EFFECT OF PH AND ATP ON THE EQUILIBRIUM DENSITY OF LYSOSOMES

Lysosomes are membrane bound structures that accumulate and hydrolyze material internalized by the endocytic pathway. A very conspicuous pro perty of this subcellular compartment is its relatively high equilibri um density. The actual mechanism that regulates lysosomal density is p oorly understood. In an attempt to gain knowledge on the factors that regulate lysosomal density we have assessed the equilibrium density of lysosomal markers after in vitro incubation of a lysosome-enriched su bcellular fraction. Incubation at pH 6 for 10 min at 37-degrees-C caus es a density shift of several lysosomal markers to light density regio ns of Percoll gradients. Addition of ATP was able to prevent the acid- induced density shift. Pretreatment of the vesicles with N-ethylmaleim ide (NEM) or trypsin inhibited the effect of ATP. Working in intact ce lls', ATP depletion, a condition that causes cytoplasmic acidification , also decreases lysosomal density. The results indicate that at low p H lysosomal density is preserved by an active process that requires AT P and membrane associated proteins. (C) 1993 Wiley-Liss, Inc.