J. Biol. Chem., Vol. 256, Issue 14, 7235-7239, 07, 1981
Protein-facilitated intermembrane transfer of squalene. Demonstration by density gradient centrifugation
Y Kojima, EJ Friedlander and K Bloch
Squalene-enriched, trypsinized microsomes display no squalene epoxidase
activity either as such or when combined with normal microsomes. On
addition of microgram quantities of supernatant protein factor to the
combined system, squalene epoxidation commences at once and continues at a
rapid rate (Friedlander, E. J., Caras, I. W., Lin, L. F., and Bloch, K.
(1980) J. Biol. Chem. 255, 8042-8045). When mixtures of trypsin-treated,
[3H]squalene-containing microsomes and normal microsomes are subjected to
isopycnic density gradient centrifugation, the two microsomal populations
separate readily. Essentially all of the radioactive squalene remains
associated with the lighter (trypsinized) fraction of microsomes. However,
if the mixture of microsomes is initially incubated with supernatant
protein factor and then centrifuged, a large fraction of labeled squalene
sediments with the denser, normal microsomes. Thus, supernatant protein
factor mediates the transfer of squalene from one microsome population to
another. This conclusion had previously been reached on the basis of less
direct experiments (Friedlander, E. J., Caras, I. W., Lin, L. F., and
Bloch, K. (1980) J. Biol. Chem. 255, 8042-8045). Evidence is presented that
the process of supernatant protein factor-mediated squalene transfer does
not involve membrane fusion and proceeds also in the reverse direction.
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