Giovanni's houseAlkaline phosphatase ALP was discovered in by Robert Robinson in young rats lombosciatalgja rabbits within ossifying bone and cartilage. For most of the past sgeroidea decades physicians have recognized the important clinical terapia steroidea lombosciatalgia that can come from measurement of ALP activity in serum. Detection and monitoring of hepatobiliary and skeletal disease are generally possible. In fact, since ALP detection terapia steroidea lombosciatalgia quantification in serum has been routine in hospital laboratories. Masten pa camping, the physiologiocal function of ALP, is largely unknown.
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Alkaline phosphatase ALP was discovered in by Robert Robinson in young rats and rabbits within ossifying bone and cartilage. For most of the past eight decades physicians have recognized the important clinical insight that can come from measurement of ALP activity in serum.
Detection and monitoring of hepatobiliary and skeletal disease are generally possible. In fact, since ALP detection and quantification in serum has been routine in hospital laboratories. Nevertheless, the physiologiocal function of ALP, is largely unknown. Bonucci and HC Anderson to be within novel extracellular structures called matrix vescicles These vescicles were found to be rich in ALP activity and later they have been demonstrated to be replenished by many enzymes and constituents such as: During early phase primary of mineralization , hydroxyapatite crystals appear and grow within these structres.
Soon after , the vescicles rupture and extravescicular secondary mineralization occurs as crystal propagation continues. Actually the proposed biological roles of ALP in mammals are numerous including: At plasmamembrane level it has been proposed that ALP can function not only such as an active P transporter but also for: Interestingly sequence analysis of ALP demonstrated that this enzyme can be coupled with other proteins, for example adhering to collagen, and it has been suggested that this physical property of ALP should be considered when we examined the action for example of ALP on skeletal matrix such as phosphoprotein phosphatase.
It has to be outlined that ALP function mainly such as a cell surface enzyme, but at some stages during embryo formation ALP may acts also intracellularly. Recently these different activities previously attribute only to Alkaline Phosphatase activity, have been studies in more details. Low levels of Pyridoxalphosphate are also observed in patients with hypophosphatemic rickets and the researchers attributed this data to increased activity of alkaline phosphatase enzymes.
However most of alkaline phosphatase values are within the normal range for children. In other words in these pathophysiological conditions no correlation exists between alkaline phosphatase activity and pyridoxalphosphate concentrations. However in these conditions plasma levels of inorganic phosphate are also higher than normal suggesting that the main factor in decreased pyridoxal 5 phosphate concentration would be low phosphate concentration rather than high levels of alkaline phosphatase.
This enzyme has been found in mineralizing tissues such as bones and teeth. Mutations in NPP1 cause the generalized arterial calcification of infancy due to inability of vascular cells to form pyrophosphate.
Another pathway for generation of pyrophosphate production is the secretion from cells by the transmembrane spanning cell surface protein Ankylosis Human homologue of the mouse progressive ankylosis protein ANKH. Two autosomal dominant human diseases have to date been reported: Data conerning the possible presence of an autosomal recessive form linked to a mutation on exon 6 of the 12 exons constituting ANKH gene has to be clarified by further studies.
Anyway ANKH protein seems to be important for mediating intracellular to extracellular channeling of pyrophosphate. Interestingly another protein called Phosphatase PHOSPHO-1, first identified in chick as a member of the haloacid dehalogenase HAD superfamily of Magnesium dependent hydrolases, is expresed at levels fold higher in mineralizing tissues compared to nonmineralizing ones.
The role of PHOSPHO-1 is to maintain the concentration of inorganic Pyrophosphate PP i so that the ratio of inorganic phosphate to inorganic pyrophosphate would be permissive of a normal mineralization process. ALP is found in nearly all plants and animals. In humans, four ALP isoenzymes are encoded by four separate genes. Three of these are expressed in a tissue-specific manner are they are called: The TNSALP chromosome structure is represented by 12 exons, 11 of which are translated into a aminoacid nascent enzyme.
From phylogenetic point of view, the TNSALP should represent an ancestral gene, whereas the tissue-specific ALPs is likely originated from a series of gene duplications. Human ALP isoenzymes gene sequence indicates that the nascent polypeptide has a short signal sequence of 17 or 21 aminoacids residues and a hydrophobic domain at its c terminal site.
The active site is coded by six exons and it is composed by 15 aminoacid residues with a nucleotidic sequence well conserved throught nature.
In summary the structure of these enzymes is formed to link a dinuclear metal cofactor structure so that a common cathalytic mechanism for enzymes involved in phosphotransfer reactions has been identified involving spin-coupled metal binding site formed by a scaffold structure at active metal linking site constituted by the same repeated tertiary spatial construct. Two metal ions are positioned at the apex of this fold forming a dinuclear metal center with 3.
Cathalytic activity require multimeric configuration of identical subunits, each monmer having an active site and two Zn atoms. The role of Zn atoms is probably those of allowing the formation of a nucleophil reactant by hydroxyl group of serine residue located on cathalitic site, that attract the phosphoric group disrupting the esteric link.
The mechanism of enzymatic reaction in ALP present in E. Coli has been elucidated for phosphate ester hydrolysis forming first an intermediate phosphoenzyme. In particular ALP of E. Coli cathalizes the transfert of phosphoryl group throught the formation of a transient link with a Serine residue located on active catalitic site.
Later this phosphate group is released and the cathalitic site left free to react with anoter phosphoester group.
