X-Message-Number: 4720
Date: Fri, 4 Aug 1995 11:08:53 +0200 (MET DST)
From: Eugen Leitl <>
Subject: nature's ways

A short (3 pages of 14, to be continued) excerpt from
"Biochemical Adaptation" (Peter W. Hochachka, George N. Somero,
Princeton University Press 1984, pp. 436-449). Apart from
raising scholary interest concerning boundaries and rates of
evolutionary adaption, this fragment elucidates how biological
systems adapt to existance in contigous cryohabitats and/or
shows some of the mechanisms enabling them to persist through
pronounced cold seasons. Especially shotgun cryoprotection
strategies utilized by some (ant)arctic organisms which involve
simultaneous use of noncolligative macromolecular peptide and
glycopeptide as well as the more conventional polyhydroxyl
alcohol (polyol) cryoprotectants with at the same time markedly
increased dehydration resistance have earned a closer look, I
think. The human ability to induce high perfusion end
concentrations at high perfusion rates yet hitherto pronounced
disability to tackle what is labeled as cryoprotectant toxicity
(Fahy et al., 1990) and nature's way of doing things appear
remarkably complementary. Perhaps a synthesis might achieve what
each isolated technique alone cannot accomplish.

-- Eugene

-----------------------------cut here---------------------------

Contents:

 List of Figures
 List of Tables
 Preface
 List of Abbreviations
 Biochemical Adaptation: Basic Mechanisms and Strategies
 Design of Cellular Metabolism
 Adaptation of Enzymes to Metabolic Functions
 Exercise Adaptations
 Limiting Oxygen Avalability
 Metabolic Adaptation to Diving
 Off-Switches in Metabolism: From Anhydrobiosis to Hibernation
 Mammalian Developmental Adaptations
 Respiratory Proteins
 Water-Solute Adaptations: The Evolution and Regulation of
  Biological Solutions
 Temperature Adaption
 Adaptations to the Deep Sea
 References
 Index

A fragment from the Temperature Adaptation chapter:

Freezing Resistance and Freezing Tolerance.

Much as organisms must closely regulate the physical states of their
lipid-based systems, for many cold climate species regulating the
physical state of the extra- and intracellular fluids assumes critical
importance during much or all of the year. With rare exceptions,
intracellular ice formation is lethal to cells. While it is true that cryo-
preservation methodologies do alloww long-term storage of frozen cells
and tissues (Ashwood-Smith and Farrant 1980), the conditions nec-
essary to achieve this feat, e.g., the addition of high concentrations of
cryoprotectant substances like dimethylsulfoxide, are not accessible
to animals in nature. Most organisms are likely to die even if ice for-
mation is confined to the extracellular spaces, although there are
striking examples of species that not only tolerate freezing of the extra-
cellular fluids, but even nucleate ice formation. In the majority of
species that experience the threat of freezing, however, mechanisms
are employed that prevent ice formation in all body compartments.
The unusual macromolecular entities, biological "antifreeze" com-
pounds, which retard ice formation and confer freezing resistance to
a varied suite of fishes and invertebrates, serve as the primary focus
of this final section of the chapter on thermal relationships.

The choice of strategy of coping with ambient temperatures that are
below the body fluid freezing points of organisms not adapted to resist
or tolerate freezing depends on several factors. One important con-
sideration relates to the ability of the organism to remain active at
these low temperatures. If an ecological factor such as lack of food
dictates that dormancy is desirable during the cold season, then the
extracellular body fluids of the animal may be modified to facilitate the
formation of ice at temperatures close to the freezing point of the blood
or hemolymph. In such cases supercooling is largely avoided. These
types of animals are termed "freeze-tolerant" because they can with-
stand ice formation in the extra-, but not the intracellular fluids. It may
seem maladaptive to produce ice-nucleating agents to trigger ice forma-
tion at relatively high temperatures, a feat common in freeze-tolerant
species (see Zachariassen, 1980) for at first glance there appears to be
little basis for concluding that ice formation in the extracellular fluids
which is followed by withdrawal of water from the intracellular fluids,
could be beneficial to the organisms. It has long been thought that
dehydration of the intracellular space, with concomitant increases in
inorganic ion concentrations and distortion of intracellular structures,
is a major cause of low temperature-induced lethality. However, the
point of using nucleating agents to foster ice formation at relatively
high freezing temperatures is that, by avoiding supercooling, ice forma-
tion will occur in the extracellular spaces at temperatures well above
those at which spontaneous ice formation can occur intracellularly. Po-
tential damage from dehydration my still exist albeit freeze-tolerant
animals have marked capacities to withstand dehydration (Kanwisher,
1955; Murphy and Pierce, 1975). Ice formation in the intracellular
space is prevented, however, and this achievement may be most critical
for a freeze-tolerant organism. The potential damage caused by ice
crystal growth within the cell appears much greater than damage due
to dehydration.

