Download serum trial
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Decreased activity of the tissue nonspecific enzyme in hypophosphatasia, a genetic disorder, causes significant alterations in the metabolism of phosphoethanolamine, pyrophosphate, and pyridoxal 5′-phosphate, suggesting that these compounds are among the normal substrates for alkaline phosphatase ( 2). The latter form is posttranslationally modified to differentiate the bone, liver, and kidney isoforms.
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In humans there are four genes for this enzyme: intestinal, placental, germ cell, and tissue nonspecific ( 2). Under the proper conditions, catalysis by the enzyme appears to be diffusion limited, making it an almost perfect enzyme ( 1). It is a nonspecific phosphomonoesterase usually linked to the external cell surface via glycosyl phosphatidylinositol. This indicates that under physiological conditions alkaline phosphatase activity toward pyridoxine 5′-phosphate is reduced approximately 50% by the normal phosphate concentration and that it will increase or decrease significantly in response to changes in phosphate concentration within the ranges observed clinically.ĪLTHOUGH assay of alkaline phosphatase activity in serum is a common clinical test, the physiological function of this enzyme remains uncertain. mL gave estimates for the K m of 56 ± 11 μmol/L and for the K i of 540 ± 82μ mol/L for phosphate.Analysis of the kinetics using pyridoxine 5′-phosphate in undiluted serum from 10 subjects with phosphorus ranging from 0.5–2.1 mmol/L and alkaline phosphatase activity ranging from 41–165 nmol/min For the bovine kidney enzyme, the K m was 0.42 ± 0.04 μmol/L, and the K i for phosphate was 2.4 ± 0.2 μmol/L. We examined the kinetics of tissue nonspecific alkaline phosphatase from bovine kidney and sera from 49 patients with a wide range of endogenous phosphate concentrations using pyridoxine 5′-phosphate as a substrate at pH 7.4. Although inorganic phosphate has been recognized as a competitive inhibitor of this enzyme for many years, the influence of phosphate on alkaline phosphatase activity in serum under physiological conditions has not been previously reported. Natural and artificial manipulation of tissue nonspecific alkaline phosphatase activity indicates that pyrophosphate, phosphoethanolamine, and pyridoxal 5′-phosphate are among the natural substrates for this enzyme.