Comparison of serum catalytic activity and immunoreactivity of bone alkaline phosphatase in hypophosphatasia

Hypophosphatasia is a rare bone disorder characterised by low levels of tissue non-specific alkaline phosphatase (TNSALP). Although TNSALP is widespread in virtually all tissues the clinical effects, when produced, seem only to affect the mineralizing tissue such as teeth and skeleton. The skeleton is severely affected in the perinatal form of the disease, when death may occur in utero, or may not be affected in the adult type variety of the disease. We therefore compared the catalytic (cBALP) and immunoreactivity (iBALP) of bone alkaline phosphatase isoenzyme in six families with hypophosphatasia. iBALP was measured using an IRMA method. cBALP was measured after electrophoretic separation of serum alkaline phosphatase isoenzymes on lectin containing agarose gel. The percentage of different isoenzymes was calculated using densitometric scanning and cBALP calculated from the known total serum alkaline phosphatase activity. Results showed cBALP=0.796+3. 269iBALP, r=0.9 p<0.01, in cases of hypophosphatasia. In general, the lower the iBALP and cBALP the more severe the skeletal disease. The bone isoenzyme level predicts the clinical severity of bone disease.