Conversion of human placental alkaline phosphatase from a high Mr form to a low Mr form during butanol extraction. An investigation of the role of endogenous phosphoinositide-specific phospholipases

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Abstract
Alkaline phosphatase in a wide range of tissues has been shown to be anchored in the membrane by a specific interaction with the polar head group of phosphatidylinositol. It has previously been suggested that the production of low Mr alkaline phosphatase during the commonly used butanol extraction procedure may result from the activation of an endogenous phosphoinositide-specific phospholipase C which removes the 1,2-diacylglycerol responsible for membrane anchoring. This conversion process was investigated in greater detail with human placenta used as the source of alkaline phosphatase. Mr and hydrophobicity of the alkaline phosphatase were determined by gel filtration on TSK-250 and partitioning in Triton X-114, respectively. Alkaline phosphatase extracted from human placental particulate fraction with butanol at pH 5.4 or released by incubation with Staphylococcus aureus phosphatidylinositol-specific phospholipase C produced a form of alkaline phosphatase of Mr approx. 170000 and relatively low hydrophobicity. By contrast, the butanol extract prepared at pH 8.3 was an aggregated form of Mr approx. 600000 and was relatively hydrophobic. The effect of a variety of inhibitors and activators on the amount of low Mr alkaline phosphatase produced during butanol extraction revealed that it was a Ca2+- and thiol-dependent process. Proteinase inhibitors had no effect. [3H]Phosphatidylinositol hydrolysis by the particulate fraction, unlike low Mr alkaline phosphatase production, was relatively sensitive to heat inactivation, indicating that the phosphoinositide-specific phospholipases C from cytosol and lysosomes were unlikely to be responsible for conversion. A butanol-stimulated activity which removed the [3H]myristic acid from the variant surface glycoprotein ([3H)mfVSG) of Trypanosoma brucei was detectable in the human placental particulate fraction. Since this activity was acid active, Ca2+- and thiol-dependent and relatively heat stable, it may be the same as that responsible for production of low Mr alkaline phosphatase. The only 3H-labelled product identified was phosphatidic acid, suggesting that the [3H]mfVSG-cleaving activity is a phospholipase D. These data strongly support the proposal that production of low Mr alkaline phosphatase during butanol extraction is an autolytic process occurring as the result of an endogenous phospholipase. However, they also suggest that the lysosomal and cytosolic phosphoinositide-specific phospholipases C that have previously been described in many mammalian tissues are not responsible for this process.