Lysolecithinase of rat brain. Further analysis of the effect of the substrate on the particulate and microsomal enzymes

The inhibition of lysolecithinase (EC 3.1.1.5) by its substrate is further analyzed. A considerable portion of lysolecithin was adsorbed onto particulate or microsomal preparations of rat brain. The adsorbed substrate could be desorbed by isotonic sucrose and, to a greater extent, by serum albumin. The degree of adsorption depended on the quantity of protein and the concentration of lysolecithin. The V/S curves were biphasic, having an ascending portion, an 'inversion point', and a descending part. When the reaction rates were related to the unadsorbed, rather than the total concentration of the substrate, the inversion point (the substrate concentration at which the peak of the biphasic V/S curve occurs) coincided with the critical micellar concentration of lysolecithin. This supports the hypothesis of Leibovitz BenGershon et al. (1972) that the enzyme utilizes molecular solutions of lysolecithin and is inhibited by its micellar aggregates. The substrate also partly inhibited the enzyme at concentrations smaller than the inversion point. Evidence is presented that this is most probably an irreversible inactivation of the enzyme, caused by lysolecithin molecules or micelles which were adsorbed onto the particles or the microsomes. This inhibition could be prevented by premixing the substrate with serum albumin before adding the enzyme.