Isolation of a delta7-cholesterol desaturase from Tetrahymena thermophila

Cell-free preparations of Tetrahymena thermophila catalyze the direct desaturation of cholesterol to delta7-dehydrocholesterol (provitamin D3). The activity was isolated in the microsomal fraction from Tetrahymena homogenates. Delta7-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 added cholesterol, thus indicating that it is constitutively expressed in T. thermophila cells.