Journal of Lipid Research, Vol 24, 290-296, Copyright © 1983 by Lipid Research, Inc.
Activation of HMG-CoA reductase by microsomal phosphatase
KR Feingold, MH Wiley, AH Moser, SR Lear and MD Siperstein
HMG-CoA reductase activity can be modulated by a reversible
phosphorylation-dephosphorylation with the phosphorylated form of the
enzyme being inactive and the dephosphorylated form, active. Phosphatases
from diverse sources, including cytosol, have been shown to dephosphorylate
and activate HMG-CoA reductase. The present study demonstrates phosphatase
activity capable of activating HMG-CoA reductase that is associated with
purified microsomes. The incubation of microsomes at 37 degrees C for 40
min results in a twofold stimulation of HMG-CoA reductase activity, and
this stimulation is blocked by sodium fluoride or phosphate. The ability of
microsomes to increase HMG-CoA reductase activity occurs regardless of
whether microsomes are prepared by ultracentrifugation or calcium
precipitation. Additionally, phosphatases capable of activating HMG-CoA
reductase are present in both the smooth and rough endoplasmic reticulum.
Freeze-thawing does not prevent microsomes from activating HMG-CoA
reductase but preincubation results in a significant decrease in the
ability of microsomes to increase HMG-CoA reductase activity. Thus, the
present study demonstrates that purified liver microsomes contain
phosphatase activity capable of activating HMG-CoA reductase.
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