| Abstract: | Skeletal abnormalities with defective formation of mature calcified bone are the most prominent clinical features of hypophosphatasia. Low concentrations of serum and tissue alkaline phosphatase and elevated plasma and urinary levels of phosphorylethanolamine (PEA) are also present. Although PEA is hydrolyzed by serum alkaline phosphatase, the relationship between PEA and the deficiency is unclear. PEA has not previously been tested as a cytochemical substrate for the in situ demonstration of human alkaline phosphatase activity. We have studied alkaline phosphatase activity in hypophosphatasia in tissue sections, utilizing PEA and adenosinetriphosphate (ATP) as well as the usual beta-glycerophosphate and naphthol phosphate substrates. Neutral and acid phosphatase activities were also examined. Our results demonstrate that PEA is a substrate for the localization of alkaline phosphatase in normal human tissue, but is not hydrolyzed in hypophosphatasia in the liver, brain or costochondral junction under alkaline conditions. In the kidney in hypophosphatasia only the straight segments of proximal tubules that rim the medullary rays are reactive with PEA. Similar results in hypophosphatasia were obtained at an alkaline pH with ATP, beta-glycerophosphate, and naphthol phosphate. However, the defect in hypophosphatasia is not a generalized deficiency of membrane-associated phosphatases because membranes that were deficient in alkaline phosphatase activity demonstrated normal reactivity with ATP at neutral pH. In addition, thiamine pyrophosphate was also split by Golgi membranes within the cytoplasm. Acid hydrolysis of beta-glycerophosphate by lysosomes was normal. |
