Supernatant protein factor, which stimulates the conversion of squalene to lanosterol, is a cytosolic squalene transfer protein and enhances cholesterol biosynthesis.

نویسندگان

  • N Shibata
  • M Arita
  • Y Misaki
  • N Dohmae
  • K Takio
  • T Ono
  • K Inoue
  • H Arai
چکیده

Squalene epoxidase, a membrane-associated enzyme that converts squalene to squalene 2,3-oxide, plays an important role in the maintenance of cholesterol homeostasis. In 1957, Bloch and colleagues identified a factor from rat liver cytosol termed "supernatant protein factor (SPF)," which promotes the squalene epoxidation catalyzed by rat liver microsomes with oxygen, NADPH, FAD, and phospholipid [Tchen, T. T. & Bloch, K. (1957) J. Biol. Chem. 226, 921-930]. Although purification of SPF by 11,000-fold was reported, no information is so far available on the primary structure or biological function of SPF. Here we report the cDNA cloning and expression of SPF from rat and human. The encoded protein of 403 amino acids belongs to a family of cytosolic lipid-binding/transfer proteins such as alpha-tocopherol transfer protein, cellular retinal binding protein, yeast phosphatidylinositol transfer protein (Sec14p), and squid retinal binding protein. Recombinant SPF produced in Escherichia coli enhances microsomal squalene epoxidase activity and promotes intermembrane transfer of squalene in vitro. SPF mRNA is expressed abundantly in the liver and small intestine, both of which are important sites of cholesterol biosynthesis. SPF is expressed significantly in isolated hepatocytes, but the expression level was markedly decreased after 48 h of in vitro culture. Moreover, SPF was not detectable in most of the cell lines tested, including HepG2 and McARH7777 hepatomas. Transfection of SPF cDNA in McARH7777 significantly stimulated de novo cholesterol biosynthesis. These data suggest that SPF is a cytosolic squalene transfer protein capable of regulating cholesterol biosynthesis.

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منابع مشابه

Studies on the conversion of enzymatically generated, microsome-bound squalene to sterol.

We have previously demonstrated that sterol carrier protein1 (SCP1) is required for the conversion of squalene to lanosterol by rat liver microsomes (Srikantaiah, M. V., Hansbury, E., Loughran, E. D., and Scallen, T. J. (1976) J. Biol. Chem. 251, 5496-5505). These studies, however, had the disadvantage that the highly waterinsoluble substrate squalene was added exogenously to incubations in sma...

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Purification and properties of sterol carrier protein1.

Previous studies have demonstrated that both the 105,000 X g soluble supernatant (S105) and microsomal membranes from rat liver are required for the enzymatic conversion of squalene to cholesterol (Scallen, T.J., Dean, W.J., and Schuster, M.W. (1968) J. Biol. Chem. 243, 5202). It was postulated that S105 contained a noncatalytic carrier protein which was required for this enzymatic process (Sca...

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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...

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عنوان ژورنال:
  • Proceedings of the National Academy of Sciences of the United States of America

دوره 98 5  شماره 

صفحات  -

تاریخ انتشار 2001