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.

In agreement with SCHLENK (I78) "nucleotide" is here taken to mean a phosphoric ester of an N-glycoside of a heterocyclic base. Although they fall outside the scope of this definition, phosphorylated derivatives of riboflavin are added in recognition of their close relation to the main body of nucleotides. A complete review of even the organic chemistry of nucleotides and their derivatives would now form a substantial monograph. The smaller and less characteristic fragments of the nucleotides, namely the hetero- cyclic bases, the component sugars and the derived sugar phosphates, are therefore excluded from this article. * Their chemistry is of longer standing and is covered in standard works. On the other hand knowledge of the "nucleosides", the N-glycosides of heterocyclic bases, has expanded very considerably in recent years and an account of the relevant work is therefore included. With regard to the nucleotides themselves attention is focussed on work published in the last few years leading to improved characterisation, more exact structural knowledge and increased availa- biIity. Only such enzymatic experiments as bear direct1y on structural problems are discussed and therefore much fascinating work by KALCKAR, FRIEDKIN et al. and by KORN BERG has been omitted. I t would be unthinkable to present a review on nucleotides without mentioning that the foundations of the subject were laid by P. A. LEVENE. His monograph with BASS (I36) was published in 1931 and two reviews by LYTHGOE (I50, I5I) cover the intervening period until 1944.