Abstract
Alkaline phosphatase, when solubilized from human liver plasma membranes by non-ionic detergents such as Triton X-100 or Nonidet P-40, has a tetrameric molecular weight when electrophoresed in polyacrylamide gradient gels under non-denaturing conditions [l]. This may be the same quaternary structure the enzyme has when bound to the plasma membrane. When the detergent-solubilized tetrameric enzyme is treated with a bacterial phosphatidylinositol phospholipase C (PI-PLC), it is converted to dimers [2]. The detergent-solubilized enzyme is hydrophobic, while the dimers formed by PI-PLC treatment are hydrophilic, as determined by their failure to adsorb to octyl-Sepharose [3]. It is likely that the interactions of the fatty acyl chains of the phosphatidylinositol (PI) anchor are a major factor for maintaining the tetrameric quaternary structure, since PI-PLC treatment resulted in a loss of these hydrophobic acyl chains and a concurrent conversion to dimers. However, the aforementioned experiments involved a very heterogeneous system since the alkaline phosphatase was obtained by direct solubilization from plasma membranes. Herein, we report the conversion by PI-PLC of pure tetrameric liver and placental alkaline phosphatases to dimers and the concomitant loss in hydrophobicity. The effect of the ionic detergent sodium dodecyl sulfate (SDS) on the quaternary structure of the two isoenzymes is also described.
Published Version
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