Biochemistry Essay, Research Paper
Picture this. A man is involved in a severe car crash in
Florida which has left him brain-dead with no hope for any
kind of recovery. The majority of his vital organs are
still functional and the man has designated that his organs
be donated to a needy person upon his untimely death.
Meanwhile, upon checking with the donor registry board, it
is discovered that the best match for receiving the heart of
the Florida man is a male in Oregon who is in desperate need
of a heart transplant. Without the transplant, the man will
most certainly die within 48 hours. The second man’s
tissues match up perfectly with the brain-dead man’s in
Florida. This seems like an excellent opportunity for a
heart transplant. However, a transplant is currently not a
viable option for the Oregon man since he is separated by
such a vast geographic distance from the organ. Scientists
and doctors are currently only able to keep a donor heart
viable for four hours before the tissues become irreversibly
damaged. Because of this preservation restriction, the
donor heart is ultimately given to someone whose tissues do
not match up as well, so there is a greatly increased chance
for rejection of the organ by the recipient. As far as the
man in Oregon goes, he will probably not receive a donor
heart before his own expires.
Currently, when a heart is being prepared for
transplantation, it is simply submerged in an isotonic
saline ice bath in an attempt to stop all metabolic activity
of that heart. This cold submersion technique is adequate
for only four hours. However, if the heart is perfused with
the proper media, it can remain viable for up to 24 hours.
The technique of perfusion is based on intrinsically simple
principles. What occurs is a physician carefully excises
the heart from the donor. He then accurately trims the
vessels of the heart so they can be easily attached to the
perfusion apparatus. After trimming, a cannula is inserted
into the superior vena cava. Through this cannula, the
preservation media can be pumped in.
What if this scenario were different? What if doctors were
able to preserve the donor heart and keep it viable outside
the body for up to 24 hours instead of only four hours? If
this were possible, the heart in Florida could have been
transported across the country to Oregon where the perfect
recipient waited. The biochemical composition of the
preservation media for hearts during the transplant delay is
drastically important for prolonging the viability of the
organ. If a media can be developed that could preserve the
heart for longer periods of time, many lives could be saved
as a result.
Another benefit of this increase in time is that it would
allow doctors the time to better prepare themselves for the
lengthy operation. The accidents that render people
brain-dead often occur at night or in the early morning.
Presently, as soon as a donor organ becomes available,
doctors must immediately go to work at transplanting it.
This extremely intricate and intense operation takes a long
time to complete. If the transplanting doctor is exhausted
from working a long day, the increase in duration would
allow him enough time to get some much needed rest so he can
perform the operation under the best possible circumstances.
Experiments have been conducted that studied the effects of
preserving excised hearts by adding several compounds to the
media in which the organ is being stored. The most
successful of these compounds are pyruvate and a pyruvate
containing compound known as
perfluoroperhydrophenanthrene-egg yolk phospholipid
(APE-LM). It was determined that adding pyruvate to the
media improved postpreservation cardiac function while
adding glucose had little or no effect. To test the
function of these two intermediates, rabbit hearts were
excised and preserved for an average of 24.5 1 0.2 hours on
a preservation apparatus before they were transplanted back
into a recipient rabbit. While attached to the preservation
apparatus, samples of the media output of the heart were
taken every 2 hours and were assayed for their content. If
the compound in the media showed up in large amounts in the
assay, it could be concluded that the compound was not
metabolized by the heart. If little or none of the compound
placed in the media appeared in the assay, it could be
concluded that compound was used up by the heart metabolism.
The hearts that were given pyruvate in their media
completely consumed the available substrate and were able to
function at a nearly normal capacity once they were
transplanted. Correspondingly, hearts that were preserved
in a media that lacked pyruvate had a significantly lower
rate of contractile function once they were transplanted.
The superior preservation of the hearts with pyruvate most
likely resulted from the hearts use of pyruvate through the
citric acid cycle for the production of energy through
direct ATP synthesis (from the reaction of succinyl-CoA to
succinate via the enzyme succinyl CoA synthetase) as well
as through the production of NADH + H+ for use in the
electron transport chain to produce energy.
After providing a preservation media that contained
pyruvate, a better recovery of the heart tissue occurred.
Most of the pyruvate consumed during preservation was
probably oxidized by the myocardium in the citric acid
cycle. Only a small amount of excess lactate was detected
by the assays of the preservation media discharged by the
heart. The lactate represented only 15% of the pyruvate
consumed. If the major metabolic route taken by pyruvate
during preservation had been to form lactate dehydrogenase
for regeneration of NAD+ for continued anaerobic glycolysis,
rather than by the aerobic citric acid cycle (pyruvate
oxidation), then a higher ratio of excess lactate produced
to pyruvate consumed would have been observed.
Hearts given a glucose substrate did not transport or
consume that substrate, even when it was provided as the
sole exogenous substrate. It might be expected that glucose
would be used up in a manner similar to that of pyruvate.
