Journal of Lipid Research, Vol 25, 497-506, Copyright © 1984 by Lipid Research, Inc.
Partial purification from rat liver microsomes of three native protein phosphatases with activity towards HMG-CoA reductase
M Sitges, G Gil and FG Hegardt
A procedure for the isolation and partial purification of three
hydroxymethylglutaryl coenzyme A reductase phosphatases in their native
high molecular weight form from rat liver microsomes is described for the
first time. Reductase phosphatase Ex (Mr 90,000), IM (Mr 75,000), and IIM
(Mr 180,000) were purified 132-, 55-, and 98-fold, respectively. Treatment
with 80% ethanol irreversibly inactivated the three enzymes contrary to
what is found for cytosolic reductase phosphatases. The three microsomal
reductase phosphatases differ among themselves and with respect to the
cytosolic reductase phosphatases in molecular weight, response to
inhibitors, thermal stability, and optimum pH. Indirect evidence that these
three proteins are phosphatases includes their inhibition by inhibitors of
phosphatase activity, such as KF, Pi, and PPi. Direct evidence includes
their ability to release 32P from highly radioactive homogeneous
32P-labeled HMG-CoA reductase, this dephosphorylation being concomitant
with activation of HMG-CoA reductase. The three phosphatases
dephosphorylate 32P-labeled phosphorylase a, but only reductase phosphatase
IIM shows glycogen synthase phosphatase activity.
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