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

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 degreesC (t(1/2) = 8-9 min at 60 degreesC) 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). (C) 2001 Elsevier Science B.V. All rights reserved.