JBC, Vol. 251, Issue 18, 5496-5504, Sep, 1976
Purification and properties of sterol carrier protein1
M. V. Srikantaiah, E. Hansbury, E. D. Loughran and T. J. Scallen
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 (Scallen, T. J., Schuster, M.W., and
Dhar, A.K. (1971) J. Biol. Chem. 246, 224). Later evidence demonstrated
that S105 contained at least two proteins which were required for the
microsomal conversion of squalene to cholesterol (Scallen, T.J.,
Srikantaiah, M.V., Seetharam, B., Hansbury, E., and Gavey, K.L. (1974) Fed.
Proc. 33, 1733). This article describes the purification and properties of
the first of these soluble proteins, sterol carrier protein1 (SCP1), which
has been purified 575-fold from rat liver S105. While SCP1 specifically
activated the enzymatic conversion of squalene to lanosterol by liver
microsomal membranes, SCP1 possessed no capacity to activate the microsomal
conversion of [3H-A14,4-dimethyl-delta8-cholestenol to C27 sterols or of
[3H]7-dehydrocholesterol to cholesterol. Lanosterol was identified by
silicic acid chromatography and mass spectrometry. The formation of
lanosterol was a hyperbolic function of the concentration of SCP1 present
in the incubation mixture. The Km observed for SCP1 was similar to the Km
observed for squalene. The formation of lanosterol from squalene required
FAD. The addition of phosphatidylserine increased enzymatic activity;
however, phosphatidylserine was not required for this conversion. SCP1 was
catalytically inactive when it was incubated with [3H] squalene and
cofactors in the absence of microsomes. Substantial evidence supports the
hypothesis that SCP1 operates as a noncatalytic carrier protein for the
water-insoluble substrate squalene in the enzymatic conversion of squalene
to lanosterol by liver microsomal membranes.
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