快猫短视频

ALMOST two thousand years after Ovid described how the witch Medea replaced
Jason鈥檚 father blood with a magic brew, blood researchers are getting close to
creating their own stand-ins for human blood. Some of their ingredients sound
almost as weird as the witch鈥檚鈥攁 by-product of nonstick pan production,
the spilt blood of cattle, plastics, mutant proteins and bits of crocodiles. But
they are pursuing their goal in the face of far stiffer competition than she
ever had: medicine already has at its disposal an elegant substitute for a
patient鈥檚 lost blood鈥攁 transfusion of someone else鈥檚.

Still, donated blood is not perfect. To prevent the patient鈥檚 immune system
mounting massive and bloody campaigns against the transfused red blood cells,
the blood must be 鈥渢yped鈥� to check that key proteins on the surface of the red
blood cell compliment the donor鈥檚, a time-consuming process when lives are
at stake. Blood also transmits known and as-yet-to-be-identified diseases, and
goes bad in just six weeks, even in a fridge, which are hard to find on the
battlefield or in Third World countries.

The ideal alternative would have none of these drawbacks, but still speed
oxygen around the body, and be cheap to boot. Early candidates for the post
included infusions of ale, wine, and opium. In the 19th century, milk
transfusions looked so promising that one John Brinton confidently predicted
that they 鈥渨ill, in a few years, have entirely superseded the transfusion of
blood, which latter operation is even now being rejected as at once dangerous
and unavailing in many parts of the country.鈥�

Milk didn鈥檛 hold up to further scrutiny, but a stand-in first investigated a
century ago has鈥攕olutions of oxygen-carrying haemoglobin minus the rest of
the red blood cell. Such solutions don鈥檛 need typing, and last for at least a
year鈥攊n some cases without a fridge, raising the possibility that one day
soldiers will be able to throw blood substitutes into their knapsacks along with
their dried rations. Half a dozen pharmaceuticals companies are testing
haemoglobin solutions in hundreds of patients, and at least one, Baxter
Healthcare in Deerfield, Illinois, will be selling their product in Europe some
time next year if the European Medicines Evaluations Agency gives their approval
as expected.

At the annual meeting of the European Society of Anesthesiologists in
Switzerland in May, Baxter announced the results of the first big clinical trial
to show that haemoglobin solutions can delay, or even do away with, the need for
blood transfusions. They gave about 100 heart bypass patients one unit of this
free haemoglobin solution during surgery instead of a blood transfusion. A day
later, 40 per cent of those patients had no need of a regular blood transfusion,
and by a week later 20 per cent had still managed without a single
transfusion.

鈥淚t鈥檚 an advance, but not the home run we are looking for,鈥� says Robert
Winslow, who heads a blood substitutes research team at the University of
California at San Diego. But even if today鈥檚 free haemoglobin solutions leave a
little to be desired, they are quite an achievement when you consider what
happens when you part haemoglobin from a red blood cell.

Usually, about 280 million haemoglobin molecules are crammed shoulder to
shoulder inside each cell. When they are set free the weak bonds that hold
together their two halves fracture, and the subunits are quickly filtered out of
the blood by the kidneys. Rats peed bright red within minutes of receiving
transfusions of early haemoglobin solutions, and the fragments clogged up their
kidneys, sometimes causing irreparable damage.

Letting go

Another problem for the early haemoglobin solutions was their reluctance to
give up their oxygen once they hit oxygen-deprived tissues. When a red blood
cell floats into a low oxygen environment, a molecule called
2,3-diphosphoglycerate, or DPG, gets between the two beta chains from each
haemoglobin subunit, loosening the protein鈥檚 grip on the oxygen. Free
haemoglobin has no DPG.

But there are ways of making free haemoglobin behave more like haemoglobin
bound up in a red blood cell. Baxter鈥檚 team, headed by Thomas Schmitz, extracts
haemoglobin from human blood that鈥檚 too old to use for a transfusion, and then
keeps the protein in one piece by using an aspirin derivative to cross-link the
two alpha chains. The cross-link also makes the haemoglobin mimic the shape
change normally triggered by DPG, so that the protein has a lower affinity for
oxygen and is more willing to give up its cargo.

BioPure Corporation in Cambridge, Massachusetts gets its haemoglobin from
cow鈥檚 blood and uses a chemical called glutaraldehyde to bind the haemoglobin
molecules into clumps of different sizes. Clumps of between two and eight
haemoglobin molecules are separated out and used for infusions. Cow haemoglobin
relies on chloride ions rather than DPG to engineer its shape change. And as
human plasma is bursting with chloride ions, persuading cow haemoglobin to
release its oxygen is not a problem. BioPure won鈥檛 be seeking approval to market
its product for humans for at least a year, but they have applied to market it
for vets to use on their patients. There are, after all, no doggie blood banks,
and few feline blood donors.

