Characterization and biogenesis of 51-methylthioxylofuranosyl adenine, a new natural analog of 51methylthioadenosine.

5?-Deoxy-5?-methylthioadenosine (MTA) is a naturally occurring sulfur nucleoside, ubiquitously distributed in nature endowed with antiproliferative activity (1-4). It derives from S-adenosylmethionine (AdoMet) metabolism through several metabolic pathways (1-4). In mammalian tissues the spermidine synthase and spermine synthase reactions represent the quantitatively most important routes for MTA formation (1-4). In spite of the occurrence of multiple biosynthetic pathways, the intracellular concentration of MTA is remarkably low when compared with that of spermidine, spermine and AdoMet (5-8). The thioether indeed does not accumulate intracellular ly because of its rapid cleavage by MTA phosphorylase into adenine and 5-methylthio-ribose-1-phosphate (9).