Rapid isolation of rat liver secondary lysosomes--autophagic vacuoles--following chloroquine administration

A technique is presented for the rapid isolation of secondary lysosomes--autophagic vacuoles (AVs)--from rat liver by a one-step centrifugation in a discontinuous Metrizamide gradient. To this end chloroquine was injected into rats in order to increase the number of AVs, a prerequisite for their isolation, since they are rare in control liver tissue. Fraction purity was some 85-90% as judged from morphological analyses. To assess the proteolytic ability of the AVs they were isolated from livers of rats injected with [14C]leucine. Proteolysis increased significantly peaking after 60 min of chloroquine exposure, by far exceeding values for lysosomes isolated from control livers. This is in contrast to AVs (secondary lysosomes) obtained after leupeptin treatment which display lowered proteolysis as compared with control. After longer exposure times to chloroquine, proteolysis gradually returned to basal level. It is surmised that the augmented proteolysis in the isolated AVs is due to increased contents of substrate. So far, the chloroquine-induced AVs (secondary lysosomes) seem to be the best candidates for further analyses of proteolytic events in these organelles.

Calcium movements and the cellular basis of gravitropism

An early gravity-transduction event in oat coleoptiles which precedes any noticeable bending is the accumulation of calcium on their prospective slower-growing side. Sub-cellular calcium localization studies indicate that the gravity-stimulated redistribution of calcium results in an increased concentration of calcium in the walls of responding cells. Since calcium can inhibit the extension growth of plant cell walls, this selective accumulation of calcium in walls may play a role in inducing the asymmetry of growth which characterizes gravitropism. The active transport of calcium from cells into walls is performed by a calcium-dependent ATPase localized in the plasma membrane. Evidence is presented in support of the hypothesis that this calcium pump is regulated by a feed-back mechanism which includes the participation of calmodulin.