Even though the blood substitutes based on animal or human haemogolobin are
cleaned up, there is still a slim risk of disease transmission. For that reason,
says William Freytag of Somatogen in Boulder, Colorado, genetically engineered
haemoglobin is the only way forward. Freytag鈥檚 team got their inspiration for
how to engineer haemoglobin so that it behaves properly in solution from a type
of anaemia that is so rare it doesn鈥檛 even have a name.

Although people with the disorder have abnormally low levels of red blood
cells, they usually suffer few of the ill-effects associated with chronic lack
of oxygen, such as shortness of breath. That鈥檚 because a mutation in their
haemoglobin gene creates a single amino acid change, enough to make the
haemoglobin protein surrender its oxygen far more willingly than normal
haemoglobin. Freytag鈥檚 team copied that gene, and modified it so that it also
codes for a glycine cross-link that will hold the haemoglobin subunits together
in solution. After that they inserted the gene into bacteria that churn out
copious quantities of the transgenic protein. The transgenic haemoglobin is
being tested in cardiac surgery patients.

And Somatogen researchers are toying with some far more bizarre
modifications. Crocodiles can stay under water for an hour or more, while they
drown their prey or look for new victims. As the crocodile runs out of oxygen,
its blood fills with bicarbonate ions, an end-product of respiration. The
bicarbonate binds to the haemoglobin, reducing its liking for oxygen, so that
the molecule surrenders more just when the animal needs it most. Molecular
biologist Kiyoshi Nagai of the University of Cambridge, who is working with
Somatogen, has inserted 12 mutations into the human haemoglobin gene so that its
protein binds to bicarbonate just like a crocodile鈥檚. The medical applications
of crocodile-like human haemoglobin are not, however, clear. 鈥淚 sort of
fantasise about giving it to Navy Seals so that they can stay under water for
long periods of time,鈥� says Freytag.

By tweaking the haemoglobin molecule, blood researchers have made haemoglobin
solutions that deliver enough oxygen, and don鈥檛 fragment and clog up the
kidneys. But other problems are proving harder to crack.

NO problem

Free haemoglobin does not last long in the body鈥攁bout half is lost
every 12 hours, compared to a life span of 40 to 60 days for transfused red
blood cells. 鈥淥nce the dust has settled, the haemoglobin gets metabolised.
Ultimately you鈥檙e going to give the patient blood bank blood anyway, you鈥檙e just
delaying it,鈥� says Lawrence Goodnough of Washington University School of
Medicine in St. Louis, Missouri, who has helped test BioPure鈥檚 haemoglobin
product.

Even more seriously, a transfusion of a haemoglobin solution can trigger an
increase in blood pressure of up to 10 per cent, hardly desirable in a sick
person. And after decades of study, researchers still aren鈥檛 exactly sure why
this rise in blood pressure is happening.

One suspicion is that the free haemoglobin messes with the mechanism
regulating blood supply to different parts of the body. When endothelial cells
lining the blood vessels are starved of oxygen, they release nitric oxide into
the smooth muscle surrounding the vessel. That makes the muscles relax, the
vessels dilate, and the supply of oxygen-carrying blood increases. But to a
haemoglobin molecule, nitric oxide looks pretty much like oxygen, and it gobbles
it up. This doesn鈥檛 matter too much if the haemoglobin is trapped inside a blood
cell, and is limited to the nitric oxide that gets there from the bloodstream.
Free haemoglobin, on the other hand, is incredibly tiny, and can squeeze between
the endothelial cells scoffing nitric oxide from the tissues before it can do
its job.

Cardiologist and lung specialist John Stamler of Duke University Medical
Center in Durham, North Carolina, has a hunch about how the problem can be
solved. Last year, he discovered that haemoglobin within the red blood cell also
plays a vital role in regulating blood pressure. As haemoglobin picks up oxygen
in the lungs it changes shape, exposing a sulphur atom which then picks up
nitric oxide, creating a molecule called S-nitrosothiol, or SNO. When the
haemoglobin releases oxygen in the tissues and flips back to its original shape
in the tissue, SNO is released and behaves just like nitric oxide, triggering
dilation of the blood vessels.