This expectation is because glucose is a precursor to
pyruvate via the glycolytic pathway however, this was not
the case. It was theorized this lack of glucose use may
have been due to the fact that the hormone insulin was not
present in the media. Without insulin, one may think the
tissues of the heart would be unable to adequately take
glucose into their tissues in any measurable amount, but
this is not the case either. It is known that hearts
working under physiologic conditions do use glucose in the
absence of insulin, but glucose consumption in that
situation is directly related to the performance of work by
the heart, not the presence of insulin.
To further test the effects of the addition of insulin to
the glucose media, experiments were done in which the
hormone was included in the heart preservation media5-7.
Data from those studies does not provide evidence that the
hormone is essential to insure glucose use or to maintain
the metabolic status of the heart or to improve cardiac
recovery. In a hypothermic (80C) setting, insulin did not
exert a noticeable benefit to metabolism beyond that
provided by oxygen and glucose. This hypothermic setting is
analogous to the setting an actual heart would be in during
transportation before transplant.
Another study was done to determine whether the compound
perfluoroperhydrophenanthrene-egg yolk phospholipid,
(APE-LM) was an effective media for long-term hypothermic
heart preservation3. Two main factors make APE-LM an
effective preservation media. (1) It contains a lipid
emulsifier which enables it to solubilize lipids. From this
breakdown of lipids, ATP can be produced. (2) APE-LM
contains large amounts of pyruvate. As discussed earlier,
an abundance of energy is produced via the oxidation of
pyruvate through the citric acid cycle.
APE-LM-preserved hearts consumed a significantly higher
amount of oxygen than hearts preserved with other media.
The higher oxygen and pyruvate consumption in these hearts
indicated that the hearts had a greater metabolic oxidative
activity during preservation than the other hearts. The
higher oxidative activity may have been reflective of
greater tissue perfusion, especially in the coronary beds,
and thereby perfusion of oxygen to a greater percentage of
myocardial cells. Another factor contributing to the
effectiveness of APE-LM as a transplantation media is its
biologically compatible lipid emulsifier, which consists
primarily of phospholipids and cholesterol. The lipid
provides a favorable environment for myocardial membranes
and may prevent perfusion-related depletion of lipids from
cardiac membranes. The cholesterol contains a bulky steroid
nucleus with a hydroxyl group at one end and a flexible
hydrocarbon tail at the other end. The hydrocarbon tail of
the cholesterol is located in the non polar core of the
membrane bilayer. The hydroxyl group of cholesterol
hydrogen-bonds to a carbonyl oxygen atom of a phospholipid
head group. Through this structure, cholesterol prevents
the crystallization of fatty acyl chains by fitting between
them. Thus, cholesterol moderates the fluidity of
membranes.8
The reason there are currently such strict limits on the
amount of time a heart can remain viable out of the body is
because there must be a source of energy for the heart
tissue if it is to stay alive. Once the supply of energy
runs out, the tissue suffers irreversible damage and dies.
Therefore, this tissue cannot be used for transplantation.
If hypothermic hearts are not given exogenous substrates
that they can transport and consume, like pyruvate, then
they must rely on glycogen or lipid stores for energy
metabolism. The length of time that the heart can be
preserved in vitro is thus related to the length of time
before these stores become too low to maintain the required
energy production needs of the organ. It is also possible
that the tissue stores of ATP and phosphocreatine are
critical factors. It is known that the amount of ATP in
heart muscle tissues is sufficient to sustain contractile
activity of the muscle for less than one second. This is
why phosphocreatine is so important. Vertebrate muscle
tissue contains a reservoir of high-potential phosphoryl
groups in the form of phosphocreatine. Phosphocreatine can
transfer its phosphoryl group to ATP according to the
following reversible reaction:
phosphocreatine + ADP + H+ 9 ATP + creatine
Phosphocreatine is able to maintain a high concentration of
ATP during periods of muscular contraction. Therefore, if
no other energy producing processes are available for the
excised heart, it will only remain viable until its
phosphocreatine stores run out.
A major obstacle that must be overcome in order for heart
transplants to be successful, is the typically prolonged
delay involved in getting the organ from donor to recipient.
The biochemical composition of the preservation media for
hearts during the transplant and transportation delays are
extremely important for prolonging the viability of the
organ. It has been discovered that adding pyruvate, or
pyruvate containing compounds like APE-LM, to a preservation
medium greatly improves post-preservation cardiac function
of the heart. As was discussed, the pyruvate is able to
enter the citric acid cycle and produce sufficient amounts
of energy to sustain the heart after it has been excised
until it is transplanted.
Increasing the amount of time a heart can remain alive
outside of the body prior to transplantation from the
current four hours to 24 hours has many desirable benefits.
As discussed earlier, this increase in time would allow
doctors the ability to better match the tissues of the donor
with those of the recipient. Organ rejection by recipients
occurs frequently because their tissues do not suitably
match those of the donors. The increase in viability time
would also allow plenty of opportunity for the organ to be
transported to the needy person, even if it must go across
the country.