Liver Supernatant or Sterol Carrier Protein1 in the Enzymatic Conversion of Farnesyl Pyrophosphate to Squalene by Rat Liver Microsomes*

Earlier studies (Rilling, H. C. (1972) Biochem. Biophys. Res. Commun. 46, 470-475) suggested the possibility that a sterol carrier protein participated in the enzymatic conversion of farnesyl pyrophosphate to squalene by liver microsomal membranes. In the present article the possible requirement for soluble proteins in liver 105,000 x g supernatant (Slos) in the enzymatic conversion of farnesyl pyrophosphate to squalene by liver microsomal membranes was investigated. This microsomal enzyme (squalene synthetase) exhibited an apparent maximum velocity at a substrate concentration of approximately 25 CM. No evidence of substrate inhibition was seen at concentrations up to 65 PM. Squalene biosynthesis was linear for 45 min, and there was no lag period at short time intervals. With a judicious choice of experimental conditions rat liver microsomal squalene synthetase can be characterized to the extent that definite conclusions can be drawn with regard to the effect of supernatant proteins upon the reaction. Under these conditions phosphatases present in the microsomes or in S105 did not interfere with squalene synthesis. The results show that S105 has no significant effect on the conversion of farnesyl pyrophosphate to squalene by liver microsomes. Also, partially purified sterol carrier protein1 (SCP1) did not activate microsomal squalene synthesis from farnesyl pyrophosphate. The experimental findings definitively demonstrate that squalene synthetase of rat liver microsomal membranes performs its function without responding to the mediating effect of any specific supernatant protein. The results described here and those of the accompanying article (Gavey, K. L., and Scallen, T. J. (1978) J. Biol. Chem. 253, 5476-5463) demonstrate that the requirement for SCPl becomes necessary only after the formation of squalene, the first water-insoluble intermediate in the biosynthesis of cholesterol.