Guanosine 5 - , occurrence

During the past 15 years data from experiments with different types of animal tissues and micro-organisms, using intact cells, crude extracts or purified enzymes, have firmly established the general occurrence of nucleotide reductases and have stressed their importance for DNA synthesis in essentially all types of rapidly growing cells [54]. It has been proposed that ribonucleotide diphosphates lose a hydroxide ion from C-2 to form a carbonium ion which is then stero-specifically reduced by a hydride ion derived from thioredoxin [54]. Adenosine diphosphate and guanosine diphosphate (as well as uridine and cytidine diphosphates) are reduced in this manner. 

Among the deoxyhexosyl esters of guanosine 5 -pyrophosphate, the most common is the /3-L-fucopyranosyl ester 22. It was isolated from extracts of Aerobacter aerogenes,90 milk,91 mammary gland,76 and blood cells,48 and was considered of probable occurrence in higher plants also (compare Ref. 92). 

The purine ring system is undoubtedly among the most ubiquitous of all the heterocyclic compounds. This arises not only from the universal occurrence of adenine and guanine in DNA and RNA and of additional modified derivatives in the various tRNAs but also from the subsidiary uses of the ring system in very many biochemical systems Indeed across the whole spectrum of biochemical reactions in living systems there is hardly a reaction sequence which does not involve in some way a purine derivative such as the adenosine or guanosine mono-, di- and tri-phosphates, associated cyclic phosphates and nucleotide coenzymes. 

The sequence of nucleotides in a chain is described by starting at the 5 end and identifying the bases in order of occurrence. Rather than write the full name of each nucleotide, though, it s more convenient to use abbreviations A for adenosine, T for thymine, G for guanosine, and C for cyti-dine. Thus, a typical DNA sequence might be written as TAGGCT. 

The dye-sensitized photo-oxidation of 1,3-diphenylisobenzofuran has been developed as a teaching experiment.286 1 The production of singlet oxygen by thiazine dye photosensitization as measured by the rate of photo-oxidation of tryptophan has been found to be very pH sensitive. The effect was shown to be related to the pJ5Ta s of the triplet dyes.2866 The occurrence of 2 in the photodynamic oxidations of guanosine,287 the reaction rates of bilirubin with 2 in the 874 877 878... 

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