Phosphatidylinositol cleavage catalyzed by the soluble fraction from lymphocytes. Activity at pH 5.5 and pH 7.0

Phosphatidylinositol breakdown by subcellular preparations of small lymphocytes from pig mesenteric lymph nodes was investigated. Activity was higher than in preparations from the tissues studied previously; it was recovered largely in the soluble fraction, which showed pH optima at both 5.4-5.6 and 7.0-7.3. As in other tissues, phosphatidylinositol cleavage produced 1,2 diacylglycerol and a mixture of myo inositol 1:2 cyclic phosphate and myo inositol 1 phosphate. It was stimulated by addition of CaCl₂ and, less effectively, by MgCl₂. On sucrose density gradient ultracentrifugation at pH 7.0, 2 peaks of activity were observed (approx. sedimentation coefficients 8S and 10S); the activity profiles on the gradients were similar when assayed at pH 7.0 and 5.5. Activity at pH 7.0 (and 0.4 mM CaCl₂) was decreased by agents, such as salts and lipophilic cations, which tend to neutralize the negative charge of phosphatidylinositol; at pH 5.5 these agents slightly stimulated activity. It is suggested that the same enzyme(s) may be responsible for activity at both pH optima and that previous workers may have underestimated the pH 7.0 activity because of the inhibitory influence of cations under the usual assay conditions.