SUMMARY: Enzyme activities of microsomal fractions have been used to examine the behaviour of cytoplasmic membranes during growth deceleration and encystment of Rough and smooth microsomal protein content increased two- to threefold during the transition from exponential growth phase to stationary phase, but declined again as the organisms encysted. Total activities of rotenone-insensitive NADH- and NADPH-cytochrome reductases, enzymes associated with endoplasmic reticulum, were two- to fivefold higher in stationary phase organisms than in exponentially growing organisms, but were 10- to 100-fold lower in mature cysts. Specific activities of these enzymes in homogenates and rough and smooth microsomal fractions showed the same pattern of change. Homogenate and microsomal activities of alkaline phosphatase, an enzyme known to be present on the contractile vacuole membrane, were highest in exponentially growing organisms, lower in stationary phase organisms and lower still, by at least eightfold relative to stationary phase cells, in mature cysts. The data indicate that during the transition to stationary phase there is extensive proliferation of cytoplasmic membranes which are subsequently broken down as the cells encyst. The changing activities of the cytochrome reductases can be attributed to this pattern of membrane synthesis followed by breakdown, although there may also be changes in the abundance of functional enzyme molecules on the membranes. The alterations in alkaline phosphatase activity portray the changing status of the membrane system responsible for water expulsion and reflect a less critical need for osmotic control in the cyst than in the trophozoite.


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