J. Biol. Chem., Vol. 267, Issue 19, 13166-13170, 07, 1992
The influence of charge and the distribution of charge in the polar region of phospholipids on the activity of UDP-glucuronosyltransferase
D Zakim and H Eibl
Division of Digestive Diseases, Cornell University Medical College, New York, New York 10021.
Studies of the mechanism of lipid-induced regulation of the microsomal
enzyme UDP-glucuronosyltransferase have been extended by examining the
influence of charge within the polar region on the ability of lipids to
activate delipidated pure enzyme. The effects of net negative charge, of
charge separation in phosphocholine, and of the distribution of charge in
the polar region of lipids were studied using the GT2p isoform isolated
from pig liver. Prior experiments have shown that lipids with net negative
charge inhibit the enzyme (Zakim, D., Cantor, M., and Eibl, H. (1988) J.
Biol. Chem. 263, 5164-5169). The current experiments show that the extent
of inhibition on a molar basis increases as the net negative charge
increases from -1 to -2. The inhibitory effect of negatively charged lipids
is on the functional state of the enzyme and is not due to electrostatic
repulsion of negatively charged substrates of the enzyme. Although the
inhibitory effect of net negative charge is removed when negative charge is
balanced by a positive charge due to a quaternary nitrogen, neutrality of
the polar region is not a sufficient condition for activation of the
enzyme. In addition to a balance of charge between Pi and the quaternary
nitrogen, the distance between the negative and positive charges and the
orientation of the dipole created by them are critical for activation of
GT2p. The negative and positive charges must be separated by the equivalent
of three -CH2- groups for optimal activation by a lipid. Shortening this
distance by one -CH2- unit leads to a lipid that is ineffective in
activating the enzyme. Reversal of the orientation of the dipole in which
the negative charge is on the polymethylene side of the lipid-water
interface and the positive charge extends into water also produces a lipid
that is not effective for activating GT2p. On the other hand, lipids with
phosphoserine as the polar region, which has the "normal" P-N distance but
carries a net negative charge, do not inhibit GT2p. This result again
illustrates the importance of the dipole of phosphocholine for modulating
the functional state of GT2p.
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