Isolation of a Delta(7)-cholesterol desaturase from Tetrahymena thermophila

Cell-free preparations of Tetrahymena thermophila catalyze the direct desat uration of cholesterol to Delta(7)-dehydrocholesterol (provitamin D-3) The activity was isolated in the microsomal fraction from Tetrahymena homogenat es. Delta(7)-Desaturase activity was stimulated fivefold by the addition of 6 mM ATP. Other cofactors assayed, including NAD, NADP, NADH or NADPH, had no significant effect. The activity was found in microsomes prepared from stationary-phase cultures of the ciliate? grown either with or without adde d cholesterol, thus indicating that it is constitutively expressed in T. th emophila cells.