development including a mineralization deficiency called osteomalacia/odontomalacia (hypomineralization of bones and teeth). [7] [8] [9] [10] Humans with inactivating mutations in the ALPL gene likewise have variable degrees of mineralization defects depending on the location of the mutation in the ALPL gene. [11] [12]
Tissue Non-Specific Alkaline Phosphatase (TNAP), encoded by the ALPL gene, exhibits an intriguing octameric structure as revealed by X-ray crystallography. [13] This distinct arrangement consists of four individual dimeric TNAP units. Structural studies on homologs of TNAP, namely human (ALPP) [14] and Escherichia coli (ecPhoA), [15] have identified the dimer as the minimal stable unit of TNAP. Notably, a single TNAP protein contains four metal ion binding sites: two Zn2+ sites and one Mg2+ site situated in the reaction center, and one Ca2+ site within the regulatory pocket. The octameric state observed in TNAP is unique compared to previously characterized alkaline phosphatases, all of which have been found in a dimeric state.
As the isozyme name “tissue-nonspecific” implies, TNAP is expressed ubiquitously and modified by post-translational glycosylation processes, to become isoforms [16] that provide significant proteomic diversity and specificity relating to various tissues and cells. The highest levels of human TNAP isoforms are expressed in bone, liver, and kidney tissues, with neutrophil granulocytes, brain and vascular cells as secondary sources of TNAP activity.
In human serum, the bone ALP (BALP) and liver ALP isoforms are the most abundant TNAP isoforms, in approximately a 1:1 ratio, comprising more than 90% of the total ALP activity. The remaining circulating ALP activity, 1–10%, is attributed mostly to intestinal ALP (IALP).
Several different analytical methods for separation and quantification of serum ALP isozymes and TNAP isoforms have been described over the years. Separation techniques like electrophoresis and chromatography are valuable for studying enzymes and proteins, revealing insights into their structure and function in pharmaceutical research and post-translational modifications (PTM) studies. [17] In particular, the development of commercial immunoassays for serum ALP has improved the usefulness and availability for clinical routine and research.
This enzyme has been linked directly to a disorder known as hypophosphatasia, a disorder that is characterized by low serum ALP and undermineralised bone (osteomalacia). The character of this disorder can vary, however, depending on the specific mutation, since this determines age of onset and severity of symptoms.
The severity of symptoms ranges from premature loss of deciduous teeth with no bone abnormalities to stillbirth [18] depending upon which amino acid [19] [20] is changed in the ALPL gene. Mutations in the ALPL gene lead to varying low activity of the enzyme tissue-nonspecific alkaline phosphatase (TNSALP or TNAP) resulting in hypophosphatasia (HPP). [21] There are different clinical forms of HPP which can be inherited by an autosomal recessive trait or autosomal dominant trait, [18] the former causing more severe forms of the disease. Alkaline phosphatase allows for mineralization of calcium and phosphorus by bones and teeth. [21] ALPL gene mutation leads to insufficient TNAP enzyme and allows for an accumulation of chemicals such as inorganic pyrophosphate [21] to indirectly cause elevated calcium levels in the body and lack of bone calcification.
The mutation E174K, where a glycine is converted to an alanine amino acid at the 571st position of its respective polypeptide chain, is a result of an ancestral mutation that occurred in Caucasians and shows a mild form of HPP. [18]
Liver function tests, also referred to as a hepatic panel or liver panel, are groups of blood tests that provide information about the state of a patient's liver. These tests include prothrombin time (PT/INR), activated partial thromboplastin time (aPTT), albumin, bilirubin, and others. The liver transaminases aspartate transaminase and alanine transaminase are useful biomarkers of liver injury in a patient with some degree of intact liver function.
