A lysosomal aminopeptidase isozyme in differentiating yeast cells and protoplasts

Cells of Saccharomyces cerevisiae that have been growing exponentially for many generations contain low activities of lysosomal enzymes. In contrast to such fully adapted cells, differentiating or resting cells contain comparatively high activities of these enzymes. Thus, the digestive enzymes seem to be involved in the process of biochemical differentiation.One of the four aminopeptidase isozymes present in extracts from yeast cells is localized in the vacuoles. This lysosomal enzyme can be separated from the other aminopeptidases by Sephadex G-150 gel filtration. Its specific activity is about 4 times higher in stationary cells than in exponentially growing cells.Upon incubating protoplasts in a buffered sorbitol medium the activities of proteases and RNase increase significantly. A corresponding increase of lysosomal aminopeptidase activity occurs in the absence of glutamic acid or casein hydrolysate. Cycloheximide and actinomycin D inhibit the increase of lysosomal hydrolase activities in differentiating protoplasts. The observed changes of enzyme activities are probably due to induced synthesis of the respective proteins.