Depuis l'isolement de la creatine par CHEVREUL, en 1832, dans les extraits de viande (44) et son identification par LIEBIG, en r847, l'attention des biochimistes a porte principalement, pendant longtemps, sur le role de l'arginine dans la production de l'uree (I28), sur le cyde de l'ureo- genese (I34) et sur celui de la phosphocreatine (7I) et de la phospho- arginine (r68, I69) dans la contraction musculaire. A la suite de la mise en evidence, d'une part des reactions de trans- amidination expliquant la convertibilite entre l'arginine et un certain nombre de derives guanidiques (28,29,33,73,2 5, 29I, 292), et, d'autre part, du mecanisme complexe de la biosynthese de l'arginine (34, 45, 46, I95, I96) , l'importance du groupement guanidique dans la fixation et le transfert de l'azote organique est devenue beaucoup plus manifeste. Par ailleurs, l'isolement de nouvelles substances dont certaines: la phosphotaurocyamine, la phosphoglycocyamine (280-282), la phospho- 10mb ricine (270), jouent le meme role de phosphagenes que le creatine- et l'argininephosphate (257), amis en evidence la part importante que les derives guanidiques prennent dans la chimie du musde. L'etude de leur biogenese a montre en outre que leur role n'est pas seulement equivalent a l'arginine dans la fixation de l'azote du groupe amidinique, mais qu'ils sont capables de jouer egalement de role de regulateurs du metabolisme azote (258). I. Structure et formation.

During the early 1950's there appeared reports, from time to time of the presence among the products elaborated by actinomycetes of antifungal antibiotics which exhibited very similar and very characteristic multipeaked ultraviolet absorption spectra. In 1954, with a good number of examples on record, these spectra were analyzed and identified as those of straight-chain conjugated polyenes, comprising tetraenes, pentaenes, hexaenes and heptaenes (85, I30). These antibiotics have since been commonly referred to as the polyene antifungal antibiotics to distinguish them from a host of other miscellaneous antibiotics which also have antifungal properties. Within the next few years, reports of discoveries of new members of this class multiplied rapidly, and almost sixty are now known. Unquestionably, a number of these will eventually be found to be identical with others, as has already happened in several instances: for example, the tetraene "tennecetin" proved to be a rediscovery of pimaricin (34), and in the methyl- pentaenes "moldcidin E" has been identified with pentamycin (83), and "lagosin" appears to be indistinguishable from fungichromin (22). Those that have been purified have turned out to be of fairly high molecular weight (ca. 700-1300) and all appear to be substances of rather similar molecular structure. So far only three, pimaricin, fungi- chromin (lagosin) and filipin, have been structurally elucidated.