ALP is linked to plasmamembrane surface, through a polar head group of a phosphatidylinositol glycan and it can be released by a specific phospholipase. Intracellualr degradation of ALPs can involve proteasomal structures. Release from plasma membrane could involve phosphatidase C or D. Clearance of circulating ALP, as for many plasma proteins is assumed to occur via uptake by the liver.
Whereas in children ALP plasma activity is mainly of bone origin and the remaining is of intestinal isotype; interestingly an old data report that blood type 0 and B are secretors influences the level of placental isoenzyme of ALP in the blood after an ingestion of a fatty meal.
In adult blood, ALP activity reflects equal amounts of hepatic and bone isotypes. Interestingly only recently on the crystal structure of placental isoform of ALP was isolated and studied on X ray crystallography.
Accordingly TNSALP partecipates in the calcification process both by restricting the concentration of extracellular inorganic pyrophosphate PPi and by contributing to the inorganic phosphate Pi pool available for calcification. The working model in bone and cartilage supposed that bone mineralization is first initiated within the lumen of Matrix Vescicles MVs.
In a second time, hydroxyapatite crystals grow beyond the confines of the MVs and become exposed to the extracellular milieu, where they continue to propagate along collagen fibrils. In other words an increased concentration of inorganic pyrophosphate PPi inhibits the crystalization process of hydroxyapatite, whereas increase increased concentration of inorganic phosphate ions Pi promote both crystalization and nucleation processes. Human diseases characterized by an abnormal decrease or increase in ALP blood levels are called respectively: In a Canadian pediatrician John Campbell Rathbun coined the term hypophosphatasia reporting a boy who developed and died from severe rickets with epilepsy, whose ALP activity in serum, bone and other tissues was paradoxically subnormal.
Present in all races, however this condition is expecially frequent in inbred Mennonite families from Mannitoba, Canada, where about 1 every 25 individuals is a carrier and 1: Six forms of hypophosphatasias have been individuated, the earlier is the presentation of symptoms and more severe is the skeletal disease and the biochemical manifestations: Laboratory findings include elevated values of phosphoethanolamine, pyridoxalphosphate, inorganic pyrophosphate.
It is characterized by deficiency in serum and bone alkaline phosphatse and defective bone and tooth mineralization. Nearly all babies with perinatal hypophosphatsia die in utero or shortly after birth. Adult hypophosphtasia typicaly manifests during middle age as recurrent, slowly healing metatarsal fractures, followed by painful nonhealing proximal femur fractures or pseudofractures. The bone symptoms are highly variable in their clinical expression, which ranges from stillbirth without mineralized bone to pathological fractures developing only late in adulthood.
Severe forms of the disease such as perinatal and infantile forms are transmitted as an autosomal recessive trait, whereas both autosomal recessive and autosomal dominant transmission may be found in milder forms, especially odontohypophosphatasia. In North American, Japanese, and European patients, indicating a very strong allelic heterogeneity in the disease. The remaining mutations are: This experiment of the nature, inherited as either an autosomal dominant or autosomal recessive trait, reveals a crucial role for TNSALP in skeletal mineralization.
There is no established medical treatment for hypophosphatasia. Augmenting circulating alkaline phosphatase activity into or even above the normal range for several months using intravenously administered ALP from various tissues sources has had no convincing beneficial effects.
Also transplantation therapy with cultured osteoblasts and bone fragments was quite unsuccessful and experiments suggested that we must lower PPi at mineralization sites. Recently a recombinant fusion protein including TNSALP ectodomain, the constant region of IgG1 Fc domain, and the terminal deca-aspartate motif has been admnistered in 11 patients with perinatal or infantile forms of hypophosphatasia.
Improvement is still being observed in patients receiving treatment for more than 3 years. The possible significance of hexosephosphoric esters in ossification.
Vescicles associated with calcification in the matrix of epiphyseal cartilage. J Cell Biol ; Structure and mechanism of alkaline phosphatase. Ann Rev Biophys Biomol Struct ; Coleman JE, Gettins P. Alkaline phosphatase p, solution structure, and mechanism. Reaction mechanism of alkaline phosphatase based on crystal structures: J Mol Biol ; Probing the role of histidine in zinc binding and the cathalitic mechanism of escherichia coli alkaline phsosphatase by site-specific mutagenesis.
Hypophosphatasia and the role of alkaline phosphatase in skeletal mineralization. Crystal structure of alkaline phosphatase from human placenta at 1. J Biological Chem ; Autosomal recessive hypophosphatemic rickets is associated with an inactivating mutation in the ENPP1 gene. Am J Hum Genet ; Tissue non specific alkaline phosphatase and plasma cell membrane glycoprotein1 are central antagonistic regulators of bone mineralization.
Loss-of-function ENPP1 mutations cause both generalized arterial calcification of infancy and autosomal recessive hypophosphatemic rickets. Collins MT, Boehm M. It ANKH necessarily so. J Clin Endocrinol Metab ; J Bone Miner Res ; N Engl J Med ; J Bone Min Res ; Childhood hypophosphatasia due to a de novo missense mutation in the tissue nonspecific alkaline phosphatase gene. Parathyroid hormone treatment improves pain and fracture healing in adult hypophosphatasia.
Enzyme replacement therapy in life threatening hypophosphatasia. Thanks to Ivy for her support. Monday, May 23, Metabolismo del collageno.