Journal of Lipid Research, Vol 21, 1143-1147, Copyright © 1980 by Lipid Research, Inc.
In vitro conversion of squalene from squalene-phospholipid liposomes into sterols by rat liver microsomes and cytosol
RJ Morin and MV Srikantaiah
When rat liver cytosol, possessing sterol carrier protein (SCP) activity
was incubated with [3H]-squalene-phospholipid liposomes, cofactors, and rat
liver microsomes, squalene from the liposomes was converted into sterols.
When cytosol was omitted from the incubation mixture, only insignificant
amounts of sterols were produced. Liposomes of squalene with either
phosphatidylserine or phosphatidylcholine were equally effective as
substrates. The liposomes were stable at 4 degrees C for 3 weeks. The ratio
of squalene to phospholipid in the liposomes could be varied over a range
of 0.004 to 0.23. Multilamellar liposomes with squalene were not effective
as a substrate for the conversion of squalene to sterols. The mechanism for
transfer of squalene from the liposomes to the enzymes appears to be
initial binding of liposomes to microsomes, with subsequent transfer of the
substrate to the enzyme site by the SCP in the cytosol. Microsome-liposome
complexes prepared in the absence or presence of cytosol are effective in
converting squalene to sterols only if cytosol is added again, indicating
that cytosol is not required for the binding of liposomes to microsomes.
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