A method for the assay of phosphatidylinositol-specific phospholipase D activity in serum

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 degrees C (t(1/2)=8-9 min at 60 degrees 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).

Glycosylphosphatidylinositol-specific phospholipase D of human serum--activity modulation by naturally occurring amphiphiles

The enzymatic properties of glycosylphosphatidylinositol-specific phospholipase D (EC 3.1.4.50) were characterized using a 6,000-fold purified enzyme. This was obtained in 100 microg amounts from human serum with a recovery of 35%. Pure alkaline phosphatase containing one anchor moiety per molecule was used as substrate. The enzyme is stimulated by n-butanol, but in contrast to other phospholipases this activation is not produced by a transphosphatidylation reaction. The previously reported non-linearity of the specific activity with respect to phospholipase concentration in the test was no longer observed upon purification, indicating inhibitor removal. The serum inhibitor(s) co-chromatograph with serum proteins and lipoproteins. The main part of the inhibitory activity was found in the lipid fraction after protein denaturation and can be subfractionated into acid phospholipids, cholesteryl esters and triacylglycerides. Added phosphatidyl-serine, phosphatidylinositol, phosphatidylglycerol, gangliosides, cholesteryl esters, and sphingomyelins turned out to be strong inhibitors, as well as phosphatidic acid. Phosphatidylethanolamine and various monoacylglycerols were found to be activators. The low glycosylphosphatidylinositol-specific phospholipase activity found in native serum did not increase significantly upon 90% removal of phospholipids by n-butanol. High serum concentrations of strongly inhibiting compounds, complex kinetic interactions among aggregates of these substances, and compartmentalization effects are discussed as possible reasons for the observed inactivity.

Physicochemical and pathophysiological factors in the release of membrane-bound alkaline phosphatase from cells

Alkaline phosphatase is bound to cell membranes by a glycan phosphatidylinositol anchoring domain. The structure of this domain and ways in which it may be cleaved by chemical and enzymatic means provide a basis for understanding the solubilization of alkaline phosphatase from tissues in vitro and in vivo and the generation of isoforms.

Release of membrane-bound enzymes from cells and the generation of isoforms

Enzymes bound to the surfaces of cells may be retained by a hydrophobic amino acid sequence (e.g. gamma-glutamyltransferase) or by a specific glycan phosphatidylinositol (GPI) anchor (e.g. alkaline phosphatase). In either case the attachment is by means of non-covalent hydrophobic interactions between the anchoring domain of the enzyme and lipid components of the cell membrane. Enzyme molecules released into the plasma or bile, complete with their hydrophobic domains, can undergo aggregation and complexation to give rise to high molecular weight isoforms of gamma-glutamyltransferase or alkaline phosphatase. However, the GPI domain of alkaline phosphatase can be degraded by an inositol-specific phospholipase in plasma, but not in bile, with production of the hydrophobic, non-aggregating isoform of alkaline phosphatase that predominates in plasma.