Effect of sonication on nucleotide-dependent light scattering changes in retinal rod outer segment suspensions

Near-infrared light scattering from suspensions of rod outer segment fragments is a useful probe of visible-light-activated changes in peripheral membrane proteins in photoreceptor cells. Limited sonication of suspensions has been shown to increase the amplitude of light induced turbidity changes in the presence of guanosine triphosphate by a factor of 2. Further sonication led to a decrease in the signal amplitude by an order of magnitude. This reduction has been puzzling, since the activity of the GTP-binding protein (as measured by GTP hydrolysis turnover number) was unaffected by the range of sonication used. This effect of sonication is investigated here using a novel, Reticon-based apparatus that measures the angular distribution of scattered light from samples as small as 1 microliter. The results show that even at high rhodopsin concentrations (125 microM) with millimeter path lengths, significant amounts of unscattered light are transmitted by the samples. A simple phenomenological theory that assumes a constant fractional change in scattering power (15%), independent of amount of sonication, explains the effect of sonication on the angle dependence data as well as the original turbidity data. The results have general relevance for optimization of light-scattering studies of membrane systems.