The Possible Significance of Hexosephosphoric Esters in Ossification. Part III: The Action of the Bone Enzyme on the Organic Phosphorus Compounds in Blood

Alkaline Phosphatase Replacement Therapy for Hypophosphatasia in Development and Practice

Hypophosphatasia (HPP) is an inherited disorder that affects bone and tooth mineralization characterized by low serum alkaline phosphatase. HPP is caused by loss-of-function mutations in the ALPL gene encoding the protein, tissue-nonspecific alkaline phosphatase (TNSALP). TNSALP is expressed by mineralizing cells of the skeleton and dentition and is associated with the mineralization process. Generalized reduction of activity of the TNSALP leads to accumulation of its substrates, including inorganic pyrophosphate (PP_i) that inhibits physiological mineralization. This leads to defective skeletal mineralization, with manifestations including rickets, osteomalacia, fractures, and bone pain, all of which can result in multi-systemic complications with significant morbidity, as well as mortality in severe cases. Dental manifestations are nearly universal among affected individuals and feature most prominently premature loss of deciduous teeth. Management of HPP has been limited to supportive care until the introduction of a TNSALP enzyme replacement therapy (ERT), asfotase alfa (AA). AA ERT has proven to be transformative, improving survival in severely affected infants and increasing overall quality of life in children and adults with HPP. This chapter provides an overview of TNSALP expression and functions, summarizes HPP clinical types and pathologies, discusses early attempts at therapies for HPP, summarizes development of HPP mouse models, reviews design and validation of AA ERT, and provides up-to-date accounts of AA ERT efficacy in clinical trials and case reports, including therapeutic response, adverse effects, limitations, and potential future directions in therapy.

Alkaline Phosphatase: Discovery and Naming of Our Favorite Enzyme

Alkaline phosphatase can be considered "our favorite enzyme" for reasons apparent to those who diagnose and treat metabolic bone diseases or who study skeletal biology. Few might know, however, that alkaline phosphatase likely represents the most frequently assayed enzyme in all of medicine. Elevated activity in the circulation is universally recognized as a marker for skeletal or hepatobiliary disease. Nevertheless, the assay conditions in many ways are nonphysiological. The term alkaline phosphatase emerged when it became necessary to distinguish "bone phosphatase" from the phosphatase in the prostate that features an acidic pH optimum. Beginning in 1948, studies of the inborn-error-of-metabolism hypophosphatasia would identify the natural substrates and establish the physiological role of alkaline phosphatase, including in biomineralization. Here, we recount the discovery in 1923 and then eventual naming of this enzyme that remains paramount in our field. © 2017 American Society for Bone and Mineral Research.

Hydrolysis of phosphoric esters by the kidney in vivo

Robison (1923) found that aqueous extracts of macerated kidney contain an enzyme which hydrolyses phosphoric esters, such as hexosephosphates and glycerophosphate. This enzyme is also present in bones and teeth, and occurs in cartilage as soon as ossification starts. It is also present in the intestinal mucosa, but other tissues contain it in traces only. It is characterised by a high optimum p H (8·4-9·4). At p H 9·3 its activity is five times as great as at p H 7·3 (Robison and Soames, 1924). It was suggested by Robison that this enzyme plays an important rôle in the calcification of bone, and the suggestion has been supported by a considerable amount of evidence (3, 4, 5). Nothing, however, was known of the function of the enzyme in the kidney. It was thought possible that it might be required for the eventual hydrolysis and excretion of those phosphoric esters, which are present in considerable amount in the red corpuscles and in small amount in the plasma. Eichholtz and Starling (1925) have recently shown that the isolated kidney wheir perfused by means of a heart-lung preparation, does not excrete inorganic phosphates. This disability they attribute to the fact that the inorganic phosphates of the serum are present in a state to which the glomerular membrane is impermeable. They suggest, therefore, that the normal urinary phosphates are secreted by the tubule cells. Moreover, under the conditions of the experiment, the phosphate excretion would seem to be more readily extinguished than the excretion of either urea or sulphate.

THE PHOSPHATASE OF FETAL BONE

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