The Space Connection

The Space Connection

DIABETES: THE
SPACE CONNECTION

Astronaut and protein
crystallographer Larry DeLucas, PhD, has launched several experimental insulins
into orbit. These include Humalog, the long-awaited, recently approved, short-acting
insulin lispro. Outer space has provided him with a laboratory environment unavailable
on Earth. In the gravity-free environment of space, DeLucas can grow flawless
crystals of complex proteins. Such crystals are easier for crystallographers
to examine, and they provide more accurate "blueprints" for pharmaceutical
manufacturers.
It may seem odd to think of insulin
as a crystal. DeLucas points out, however, that the cloudiness in vials of NPH
and ultralente insulin is nothing more than undissolved insulin crystals. Regular
insulin is clear because the insulin is already dissolved--and thus faster acting.
Some animals--rats, for instance, actually have insulin crystals undissolved
in their pancreas.
For more than four decades, scientists
have experimented with new forms of insulin. Most of these variants, or analogs,
offer insulin users different durations of action. Insulin lispro, for instance,
begins acting within minutes after injection to control the rise of blood glucose
immediately after meals. On the other hand, researchers are developing an extra-long
acting variety of insulin. Such an insulin analog might deliver a basal dose
lasting 2 days or more, instead of the maximum 28-hour cover now available with
ultralente.
Eli Lilly's Humalog was approved for
sales in the U.S. market last June. A long-acting analog, however, is on the
more distant horizon--literally.
Why Grow Crystals in Space?
In 1992, DeLucas donned a spacesuit
himself as a payload specialist for the National Aeronautics and Space Agency
(NASA). He wanted to learn the rigors and practical realities of research in
a weightless environment. Now that he has designed a successful crystal-growth
system, he prepares experiments on Earth and ships them to space aboard space
shuttles, where other astronauts tend the projects.
"Crystals produced on Earth are
usually flawed because of gravity-induced movements within the liquid, crystal-growing
medium," explains DeLucas. "But in space, the liquid is motionless,
which greatly slows the growth of some protein crystals. The more slowly you
grow crystals, the fewer the flaws in their structure."
The atoms and molecules of space-grown
crystals are aligned more perfectly. This allows their structures to be seen
at higher resolutions.
DeLucas, who studies crystals at the
Center for Macromolecular Crystallography at the University of Alabama at Birmingham,
said he could not have designed his system without his actual space trip in
1992. "I could see what changes I needed to make," he said. "So
many simple things happen differently in space, like stirring a drop of liquid
with a syringe tip -- the drop of liquid wouldn't stay in position in the container.
Instead the drop of liquid stuck to the syringe tip, moving with the tip as
I used it to try to mix the liquid drop. Another nuisance was bubbles in the
syringe. To get rid of them, I had to create artificial gravity -- using my
arm to swing the syringe around in a bag."
DeLucas said that he has worked with
insulin for years as a "practice" crystal to develop his techniques
and equipment for crystal-growing in space. Insulin stopped being just for practice,
however, when DeLucas began collaborating with Dr. G. David Smith. Smith, at
the Hauptman-Woodward Medical Research Institute in Buffalo, New York, has worked
extensively on the insulin lispro analog with Hauptman-Woodward colleague Dr.
Ewa Ciszak, PhD. They grew insulin lispro crystals on two space flights during
1995. Smith published the structure of insulin lispro recently and has answered
some of the difficult questions explaining its fast-acting behavior.
Smith spent 14 years studying insulin
structures with the goal of developing an ultra-slow-dissolving insulin. Smith
had experimentally altered ordinary insulin by adding a small "guest molecule."
This, in turn, "buries" the insulin's zinc ions, which play an important
role in stabilizing the insulin. Isolating the zinc makes insulin harder to
dissolve. Smith tried two guest molecules-- Tylenol and p-hydroxybenzamide (BZN).
Unfortunately, the Tylenol made the insulin slightly toxic. While Earth-grown
BZN-altered crystals were adequate to determine the insulin structures, it was
hoped that the space-grown crystals of BZN could be studied more closely.
Surprises From Space
After careful review to choose the most
promising experiments, a BZN-insulin growing chamber was sent into orbit on
the NASA-sponsored Space Habitat mission number STS60, in 1994. Back on Earth,
the crystal surprised Smith, even after 10 years of insulin research. To his
amazement, the BZN additive had actually attached itself in pairs to an insulin
molecule.
"This was the first time something
like this had ever been observed," Smith noted. "In hindsight, there
were indications from the Earth-grown crystals, but we might have missed the
paired structure without the space-grown crystals." Smith said that the
BZN-insulin is unlikely to result in a new product at this time. However, he
says that he and DeLucas will probably be asked to grow other insulin crystals
in upcoming space flights.
DeLucas has grown several other diabetes-related
proteins in space. Space-grown variants of aldose reductase crystals have also
been produced. Aldose reductase, an enzyme, has been implicated as a cause of
various diabetes complications. By building perfect, space-grown crystals of
aldose reductase bound to different inhibitors, DeLucas says scientists hope
to design a matching compound to more effectively block the enzyme's activity.
Previous attempts to design such a drug have not been completely successful.
The precision of space-grown crystals offers another chance of finding a medication
that actually works.
(Note: This article appeared in
DIABETES DATELINE, Fall 1996, published by the National Diabetes Information
Clearinghouse, National Institute of Diabetes and Digestive and Kidney Diseases,
National Institutes of Health.)