which transport and secretion occur more or less continuously, as with ondary lysosomes). Two classes of proteins participate in lysosomal
immunoglobulins produced by plasma cells, or it is regulated by exter- function: soluble acid hydrolases and integral lysosomal membrane
nal signals, as in the control of salivary secretion by autonomic neural proteins. Each of the 50 known acid hydrolases (including proteases,
stimulation. In regulated secretion, the secretory product is stored tem- lipases, carbohydrases, esterases and nucleases) degrades a specific sub-
porarily in membrane-bound secretory granules or vesicles. Exocytosis strate. There are about 25 lysosomal membrane proteins participating
is achieved by fusion of the secretory vesicular membrane with the in the acidification of the lysosomal lumen, protein import from the
plasma membrane and release of the vesicle contents into the extracel- cytosol, membrane fusion and transport of degradation products to the
lular domain. In polarized cells, e.g. most epithelia, exocytosis occurs cytoplasm. Material that has been hydrolysed within secondary lyso-