The enzyme alkaline phosphatase is a phosphatase with the physiological role of dephosphorylating compounds. The enzyme is found across a multitude of organisms, prokaryotes and eukaryotes alike, with the same general function, but in different structural forms suitable to the environment they function in. Alkaline phosphatase is found in the periplasmic space of E. coli bacteria. This enzyme is heat stable and has its maximum activity at high pH. In humans, it is found in many forms depending on its origin within the body – it plays an integral role in metabolism within the liver and development within the skeleton. Due to its widespread prevalence in these areas, its concentration in the bloodstream is used by diagnosticians as a biomarker in helping determine diagnoses such as hepatitis or osteomalacia.
In biochemistry, isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters, or are regulated differently. They permit the fine-tuning of metabolism to meet the particular needs of a given tissue or developmental stage.
Aldolase A, also known as fructose-bisphosphate aldolase, is an enzyme that in humans is encoded by the ALDOA gene on chromosome 16.
Hypophosphatasia (; also called deficiency of alkaline phosphatase, phosphoethanolaminuria, or Rathbun's syndrome; sometimes abbreviated HPP) is a rare, and sometimes fatal, inherited metabolic bone disease. Clinical symptoms are heterogeneous, ranging from the rapidly fatal, perinatal variant, with profound skeletal hypomineralization, respiratory compromise or vitamin B6 dependent seizures to a milder, progressive osteomalacia later in life. Tissue non-specific alkaline phosphatase (TNSALP) deficiency in osteoblasts and chondrocytes impairs bone mineralization, leading to rickets or osteomalacia. The pathognomonic finding is subnormal serum activity of the TNSALP enzyme, which is caused by one of 388 genetic mutations identified to date, in the gene encoding TNSALP. Genetic inheritance is autosomal recessive for the perinatal and infantile forms but either autosomal recessive or autosomal dominant in the milder forms.
Boron, Walter F.; Boulpaep, Emile L., eds. (2017). Medical Physiology (3rd ed.). Philadelphia, PA: Elsevier. ISBN 978-1-4557-4377-3.
Corticosteroid 11-β-dehydrogenase isozyme 2 also known as 11-β-hydroxysteroid dehydrogenase 2 is an enzyme that in humans is encoded by the HSD11B2 gene.
X-linked hypophosphatemia (XLH) is an X-linked dominant form of rickets that differs from most cases of dietary deficiency rickets in that vitamin D supplementation does not cure it. It can cause bone deformity including short stature and genu varum (bow-leggedness). It is associated with a mutation in the PHEX gene sequence (Xp.22) and subsequent inactivity of the PHEX protein. PHEX mutations lead to an elevated circulating (systemic) level of the hormone FGF23 which results in renal phosphate wasting, and local elevations of the mineralization/calcification-inhibiting protein osteopontin in the extracellular matrix of bones and teeth. An inactivating mutation in the PHEX gene results in an increase in systemic circulating FGF23, and a decrease in the enzymatic activity of the PHEX enzyme which normally removes (degrades) mineralization-inhibiting osteopontin protein; in XLH, the decreased PHEX enzyme activity leads to an accumulation of inhibitory osteopontin locally in bones and teeth to block mineralization which, along with renal phosphate wasting, both cause osteomalacia and odontomalacia.
Pyruvate dehydrogenase kinase is a kinase enzyme which acts to inactivate the enzyme pyruvate dehydrogenase by phosphorylating it using ATP.
Aldolase B also known as fructose-bisphosphate aldolase B or liver-type aldolase is one of three isoenzymes of the class I fructose 1,6-bisphosphate aldolase enzyme, and plays a key role in both glycolysis and gluconeogenesis. The generic fructose 1,6-bisphosphate aldolase enzyme catalyzes the reversible cleavage of fructose 1,6-bisphosphate (FBP) into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP) as well as the reversible cleavage of fructose 1-phosphate (F1P) into glyceraldehyde and dihydroxyacetone phosphate. In mammals, aldolase B is preferentially expressed in the liver, while aldolase A is expressed in muscle and erythrocytes and aldolase C is expressed in the brain. Slight differences in isozyme structure result in different activities for the two substrate molecules: FBP and fructose 1-phosphate. Aldolase B exhibits no preference and thus catalyzes both reactions, while aldolases A and C prefer FBP.
