Further observations on the differential precipitation of alkaline phosphatase isoenzymes by ethanol

The proportions of the total activities of different isoenzymes of human alkaline phosphatase precipitated from serum by ethanol (20% v/v) were: liver phosphatase, 37%; placental phosphatase, 23%; bone phosphatase, 8.0%, and small-intestinal phosphatase, 3.7%. Treatment of the isoenzymes with neuraminidase reduced the percentages of non-intestinal phosphatases precipitated by ethanol to below 10%. Precipitation of intestinal alkaline phosphatase was unaffected by this treatment. The degree of solubility in ethanol therefore appears to be largely determined by the content of terminal sialic acid residues in the alkaline phosphatase molecules. In contrast the stabilities of the isoenzymes to heating at 56 degrees C were not significantly altered by neuraminidase digestion.