X-Message-Number: 29839
Date: Sun, 16 Sep 2007 17:57:20 -0700 (PDT)
From: 
Subject: urea prospective additive to vitrification solutions

[Urea is a small rapidly penetrating molecule, which is freely soluable in
water. Vitrification solutions tend to destabilize cell membranes, while
urea acts to stabilize them. Such solutions may therefore exhibit a
reduced toxicity, if they were to incorporate urea.]

J Phys Chem B. 2006 Nov 30;110(47):23845-52.
How small polar molecules protect membrane systems against osmotic stress:
the urea-water-phospholipid system.
    Costa-Balogh FO, Wennerstr m H, Wads  L, Sparr E. Physical Chemistry 1,
Chemical Center, Lund University, P.O. Box 124, SE-22100 Lund, Sweden.
    We investigate how a small polar molecule, urea, can act to protect a
phospholipid bilayer system against osmotic stress. Osmotic stress can be
caused by a dry environment, by freezing, or by exposure to aqueous systems
with high osmotic pressure due to solutes like in saline water. A large
number of organisms regularly experience osmotic stress, and it is a common
response to produce small polar molecules intracellularly. We have selected
a ternary system of urea-water-dimyristoyl phosphatidylcholine (DMPC) as a
model to investigate the molecular mechanism behind this protective effect,
in this case, of urea, and we put special emphasis on the applications of
urea in skin care products. Using differential scanning calorimetry, X-ray
diffraction, and sorption microbalance measurements, we studied the phase
behavior of lipid systems exposed to an excess of solvent of varying
compositions, as well as lipid systems exposed to water at reduced relative
humidities. From this, we have arrived at a rather detailed thermodynamic
characterization. The basic findings are as follows: (i) In excess solvent,
the thermally induced lipid phase transitions are only marginally dependent
on the urea content, with the exception being that the P(beta) phase is not
observed in the presence of urea. (ii) For lipid systems with limited access
to solvent, the phase behavior is basically determined by the amount
(volume) of solvent irrespective of the urea content. (iii) The presence of
urea has the effect of retaining the liquid crystalline phase at relative
humidities down to 64% (at 27 degrees C), whereas, in the absence of urea,
the transition to the gel phase occurs already at a relative humidity of
94%. This demonstrates the protective effect of urea against osmotic stress.
(iv) In skin care products, urea is referred to as a moisturizer, which we
find slightly misleading as it replaces the water while keeping the physical
properties unaltered. (v) In other systems, urea is known to weaken the
hydrophobic interactions, while for the lipid system we find few signs of
this loosening of the strong segregation into polar and apolar regions on
addition of urea.
PMID: 17125350

 J Exp Biol. 2005 Nov;208(Pt 21):4079-89.
Cryoprotection by urea in a terrestrially hibernating frog.
    Costanzo JP, Lee RE Jr. Department of Zoology, Miami University,
Oxford, OH 45056, USA.
    The role of urea as a balancing osmolyte in osmotic adaptation is well
known, but this 'waste product' also has myriad other functions in diverse
taxa. We report that urea plays an important, previously undocumented role
in freezing tolerance of the wood frog (Rana sylvatica), a northern woodland
species that hibernates terrestrially in sites where dehydration and
freezing may occur. Wood frogs inhabiting an outdoor enclosure accumulated
urea to 65 mmol l-1 in autumn and early winter, when soil moisture was
scarce, but subsequently urea levels fell to approximately 2 mmol l-1 as the
availability of environmental water increased. Laboratory experiments showed
that hibernating R. sylvatica can accumulate at least 90 mmol l-1 urea under
relatively dry, warm conditions. During experimental freezing, frogs
synthesized glucose but did not accumulate additional urea. Nevertheless,
the concentrations of urea and glucose in some tissues were similar. We
tested urea's efficacy as a cryoprotectant by measuring lysis and lactate
dehydrogenase (LDH) leakage in samples of R. sylvatica erythrocytes
frozen/thawed in the presence of physiological levels of urea or other
osmolytes. In conferring protection against freeze/thaw damage, urea was
comparable to glycerol and as good as or better than glucose,
cryoprotectants found in freeze-tolerant frogs and other animals. Urea
treatment also improved the viability of intact tissues frozen in vitro, as
demonstrated by post-thaw measures of metabolic activity and LDH leakage.
Collectively, our findings suggest that urea functions both as an
osmoprotectant and a cryoprotectant in terrestrially hibernating amphibians.
PMID: 16244167

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