ANNETTE ALFSEN
Directeur de Recherche,
Centre National de la Recherche Scientifique.
U.P.R. 64 CNRS, 45 rue des Saints Pres 75006 Paris.
The shortage of animal proteins is one of the most serious problems facing the population of Asia. Thus, finding a source of vegetal proteins and increasing their nutritional value should be an important goal for the Asian countries. Marine algae used as food for a long time in Asia are now largely cultivated by some countries and could be a very large source of protein as known from recent studies.
In order to develop and extend the use of algal protein in human food and animal feed, three main steps should be followed:
increase the total production of marine algae most commonly used as food such as Porphyra, Undaria, Laminaria and Gracilaria.
genetically improve the quality of the proteins obtained from the different algae.
process the harvest in order to obtain a product easy to use, keep or export to other Asian countries and those outside the region.
To accomplish the above, more has to be known about the biology of algae and their relation with the external medium. Our group in Paris, in collaboration with the Academia Sinica's Institute of Oceanology in Qingdao, China has been conducting fundamental research on marine algae, mainly on the internalization and intracellular traffic of macromolecules like proteins.
Using our experience in animal and plant cell biology, we have described the process of endocytosis in two species of pluricellular algae, namely, the green alga, Ulva lactuca (Ulvophyceae), and brown alga, Laminaria digitata (Pheophyceae). Such a process has been largely studied in animal cells and is known to govern all the exchanges of the organism, the organs and the cells with their external medium. It is also responsible for the intracellular traffic of the internalized molecules and therefore for the growth and multiplication of the cells. The cell organelles involved in this process are membrane organelles called coated pits (CP) and coated vesicles (CV), because of their being surrounded with a lipid vesicle of a fuzzy protein coat, which is very characteristic. The demonstration of the existence of the same process involving similar organelles and mechanisms in the vegetal (particularly in marine algae) as in the animal kingdom, allowed us to suggest a common role for these organelles in the cells throughout the evolution.
Biochemical analyses of whole algae and of algal coated vesicles demonstrates the presence of phospholipids (PE, PC, PS, PI and PG) which are all present in mammalian CV. From different sources of animal tissue and different methods of preparation, 150–430 nmoles of lipids phosphorus/mg protein have been reported in the CV. The values obtained for L. digitata and U. lactata CV enriched fractions, which are 312 nmoles and 103 nmoles /mg of protein respectively, were in the same range. The amino sugars GalNAc and GlcNAc represented 25% of the total carbohydrate content detected and measured in CV from the two species, after acid hydrolysis of the samples. It suggests the presence in these organelles of a high content of glycolipids or of glycoproteins. Indeed, in the whole cell extract of Phaeophycae and Chlorophycae which exhibited a high content of glycolipids, neither GalNAc nor GlcNAc was detected. The detection of these sugars only in CV suggests involvement of these organelles in molecules segregation and concentration in seaweeds as well as in animal cells. As in other plant cells, algal CV may therefore be involved in polysaccharides and glycoproteins transport for cell plasma membrane and wall edification. In Ulva lactuca in which growth mostly takes place in only five months, the amount of clathrin decreased as a function of plant growth stage. This is in agreement with an involvement of CV in the intracellular traffic of the young tissue with an active metabolism, as further suggested.
Related to the biochemical composition of the whole algae, it is worth noting the important differences in the protein content of the different species: from 40% of dry weight for Porphyra sp. to 20% for Laminaria. Compared to L. digitata, the total amount of protein and carbohydrate measured in U. lactuca was higher and the lipid content lower. In both Laminaria digitata and Ulva lactuca, the lipids identified were: PE, PC, PI, MGDG, and DGDG. These two types of lipids are characteristic of plants and very interesting for their content of polyunsaturated fatty acids.
Analysis of sugar content by gas chromatography revealed the presence, in both Ulva lactuca and Laminaria digitata, of GalNac, GlcNac, galactose, mannose, fucose, glucose and xylose.
From these data on two species of marine algae we can propose a programme of algae utilization in two domains: (i) as a source of protein in human and animal food and (ii) as a source of drug and of drug carrier.
Some species of marine algae like Porphyra and Undaria are very rich in protein, and extraction of these proteins by industrial means will be developed in relation with processing plants like France Luzerne who are interested in this programme. The processes for extracting concentrated purified proteins would be similar to those used for higher plant leaf proteins. To make the protein extraction from some species of algae economically valuable, a preliminary step in the processing should be the extraction of the phycocolloids agar, carrageenan or alginate. A method of extraction allowing alginate recovery from Laminaria digitata without destroying the plant protoplast has been developed and could be used for other species.
On the other hand fundamental research should be continued on genetic and molecular biology of algae. It could be highly profitable for countries producing algae to be able to substitute soyabean protein, which dominates the protein field in food, with the proteins from this source. The characteristics of soyabean protein, i.e. hydration, absorption and texture, should be as far as possible found in the proteins from algae and this might be possible by genetic manipulations of the algal plants.
Finally our knowledge on the composition and function of coated vesicles in algae as well as in higher plants and in animal cells allows us to propose a program of preparing the organelles from different species of algae cultivated in Asia and utilizing them as natural drug carrier. A list of compounds that could be of medical and pharmaceutical use has been established and, according to our data, these compounds should be naturally concentrated in the carrier vesicles. Furthermore these vesicles by their structure are natural liposomes which could carry targeted drug in the organism and, as such, can be charged with specific drug molecules.
The possibilities of algal utilization has been until now largely underestimated, and not scientifically examined. Our proposal is to consider marine algae as a large source of proteins and of drugs, natural or synthetic, without losing the production of the phycocolloids which have still a large world market.
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