In 1602, a Spanish fleet was sailing up the Pacific coast of Mexico when the crew became deathly ill. “The first symptom is pain in the whole body that makes it sensitive to touch,” wrote Antonio de la Ascensión, a priest on the expedition. “Purple spots begin to cover the body, especially from the waist down; then the gums become so swollen that the teeth cannot be brought together, and they can only drink, and finally they die all of a sudden, while talking.”
The crew was suffering from scurvy,
a disease that was then both bitterly familiar and deeply mysterious.
No one knew why it struck sailors or how to cure it. But on that 1602
voyage, Ascensión witnessed what he considered a miracle. While the crew
was ashore burying the dead, one sick sailor picked up a cactus fruit
to eat. He started to feel better, and his crewmates followed his
example.
“They all began to eat them and bring them back on board so that, after
another two weeks, they were all healed,” the priest wrote.
Over the next two centuries, it gradually became clear that scurvy was
caused by a lack of fruits and vegetables on long-distance voyages. In
the late 1700s, the British Navy started supplying its ships with
millions of gallons of lemon juice, eradicating scurvy. But it wasn’t
until 1928 that the Hungarian biochemist Albert Szent-Gyorgyi discovered the ingredient that cured scurvy: vitamin C.
Szent-Gyorgyi’s experiments were part of a wave of early-20th-century
research that pulled back the curtain on vitamins. Scientists discovered
that the human body required minuscule amounts of 13 organic molecules. A deficiency of any of the vitamins led to different diseases — a lack of vitamin A to blindness, vitamin B12 to severe anemia, vitamin D to rickets.
Today, a huge amount of research goes into understanding vitamins, but
most of it is focused on how much of them people need to stay healthy.
This work does not address a basic question, though: How did we end up
so dependent on these peculiar little molecules?
Recent research is providing new answers. It appears that vitamins were
essential to life from its earliest stages some four billion years ago.
Early life-forms could make their own vitamins, but some species —
including ours — later lost that ability. Species began to depend on
each other for vitamins, creating a complex flow of molecules that scientists have named “vitamin traffic.”
A Universal Chemistry
Every vitamin is made by living cells — either our own, or in other
species. Vitamin D is produced in our skin, for example, when sunlight
strikes a precursor of cholesterol. A lemon tree makes vitamin C out of
glucose. Making a vitamin is often an enormously baroque process. In
some species, it takes 22 different proteins to craft a vitamin B12
molecule.
While a protein may be made up of thousands of atoms, a vitamin may be
made up of just a few dozen. And yet, despite their small size, vitamins
expand our chemical versatility. A vitamin cooperates with proteins to
help them carry out reactions they couldn’t manage on their own. Vitamin B1, for example, helps proteins pull carbon dioxide from molecules.
Vitamins carry out these chemical reactions not just in our own bodies
but in all living things. “If you talk about bacteria, fungi, plants,
humans — everybody needs them,” said Harold B. White III, a biochemist at the University of Delaware.
This universal chemistry is likely the result of evolution. Scientists
generally agree that life on earth today evolved from a chemically
simpler form perhaps four billion years ago. Those primordial organisms
relied on a single-stranded variant of DNA, called RNA. Back then, RNA
did double duty, carrying genes, the way DNA does today, and catalyzing
chemical reactions, as proteins do now.
Dr. White was one of the first scientists to think seriously about this
primordial “RNA world.” In 1975, he proposed that vitamins helped RNA
molecules carry out their chemical reactions. While proteins took over
those reactions, they still rely on the same vitamins. “There’s no way
we’re going to get rid of them now,” he said.
When Dr. White offered up his theory, other scientists were skeptical.
“People were saying, ‘How are you going to test it?’ ” he recalled. “I
said, ‘I can’t.’ I didn’t see any way to do that work at the time.”
It took nearly four decades for technology to catch up. Dipankar Sen, a biochemist at Simon Fraser University in British Columbia, set out in 2007 to test Dr. White’s idea.
After six years of tinkering and testing, Dr. Sen and a graduate
student, Paul Cernak, found an RNA molecule that could use vitamin B1 to
pull carbon dioxide from another molecule. That is what proteins use B1
for today, just as Dr. White had predicted. Dr. Cernak and Dr. Sen described their experiment in Nature Chemistry.
The Ability We Lost
Once the ability to make vitamins evolved, some species became
especially good at making them. Plants, for example, evolved into
vitamin C factories, packing their leaves and fruits with the molecule.
