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Claire de Thoisy-Méchin

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Email: claire.de_thoisy-mechin@upmc.fr

 

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Katherine Tyrka

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Why can the Japanese easily digest sushi?

Colonies de la flavobactérie marine © Tristan Barbeyron - CNRS

The porphyran, a polymer made up of sugars present in the cell walls of a red alga that is used notably in the preparation of sushi, is broken down specifically by an enzyme called porphyranase. This new enzymatic activity has been identified in marine bacteria and, surprisingly, in the bacteria that populate the gut of the Japanese. Scientists from CNRS and UPMC have explained this discovery by a transfer of genes between the bacteria, which would allow the gut microbiota of the Japanese to acquire all the "machinery" it needs to consume the algae that surround sushi. Their results are published in the journal Nature on April 8, 2010.

 

Without intestinal flora, humans cannot break down the polysaccharides in their diet, which are one of the main sources of energy for the brain. Indeed, intestinal bacteria contain enzymes known to "break down" the polysaccharides, which are polymers made up of sugars. They are essential because the human genome is not endowed with such enzymes.

Two research teams working at the Station Biologique in Roscoff (CNRS / UPMC) have been investigating  the porphyranase, an enzyme that breaks down polysaccharides but whose true activity was previously unsuspected. Scientists have thus discovered that porphyranase breaks down a highly specific molecule:  the porphyran, and not another substrate, as had previously been assumed.

Porphyran is a polysaccharide, one of the components in the walls of red-colored algae called Porphyra. These algae are used to prepare the "well-known" sushi. According to historical documents, this alga has been consumed for many generations by the Japanese. Of considerable cultural importance in Japan, it has sometimes served as a gift or to pay certain taxes.

The researchers then demonstrated the process of recognition between the enzyme (porphyranase) and its substrate (porphyran). They were able to identify the "signature" of the sequence involved in this recognition (the specific site on the the enzyme to which the reagent binds). As expected, this novel enzymatic activity was detected in marine bacteria. Further investigations led the scientists to compare genomic data regarding the gut flora of 13 Japanese individuals and 18 North Americans. They thus discovered that porphyranase was also present in the gut flora of the Japanese (but not in that of the North Americans).

The scientists suppose that the presence of the enzyme in the gut flora of the Japanese is directly linked to their dietary habits. As major consumers of Porphyra for several centuries, the Japanese have thus been in contact with the marine bacteria that contain the porphyranases via their diet. Mirjam Czjzek and her team presume that a transfer of genes from marine bacteria to intestinal bacteria must have allowed the micorbiota of the Japanese to accept the "machinery" required to break down the polysaccharides in Porphyra algae. These findings suggest that food associated with marine bacteria may constitute a mean for the human intestinal flora to acquire new enzymes, which may, among other factors, explain their diversity.

 

Source

Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota. Jan-Hendrik Hehemann, Gaëlle Correc, Tristan Barbeyron, William Helbert, Mirjam Czjzek & Gurvan Michel. Nature. 8 avril 2010.



11/05/10