The autorelease of alkaline phosphatase from the plasma membrane during the incubation of cultured liver cell homogenates.

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When a rat hepatoma cell (R-Y121B) homogenate was incubated at 37°C, 30-70% of the total alkaline phosphatase was released into the supernatant fluid from the precipitate fractions. The release reached a plateau level after 10 h of incubation at 37°C. The optimum pH value for the release was 7. 4. Alkaline phosphatase activity increased during the incubation of the cell homogenates, but this increase was independent of the enzyme release. Serum increased not only alkaline phosphatase activity in the cultured cells but also enzyme release in their homogenates. In addition, we examined a rat liver homogenate and the following 11 cell lines: 3 hepatoma cell lines, including the R-Y121 B cell line, 4 liver cell lines, 2 human urinary bladder carcinoma cell lines, a kidney cell line, and a mouse adrenal tumor cell line. Only in the cultured liver cell line and hepatoma cell lines, 30-60% of the total enzyme was released into the soluble fraction from the precipitate fractions; the release was not observed in the other cell lines, nor in the rat liver homogenate. The release of alkaline phosphatase took place in both heat-stable and heat-labile alkaline phosphatases. Alkaline phosphatase, extracted from cell homogenates, showed two bands during polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The mobilities of the two bands changed inversely with or without sodium dodecyl sulfate. In general, the alkaline phosphatase which showed slow mobility with sodium dodecyl sulfate was more readily released from the plasma membrane. It is suggested that the physicochemical state of the form of the enzyme which binds to the plasma membrane of cultured liver cells might be different from that of the other tissue cell lines and rat liver homogenate.

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