Introduction

Lysosomes are organelles that contain many hydrolases with an acidic optimum pH.1'7 Lysosomes constitute 1-15% of the total cell volume and total cell protein in most mammalian cells. The existence of such organelles was suggested more than 100 years ago by the work of Metchnikoff and others concerning phagocytosis by protozoa.1,2 They observed that bacterial aggregates could be taken up by certain protozoa and were delivered to other regions of the cell, now known to be lysosomes or vacuoles, where they were digested.