Effects of urea and trimethylamine n-oxide on fluidity of liposomes
and membranes of an elasmobranch.
Buhr, Kimby N. Barton Mary M., and James S. Ballantyne.
1Department of Zoology, University of Guelph, Guelph, Ontario, N1G
2W1, 2Department of Animal and Poultry Science, University of Guelph,
Guelph, Ontario, N1G 2W1
APStracts 5:0385R, 1998.
The effects on membrane fluidity of two solutes of biological
importance in elasmobranch fishes, urea and trimethylamine oxide
(TMAO), were determined using elasmobranch red blood cell plasma
membranes and artificial liposomes. Fluorescence polarizations of
three probes with differing sites of insertion (1,6
-diphenylhexatriene (DPH), cis-parinaric acid (cPNA) and trans
-parinaric acid (tPNA)) were used to study the effects of
physiological levels of urea (400 mM) and TMAO (200 mM), separately
and together in a 2:1 urea:TMAO ratio (400:200 mM). In the
elasmobranch erythrocyte membrane, there was a trend towards an
increase in the order of the gel phase domains when treated with
urea, although this was not statistically significant. This effect
was counteracted by the presence of TMAO. To determine if the organic
solutes were acting directly on the membrane lipids or on the
integral proteins, phase transition profiles of protein-free
dipalmitoyl phosphatidylcholine (DPPC) liposomes were determined.
These profiles showed that urea again increased the order of the gel
phase domains of the bilayer however, this effect was not
counteracted by the presence of TMAO. We suggest that the increased
order in the gel phase domains may be an indirect effect of a
decrease in the order of the fluid phase domains. This increase in
fluidity may be due either to a disruptive effect of urea on the
hydrophobic core of the membrane or to indirect effects mediated by
changes in the integral membrane proteins. This study is the first to
demonstrate that urea and TMAO may act as counteracting solutes in
the elasmobranch erythrocyte membrane and that the counteraction
appears to be at the level of the integral proteins rather than on
membrane lipids.
Received 17 October 1997; accepted in final form 12 October 1998.
APS Manuscript Number R656-7.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1998 The American Physiological Society.
Published in APStracts on 10 November 1998