1. Rat liver contains a phosphatidylinositol kinase.
2. The enzyme requires phosphatidylinositol, ATP and Mg2+ or Mn2+ for activity.
3. The subcellular distribution of the enzyme does not parallel that of nuclei, mitochondria, lysosomes or endoplasmic reticulum.
4. The kinase and 5'-nucleotidase (a plasma membrane enzyme) are similary distributed in subcellular fractions.
5. The activity of phosphatidylinositol kinase is at least twice as high in purified plasma membrane as in nuclei prepared from the same crude nuclear fraction.
6. The role of the kinase at the cell surface is discussed.
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