and 18 PM, respectively, the only lipid substrate was phosphatidic acid; neither lysophosphatidic acid nor any other lipid tested was phosphorylated. With 3zP-and 14C-labeled diacylglycerol pyrophosphate, the product of the enzyme, it was shown that the kinase catalyzes a reversible reaction. The activity of the extraded enzyme depended on the presence of surfactants such as Triton X-100 or 8-octylglucoside, whereas deoxycholate was strongly inhibitory. Kinetic analysis with lriton X-lOO/phosphatidate mixed micelles performed according to the "surface dilution" kinetic model showed saturation kinetics with respect to both bulk and surface concentration of phosphatidate. l h e interfacial Michaelis constant for phosphatidate was determined as 0.6 mo1 %.

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