Phosphatidylinositol kinases play a crucial role in signal transduction in many cell types. The 55 kDa isoform of phosphatidylinositol 4-kinases is a key enzyme in the metabolism of phosphoinositides, which work as regulators of cell function itself or as precursors of signal molecules. The experiments with HaCaT cells presented suggest that the phosphatidylinositol 4-kinase activity in this cell line corresponds to the 55 kDa isoform concerning kinetic parameters and specific activity in comparison with the malignant cell line A431. Km (for ATP and phosphatidyl-inositol) and Ki values (for Ca2+ and adenosine) are in good agreement with the parameters described for other cells. The findings support the idea that the 55 kDa phosphatidylinositol 4-kinase represents a key enzyme in the inositide signal transduction pathway.

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