Quantitation of soluble and skeletal alkaline phosphatase, and insoluble alkaline phosphatase anchor-hydrolase activities in human serum

Abstract

Background: The current studies were intended to compare the circulating levels of total and anchorless (soluble) skeletal and hepatic ALP isoenzyme activities, and insoluble ALP anchor-hydrolase activity in serum of postmenopausal women. Methods: Preliminary studies of the insoluble ALP anchor-hydrolase activity in serum revealed a pH optimum of pH 5–6.5, a sensitivity to inactivation by heat at temperatures >45 °C ( t 1/2=8–9 min at 60 °C), and an apparent K M (at pH 7.5) of 40–45 mU/ml of insoluble skeletal ALP activity. Results: Serum analyses showed that 94.5±0.5% (mean±SEM) of the ALP activity in serum was in the anchorless, soluble form. The data were also consistent with the notion that the amount of insoluble ALP anchor-hydrolase activity in serum, 52.8±0.8 U/l (mean±SEM), was sufficient for the conversion of anchor-intact (insoluble) ALP into the anchorless, soluble form, assuming activation by serum lipids and/or bile salts. Distributions of results for total, skeletal, hepatic, and insoluble ALP anchor-hydrolase activity were skewed toward the higher range and leptokurtotic ( p<0.01 for each). Total ALP activity ranged from 42% to 208% of the group mean value; skeletal, hepatic, and insoluble ALP anchor-hydrolase activities ranged from 5% to 306%, 33% to 277%, and 2% to 325%, respectively. In contrast, the soluble ALP fraction only ranged from 71% to 106% of the group mean value. Conclusions: The correlations between the total and both skeletal ( r=0.711, p<0.001) and hepatic ( r=0.782, p<0.001) ALP isoform activities were predictive. Although correlations were also observed between insoluble ALP anchor-hydrolase activity and total ( r=0.197, p<0.001), hepatic ( r=0.184, p<0.001) and skeletal ALP activities ( r=0.118, p<0.05), those relationships were not predictive ( r 2<0.04).