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 isop entenyl diphosphate and Z,E,E-geranylgeranyl diphosphate. Neither doli chyl 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, dehydrodolic hyl phosphate, or dehydrodolichyl diphosphate as a substrate, dehydrod olichol was predominantly converted into dolichol. The enzyme that cat alyzes the conversion of dehydrodolichol to dolichol was localized in microsomes. The reductase activity was stimulated 9-fold by the additi on of a 100,000 x g soluble fraction. The reductase had an opimal pH a t 8.0. These results indicate that dolichol is formed from dehydrodoli chol in rat liver microsomes. The formation of dolichol from dehydrodo lichol was also catalyzed by 10,000 x g supernatant of rat or pig test is homogenates.