UTP—glucose-1-phosphate uridylyltransferase also known as glucose-1-phosphate uridylyltransferase is an enzyme involved in carbohydrate metabolism. It synthesizes UDP-glucose from glucose-1-phosphate and UTP; i.e.,
Phosphate-regulating endopeptidase homolog X-linked also known as phosphate-regulating gene with homologies to endopeptidases on the X chromosome or metalloendopeptidase homolog PEX is an enzyme that in humans is encoded by the PHEX gene. This gene contains 18 exons and is located on the X chromosome.
Phosphorylase b kinase regulatory subunit alpha, liver isoform is an enzyme that in humans is encoded by the PHKA2 gene.
Adenylate kinase 2 is an enzyme that is encoded in humans by the AK2 gene. The AK2 protein is found in the intermembrane space of the mitochondrion.
Alkaline phosphatase, placental type also known as placental alkaline phosphatase (PLAP) is an allosteric enzyme that in humans is encoded by the ALPP gene.
Elevated alkaline phosphatase occurs when levels of alkaline phosphatase (ALP) exceed the reference range. This group of enzymes has a low substrate specificity and catalyzes the hydrolysis of phosphate esters in a basic environment. The major function of alkaline phosphatase is transporting chemicals across cell membranes. Alkaline phosphatases are present in many human tissues, including bone, intestine, kidney, liver, placenta and white blood cells. Damage to these tissues causes the release of ALP into the bloodstream. Elevated levels can be detected through a blood test. Elevated alkaline phosphate is associated with certain medical conditions or syndromes. It serves as a significant indicator for certain medical conditions, diseases and syndromes.
Arterial calcification due to deficiency of CD73 (ACDC) is a rare genetic disorder that causes calcium buildup in the arteries and joints of the hands and feet, and other areas below the waist. Although patients exhibiting these symptoms have been identified as early as 1914, this disorder had not been studied extensively until recently. The identification of the specific ACDC gene and mutations occurred in 2011. ACDC is caused by a mutation in the NT5E gene, which prevents calcium-removing agents from functioning,. Patients with this mutation experience chronic pain, difficulty moving, and increased risk of cardiovascular problems. In experiments at the molecular level, treatment with adenosine or a phosphatase inhibitor reversed and prevented calcification, suggesting they could be used as possible treatment methods. There is currently no cure for ACDC, and patients have limited treatment options which focus primarily on removal of blood calcium and improving mobility.
Alkaline phosphatase, placental-like 2 is a protein that in humans is encoded by the ALPPL2 gene.
Glycogen phosphorylase, liver form (PYGL), also known as human liver glycogen phosphorylase (HLGP), is an enzyme that in humans is encoded by the PYGL gene on chromosome 14. This gene encodes a homodimeric protein that catalyses the cleavage of alpha-1,4-glucosidic bonds to release glucose-1-phosphate from liver glycogen stores. This protein switches from inactive phosphorylase B to active phosphorylase A by phosphorylation of serine residue 14. Activity of this enzyme is further regulated by multiple allosteric effectors and hormonal controls. Humans have three glycogen phosphorylase genes that encode distinct isozymes that are primarily expressed in liver, brain and muscle, respectively. The liver isozyme serves the glycemic demands of the body in general while the brain and muscle isozymes supply just those tissues. In glycogen storage disease type VI, also known as Hers disease, mutations in liver glycogen phosphorylase inhibit the conversion of glycogen to glucose and results in moderate hypoglycemia, mild ketosis, growth retardation and hepatomegaly. Alternative splicing results in multiple transcript variants encoding different isoforms [provided by RefSeq, Feb 2011].
Asfotase alfa, sold under the brand name Strensiq, is a medication used in the treatment of people with perinatal/infantile- and juvenile-onset hypophosphatasia.