The activity of microsomal delta 12-desaturase in Acanthamoeba castellanii was increased after growing cultures were chilled from the optimal growth temperature (30 degrees C) to 15 degrees C. This increase was detectable in microsomes isolated from organisms subjected to only 10 min chilling. The mechanism of induction was investigated. The increase in activity on chilling was greatly reduced when protein synthesis was blocked before the temperature shift. Thus the major mechanism for the induction of delta 12-desaturase is increased protein synthesis. delta 12-Desaturase activity was higher when assayed at 20 degrees C than when assayed at 30 degrees C, but these changes were not due to the increased solubility of O2 at 20 degrees C. The major substrate of delta 12-desaturase was found to be 1-acyl-2-oleoyl phosphatidylcholine.

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