Formation of dolichol from dehydrodolichol is catalyzed by NADPH-dependent reductase localized in microsomes of rat liver.

The alpha-saturation reaction involved in the biosynthesis of dolichol has been investigated with rat liver preparations. Under improved in vitro conditions with 10,000 x g supernatant of rat liver homogenates in the presence of NADPH at pH 8.0, dolichol was synthesized from isopentenyl diphosphate and Z,E,E-geranylgeranyl diphosphate. Neither dolichyl diphosphate nor dolichyl phosphate was detected. The chain length distribution of the dolicohol was the same as that of dehydrodolichyl products. In an assay system containing dehydrodolichol, dehydrodolichyl phosphate, or dehydrodolichyl diphosphate as a substrate, dehydrodolichol was predominantly converted into dolichol. The enzyme that catalyzes the conversion of dehydrodolichol to dolichol was localized in microsomes. The reductase activity was stimulated 9-fold by the addition of a 100,000 x g soluble fraction. The reductase had an opimal pH at 8.0. These results indicate that dolichol is formed from dehydrodolichol in rat liver microsomes. The formation of dolichol from dehydrodolichol was also catalyzed by 10,000 x g supernatant of rat or pig testis homogenates.