Lysophospholipases of rat brain.

1. The properties of rat brain lysophospholipase were investigated. Three subcellular fractions were employed: a particulate preparation which sedimented at 26,000 X g, a microsomal preparation which sedimented at 100,000 X g, and the supernatant of the above. The soluble enzyme represented only a small fraction of the total activity; it was further purified by treatment with protamine and ammonium sulfate and by gel filtration through Sephadex G-50. Attempts to solubilize the particulate or microsomal enzymes yielded only 1% or less of their activity as a soluble enzyme. 2. When reaction rates were plotted as a function of enzyme concentration, straight lines were obtained with the soluble enzyme. With the particulate or microsomal enzymes, these curves were straight lines only at lysolecithin concentrations below 0.05 to 0.1 mu. Above these concentrations, the curves were parabolic. 3. When reaction rates were plotted as a function of substrate concentration, the curves were biphasic. Assymmetrical bell-shaped curves were obtained with the particulate or microsomal enzymes. When using the soluble enzyme, the curve ascended as a hyperbola, but reached a maximal value above which the reaction rates were constant. With the particulate or microsomal enzymes the concentration of substrate at which the maximal activity was obtained increased with increasing protein concentrations. 4. Albumin increased the reaction rates catalyzed by the particulate or microsomal enzymes at all substrate concentrations. It changed the parabolic V/E curves into straight lines and the biphasic Y/S curves into rectangular hyperbolas. When using the soluble enzyme, low concentrations of albumin had no effect, while higher concentrations decreased the reaction rates. 5. Lysolecithin was adsorbed onto the particulate or microsomal enzymes; this was counteracted by serum albumin. 6. These data are discussed in light of the hypothesis that rat brain lysophospholipase utilizes monomers but probably not micelles of the substrate.