Nucleoside Antibiotics

By a combination of 15N-, 3H-, and 14C-labeled experiments with adenosine and d-ribose, it has been possible to demonstrate that adenosine is directly involved in the biosynthesis of 9-β-d-arabinofuranosyladenine by Streptomyces antibioticus. This conclusion is based on the following observations. (a) The ratio of 15N:14C in [6-amino-15N, U-14C]arabinofuranosyladenine and in the adenosine isolated from the RNA are the same as the ratio of 15N:14C in [6-amino-15N, U-14C]adenosine added to cultures of S. antibioticus. (b) All of the tritium on C-2' of [2'-3H]adenosine is lost in the epimerization. (c) Arabinofuranosyladenine derived from d-[3-3H]ribose contained only one-third of the tritium label on C-3'. (d) The conversion of adenosine to 9-β-d-arabinofuranosyladenine does not proceed via 2'-deoxyadenosine. (e) [U-14C]Arabinofuranosyladenine is rapidly taken up by S. antibioticus, but it is hydrolyzed to adenine. This latter observation was determined by the isolation of 14C-labeled AMP from the cellular RNA. These data suggest that adenosine is converted to a 2'-ketonucleoside or 2'-ketonucleotide. Enolization of this 2'-keto compound might explain the loss of tritium on the 3' carbon of arabinofuranosyladenine. The metabolism of 9-β-d-arabinofuranosyladenine and 2'-deoxyadenosine differs markedly from that of adenosine in that the first two nucleosides are hydrolyzed to adenine while adenosine is not hydrolyzed. If the conversion of adenosine to arabinofuranosyladenine proceeds via an epimerase, this reaction would differ from the UDP-d-glucose 4'-epimerase and l-ribulose-phosphate 4'-epimerase in which the hydrogen on C-4' of UDP-glucose or d-xylulose 5-phosphate is found intact in the same position following epimerization. The biosynthesis of arabinofuranosyladenine could be similar to the UDP-acetyl-d-glucosamine 2'-epimerase, cellobiose-2'-epimerase, d-ribulose-phosphate 3'-epimerase, or dTDP-l-rhamnose synthetase in which hydrogen from water is incorporated into the product.