Abstract
This chapter focuses on hypophosphatasia, which is an inherited metabolic bone disease that clarifies the critical role that alkaline phosphatase (ALP) plays in skeletal mineralization. Subnormal activity of ALP in serum (hypophosphatasemia) is the biochemical hallmark and it reflects a generalized deficiency of tissue-nonspecific (liver/bone/kidney) ALP isoenzyme (TNALP), with attendant clinical effects of skeletal undermineralization, pathologic fractures, and loss of teeth. ALP denotes a group of isoenzymes, orthophosphoric monoester phosphohydrolases (alkaline optimum), characterized by optimal hydrolytic activity toward artificial phosphomonoesters at alkaline pH. In vitro measurements of enzyme activity for the assessment of bone and liver disease have been a part of clinical laboratory medicine for decades, but there is still some uncertainty about the precise composition of these proteins. Studies in different organisms and molecular se- quencing of the structural genes have led to a better understanding of their structure and function. In the infant, bone histomorphometry reveals a marked excess of osteoid volume and an osteomalacic pattern of tetracycline labeling in dynamic studies. Bone ALP is usually undetectable, and electron microscopy shows otherwise normal subcellular architecture of osteoblasts and their associated matrix vesicles.
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