A second major strategy for dealing with low body temperatures is
found in freeze-resistant species, organisms that employ biochemical
mechanisms to prevent ice formation in both the extra- and intracellu-
lar fluids. In many cases the species that utilize this strategy remain
active at potentially freezing temperatures. Sustained activity and the
presence of ice-containing extracellular fluids and dehydrated intra-
cellular spaces are conditions that seem incompatible. Polar fishes
offer especially good examples of freezing resistance, and the peptide
and glycopepide antifreezes in the body fluids of these species allow
them to remain active in the presence of ice at seawater temperatures
of -1.86 deg C (DeVries, 1980, 1982).

While many fishes do erect biochemical defences against freezing,
it is appropriate to note that many species employ seasonal migrations
which lead to removal of the threat of freezing. For example, the long
horn sculpin, Myoxocephalus octodecemspinosus, migrates out of near-
shore waters in the winter where ice formation occurs, and seeks a
deeper, ice-free habitat during those cold months. This behaviour eli-
minates the danger of freezing via the seeding of the body fluids (freez-
ing points are more than 1 deg C above the freezing point of seawater for
fishes lacking antifreezes). However, migration into deeper water would
appear to present this sculpin with a reduced food supply as well as
exposure to another set of predators. Thus, behavioral avoidance of
freezing may carry the costs of existance in a suboptimal habitat for
at least part of the year. The development of antifreze molecules has
allowed organisms to select habitats on criteria other than than the presence
or absence of temperatures that are lethal to freeze-susceptible species.

Examples of Freeze-Tolerant Organisms. Prior to reviewing the
biochemistry of antifreeze molecules, we shall consider briefly a variety
of organisms that tolerate, and often induce, ice formation in their
extracellular fluids. These organisms typically are terrestrial species
that are dormant in winter. Numerous examples of insects are known
that contain ice-nucleating agents that effectively prevent supercooling
of the hemolymph (Duman, 1980; Zachariassen, 1980). As mentioned
above, this relatively high-temperature freezing prevents more deletorious
low-temperature freezing of the cytosol.

A somewhat similiar adaptive strategy has recently been reported in
the plant, Lobelia telekii, which is native to an Afro-alpine environment
where temperatures near -10 deg C may occur at night throughout the
year (Krog et al., 1979). This plant, unlike an Arctic beetle, cannot go
dormant for weeks to months at a time, but instead must find ways to
achieve tolerance for freezing at night while remaining metabolically
active and growing during warmer periods of the day. The key factor
of this plant's diurnal thermal regulatory strategy is a large, fluid-filled
compartment in the inflorscence of the plant. This cavity is filled with
a viscous fluid that serves at least two thermally related functions. First,
the aqueous fluid has a large heat capacity and thermal inertia, so some
of the changes in ambient temperature will be buffered by this fluid.
Second, this fluid behaves like the blood of freeze-tolerant insects in
that it contains an ice-nucleating agent that triggers freezing near 0 deg C.
This ice formation can be beneficial in two ways. As in the case of
freeze-tolerant animals, the prevention of supercooling acts as a means
for reducing the dangers of intracellular ice formation which could
occur if the body temperature of the plant fell well below 0 deg C. In addi-
tion, at higher subzero temperatures the heat of fusion released during
ice formation can actually warm the plant. Krog et al. (1979) found
that as the ambient temperature decreased from about 12 deg C during
daytime highs to nearly -8 deg C during the late night, the temperature
of the central part of the plant did not fall below 0 deg C. Ice was gener-
ated at a sufficient rate to buffer the central temperature. These authors
calculated that approximately 2 percent of the central cavity froze
under these conditions, suggesting that a significant "heat reservoir"
remained should more extreme temperature conditions be experienced.

It seems highly unlikely that similiar heat-generating functions of
ice-nucleating agents occur in animal body fluids. The relative fluid
volumes of L. telekii and animals are grossly different, and the diffi-
culsties of controlling heat flow in small animals are obvious. Moreover,
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Hang on, the next stuff is a lot more interesting (but my
fingers bleed from typing...). Look at the thing with a
proportional font to somewhat reduce layout raggedness.

-- Eugene

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