At first, vitamin C probably defended plants against stress — a function
it carries out in other species, including us. But over time, the
vitamin took on new jobs in plants, like helping control the development
of fruit.
It took hundreds of millions of years for plants to become such
proficient vitamin C manufacturers, but vitamin production can change in
far less time. Our own ancestors needed just thousands of years to
alter their production of vitamin D. When humans left equatorial Africa
and spread to higher latitudes, the sun was lower in the sky and
supplied less ultraviolet light. By evolving lighter skin, Europeans and
Asians were able to continue making a healthy supply of vitamin D.
Aside from vitamins D and K, we humans can’t make any of the vitamins we
need to stay healthy. In some cases, our ancestors could make them, but
lost that ability. Our mammalian ancestors 100 million years ago never
got scurvy, for example, because they could make their own vitamin C.
Many vertebrates can make vitamin C, and use an identical set of genes
to do so. “We should be able to make it, too, since we have all the
genes,” said Rebecca Stevens of the French National Institute for Agricultural Research.
Unlike a frog or a kangaroo, however, we have crippling mutations in one of those genes, known as GULO.“It’s not just us — it goes back a long time,” said Guy Drouin, a molecular evolutionary biologist at the University of Ottawa. He and other researchers have found that apes and monkeys, our closest primate relatives, have disabled GULO genes, with many of the same mutations. Dr. Drouin has concluded that the common ancestor we share with those other primates lost the ability to make vitamin C around 60 million years ago.
Podcast: Science Times
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It Wasn’t Just Us
Primates are not the only animals with a damaged GULO gene, however, and
that’s why scientists were able to discover vitamin C in the first
place. Dr. Szent-Gyorgyi made his breakthrough thanks to a discovery
that guinea pigs, unlike other rodents, get scurvy. It turns out that
their GULO gene is disabled by a different set of mutations from the
ones we carry.
As it did in primates and guinea pigs, the GULO gene became disabled in a
few other lineages, like bats and songbirds. Scientists have found that
animals tend to lose vitamin C after a switch to a diet rich in it. Our
primate ancestors, for example, started eating fruit and leaves that
supplied them with far more vitamin C than they needed.
“It may seem counterintuitive that you would lose a gene that enables you to be independent,” said Katherine E. Helliwell of the University of Cambridge, co-author of an August review about vitamin decay in Trends in Genetics. “But if you’re always surrounded by a vitamin for a long period of time, then you don’t need to use the gene.”
Now that scientists can scan genomes of thousands of species, they’re
discovering many more cases in which vitamin genes have either decayed
or disappeared altogether. Sergio Sanudo-Wilhelmy
of the University of Southern California and his colleagues recently
surveyed the genomes of 400 of the most abundant species of bacteria in
the oceans. As they report in a paper to be published in the Annual Review of Marine Science, 24 percent of the bacteria lack genes to make B1, and 63 percent can’t make B12.
These recent studies are especially surprising because bacteria have
long been considered self-sufficient when it comes to vitamins. Now
scientists need to figure out why many species of bacteria in the ocean
aren’t dead from a microbial version of scurvy.
“Somebody’s making it for the good of the community, but we don’t know who,” Dr. Sanudo-Wilhelmy said.
Only recently have scientists made measurements of vitamins in the sea.
They are finding some places that are abundant with them and others that
are vitamin deserts. It is possible that the difference influences not
just bacteria and algae, but the animals that feed on them.
Vitamins flow in complex routes, not just in the ocean, but on land. We
humans can’t make our own supply of vitamin B12, for example, so we need
to get it from food. One way is to eat meat like beef, which contains
B12. It turns out that the cows and other animals that we consume don’t
make B12 in their own cells. Instead, the bacteria in their guts
manufacture it for them.
We are also home to thousands of species of bacteria, which synthesize
vitamins as they eat our food. Does that mean we depend on our internal
vitamin traffic? “It’s still theoretical,” said Douwe van Sinderen,
a microbiologist at University College Cork in Ireland. “But evidence
is building that bacteria can provide some vitamins that we need.”
If that’s so, we may need to think of our bodies as self-contained
oceans of vitamin traffic — a continuation of the traffic that has
occurred on earth for four billion years.
Source: http://www.nytimes.com/2013/12/10/science/vitamins-old-old-edge.html?smid=fb-nytimes&WT.z_sma=SC_VOO_20131210&bicmp=AD&bicmlukp=WT.mc_id&bicmst=1385874000000&bicmet=1388638800000&fblinkge0&_r=0#h[]
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