Studies on Tetrahymena membranes. Palmitoyl-coenzyme a desaturase, a possible key enzyme for temperature adaptation in Tetrahymena microsomes.

(1) Microsomes from a thermotolerant Tetrahymena NT-1 catalyze the conversion of palmitoyl-CoA to palmitoleate. (2) Palmitoyl-CoA desaturase enzyme requires molecular oxygen and NADH or NADPH as cofactor and its activity is inhibited by cyanide. A pH optimum range 7.0--7.3 is observed. (3) There is a clear break at 30 degrees C and a slight bend around 15 degrees C in the Arrhenius plots of palmitoyl-CoA desaturase activity. (4) After quenching from 39.5 degrees C, at 26 degrees C microsomal membranes show small particle-free areas, when examined by freeze-fracture electron microscopy, indicating the onset of phase separation. Larger smooth areas devoid of membrane-intercalated particles are observed in microsomes at 23 and 15 degrees C. The results support evidence that the thermally induced transition of desaturase enzyme activity in related to the altered membrane properties due to temperature change.