Acuaporinas

Issue 11, December 2003

Aquaporins : « A Nobel Prize for the water ferries »

To say that water is the source of life is not just a convenient catchword for the media. The first signs of life stirred in the depths of primitive oceans 3.5 billion years ago and as a result, water is the main constituent of all living creatures. Without water it would be impossible to digest food, think, walk,breathe, in short exist. With 2003 nominated Year for Water by the United Nations, it was only fitting to award the Nobel Prize to those who discovered the aquaporins – a family of proteins whose function is to ferry water.

All living things are essentially composed of water: be it a hippopotamus, a rhododendron, a human or bacteria. The human body for instance is made up of 65% of water, i.e.45 liters for a person who weighs 70 kg! Of course, water concentration varies from one organ to another – tooth enamel represents only l% whilst blood sports almost 90%! Yet the body cannot stock its water – it is constantly eliminated via excretions (in the urine mainly), breathing (when we exhale) and especially perspiration. So it is a good thing to make up for this daily loss by drinkinglarge amounts of liquid since we experience physical fatigue after a fall of only l% of our water content. This also applies for our intellectual capacities: short-term memory is affected after a water loss of 2%. It follows that it is possible to survive several weeks without food, but only a few days without water. A large amount of this water is inside our cells but a certain quantity resides inthe intercellular spaces as a reserve supply for the cells and blood vessels. The remainder is in the blood and the

lymph and is thus continuously circulating in the body. To understand why we are composed of so much water, we must return to the origins of Life.

And the Lord said: “Let there be water…” Scientists differ as to how Life originated on earth but they do agree on one point: it allstarted in water. When the Earth was formed 4.5 billion years ago, the conditions were not particularly welcoming for Life to develop. Indeed, the first traces only appeared one billion years later! During this lapse of time, in shallow pools along the ocean shores, molecules slowly organized themselves to form life. Then the first cells appeared. But as yet there was no ozone layer and theearth’s surface was continuously subjected to an intense stream of ultra violet rays which would have destroyed any living organism that had dared stray out of the water. The protective screen afforded by the ozone layer was formed only 2 billion years later. Then only did the aquatic creatures emerge and venture onto dry land,

finally overrunning the continents only 700 million years ago. BrainLungs Heart Stomach & intestines Kidney s Bones Muscles Blood Skin

instance, each deal with several hundreds of liters of water every day as they filter the blood to cleanse it of waste material. It has long been known that water is quite easily diffused through cell membranes but such a mechanism – although effective – is far too slow and insufficient to explain the flow of such large quantitiesof water. Another means must then exist. But which? The question arose as early as the 1950s but it was only due to an accidental observation, as is so often the case in science, that the mystery was actually solved in 1988 – over thirty years later.

The great Aquaporin family The existence of canals that allowed the rapid flow of large quantities of water through certain tissues had long beensuspected. This process had to be extremely selective to retain only tiny essential molecules within the cells. But since no one had, as yet, identified these canals most scientists purely and simply denied their existence. In 1988, intent on their work on red blood cells, young scientists of Peter Agre’s team in Baltimore came across a strange protein lodged in the cells’ membrane, i.e. the...