Linked pools of processed alpha-mannosidase in Dictyostelium discoideum

We are studying the fate of alpha-mannosidase, a lysosomal enzyme, in Dictyostelium discoideum. alpha-Mannosidase is synthesized as a 150,000-dalton precursor which becomes proteolytically cleaved to mature (56,000-62,000 dalton) forms. When cells are shifted into starvation buffer (5 mM phosphate, pH 6.5), the enzyme is secreted. We compared the kinetics of secretion of newly processed alpha-mannosidase with that of bulk forms. After 2 h in phosphate buffer less than 20% of the newly processed forms but as much as 50% of the bulk enzyme activity was secreted. During the course of the experiments, the total alpha-mannosidase activity remains constant, cells remain viable, and there is no evidence of degradation of enzyme. Furthermore, a 4-h chase period prior to starvation is required before the newly processed forms are secreted to the same extent as the bulk forms. On the basis of these results we propose that the enzyme is present in at least two pools and that it is transferred from a newly processed to an efficiently secreted pool.