Abstract
The enzyme lysolecithin:lysolecithin acyltransferase has been isolated from the soluble fraction of rabbit lung and it has been found responsible for two different reactions: (a) transacylation between two molecules of lysolecithin to give dipalmitoylphospha-tidylcholine and glycerophosphorylcholine and (b) hydrolysis of the substrate yielding free fatty acid and glycerophosphorylcholine. Both activities copurify 80-fold with a hydrolysis/transacylation ratio about 2 in all purification steps. The enzyme shows a strong tendency to aggregate with other proteins without loss of activity. The hydrolysis/transacylation ratio is strongly dependent on experimental conditions and these must be carefully controlled if reproducibility is desired. The dependence of this ratio on several parameters is discussed. Based on identical behavior against temperature, β-mercaptoethanol, and iodoacetate and on the appearance of a unique band on electrophoresis, it can be concluded that only one protein is responsible for both activities. The molecular weight ranges between 58,000 and 60,000 D and the amino acid composition shows its acidic character, as described for the enzyme from other sources. The kinetic pattern of both reactions is different and depends on the physical state of the substrate; when the enzyme binds to monomers, the substrate is hydrolyzed to free fatty acid, whereas the binding to micelles favors the transacylation giving phosphatidylcholine. The kinetic constants have been evaluated and a model is proposed for both reactions. The activation energy has been measured and the proximity of the values for the two reactions suggests that the limiting step is the same for hydrolysis and transacylation. The implication of this enzyme in surfactant synthesis is discussed.