According to Stamler, SNOs are essential for maintaining normal blood
pressure, something that the blood substitute people have only just realised.
When haemoglobin is purified for use as a substitute, the SNOs are stripped off.
In some cases, the haemoglobin is even modified so that it can鈥檛 make new SNOs
when it passes through the lungs.

Such limitations have helped convince Thomas Chang, director of the
Artificial Cells and Organs Research Center at McGill University in Montreal,
that the future lies with artificial blood that more closely resembles the real
thing. His team has developed a technique for wrapping up haemoglobin, DPG and
other red blood cell enzymes, in a thin lipid membrane. The artificial red blood
cells readily give up their oxygen, at least in the animal experiments that have
been done so far, and they are so small they are difficult for the lymph nodes,
liver and spleen to trap, and can survive up to 36 hours in the body.

Eating blood

Chang and his team have replaced up to 90 per cent of a rat鈥檚 blood with
artificial cells to no obvious adverse affect. Now, the team has created
artificial red cells less than 200 nanometres in diameter using a biodegradable
polymer membrane, in the hope that they will last even longer. The one major
drawback is that no one so far has figured out how to make the artificial cells
cheaply enough for mass production.

Peter Keipert at Alliance Pharmaceuticals in San Diego is testing a totally
synthetic blood substitute, which is also cheap because it is a byproduct of the
Teflon industry. The substance is perfluorocarbon, a chemical that can dissolve
large amounts of oxygen鈥攕ometimes to great dramatic effect. In the movie,
The Abyss, the hero Bud uses a perfluorocarbon 鈥渓iquid breathing
system鈥� to retrieve a nuclear warhead, pitched into an oceanic trench by a
crazed Navy Seal who is trying to destroy undersea intelligent life forms.
Though liquid breathing for scuba diving remains science fiction, humans can
breathe perfluorocarbons, which have even been tested as a way of helping
premature infants to breathe.

Keipert鈥檚 team has transfused hundreds of cardiac surgery patients with
perfluorocarbon beads wrapped in a milky-white surfactant made from egg yolks.
The surfactant, which lets the chemical mix in the bloodstream, is metabolised
by the body, and the lungs exhale the perfluorocarbons as a vapour. In a study
of 256 patients undergoing surgery, one unit of perfluorocarbons delayed the
need for a later blood transfusion by a significant amount (the company refused
to give an exact time) compared to an initial transfusion of blood.

But perfluorocarbons have a major drawback. They provide the body with only a
fraction of the oxygen that a haemoglobin solution does, unless the patient
breathes pure oxygen through a mask鈥攚hich more or less limits their use to
the operating table.

As the companies鈥擬edea-like鈥攁dd a pinch of this or a dash of that
in attempts to perfect the blood substitutes, it is becoming clear that their
future lies mainly with the approximately 20 per cent of blood transfusions that
happen in an emergency. For patients whose surgery is planned, another option is
emerging. Leviticus 7:26 forbids the eating of blood, which Jehovah鈥檚
Witnesses take to mean a ban on blood transfusions as well. The physicians who
treat these patients have pioneered new ways for them to do without. Their
techniques include using the hormone erythropoietin to trick the body into
making extra red blood cells, and surgical tools and techniques that reduce the
blood letting. Says Goodnough: 鈥渨e鈥檝e been able to show that you can get nearly
to bloodless surgery.鈥�

* * *

Stealth Red Blood Cells

PEOPLE with sickle cell disease and thalassemia need numerous blood
transfusions. Although the red blood cells in transfusions are 鈥渢yped鈥� to make
sure the large antigens on their surface match those of the recipient, it is not
possible to do the same for the myriad of smaller antigens. In some patients,
repeated exposure to foreign antigens leads to life-threatening immune
hypersensitivity to blood transfusions.

Now, Mark Scott at Albany Medical College in New York and John Eaton at
Baylor College of Medicine in Houston, have found a way to camouflage the
surface of red blood cells so that the body鈥檚 immune system won鈥檛 detect them in
the first place. This technique could create a universal blood type suitable for
everyone.

Scott and Eaton coat their red blood cells with a polymer called polyethylene
glycol that hides the antigens. 鈥淧EG is a very simple compound. Because it鈥檚 not
very exotic, it鈥檚 hard for the body to see,鈥� says Scott. The long polymer
strands stick to the red blood cell and move around like spinning fan blades,
stopping immune cells and antibodies reaching the surface in the first place.
鈥淎n antibody can never quite get down there, but smaller molecules like oxygen
and glucose go through just fine,鈥� he says.

When the researchers transfused PEG-coated sheep blood cells into mice, the
cells lasted for a day or more, 360 times longer than untreated sheep cells.