Bone Alkaline Phosphatase
المؤلف:
Marcello Ciaccio
المصدر:
Clinical and Laboratory Medicine Textbook 2021
الجزء والصفحة:
p510-511
2025-12-16
36
Alkaline phosphatase (ALP) is a ubiquitous enzyme on the outer surface of all membranes, and this localization would make the hypothesis plausible that it is involved in the trans port of specific substances across the cell membrane into the extracellular space. Its function is still unclear, but it has long been known that the enzyme plays an essential role in bone formation and mineralization.
The total circulating concentration is the sum of a group of different isoforms produced by various tissues (liver, bone, intestine, spleen, kidney, placenta), encoded by four genetic loci, three tissue-specific and one tissue-non-specific gene, located on chromosome 1. The hepatic/osseous/renal gene locus encodes for nonspecific ALP, which is present in liver cells, osteoblasts, and kidney cells. Since the phosphatases are encoded by the same gene and have the same protein nature, they should not be classified as isoenzymes. However, since they undergo specific, post-synthetic modifications of glycosylation and sialylation depending on the tis sue involved, they give rise to three distinct isoforms, bone, liver, and intestinal, which have different chemical and physical characteristics.
In a healthy adult subject, about 50% of the enzyme activity in serum originates from the liver, while the remaining 50% is bone-derived, with a ratio of 1/1. In contrast, this ratio is modified in children and adolescents due to the significant bone growth activity, with a high serum concentration of bone ALP (BAP), which represents about 90% of the total activity. The bone isoenzyme is produced by osteoblasts and catalyzes the hydrolysis of phosphate esters by increasing the concentration of phosphorus, which is necessary for the mineralization process.
Currently, the determination of bone alkaline phosphatase is preferred to the measurement of total circulating ALP, which has been widely used in clinical practice as a marker of bone metabolism because it precisely reflects osteoblast activity. The relationships among ALP, sex hormones (e.g., estrogens), and bone growth are responsible for gender- related differences. Specifically, in adolescence, the peak of bone mass is different between males and females; moreover, in men compared to women, there is a higher activity between 20 and 50 years (mainly due to differences in the concentration of the bone isoenzyme), while in women, in menopausal age, there is an increase in total activity, maintaining an equal concentration ratio between the liver and the bone form.
Many techniques have been proposed for the separation of the two main circulating isoforms based on separative (electrophoresis), non-separative (heat inactivation), and immunochemical approaches. These methods exploit the principle of the different physicochemical properties of the two forms due to variations in the carbohydrate-binding site in the chain and the degree of sialylation. Immunochemical methods, which allow rapid quantification of enzyme activity or enzyme mass, show a certain degree of cross-reactivity between the bone and liver forms (15–20%). An immunochemical method that allows the determination of BAP with an automated system at high precision and sensitivity has been developed and is spreading in clinical practice.
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