An Unusual Pathwav for the Svnthesis of Adenosine

Brine shrimp are incapable of synthesizing purines de nova and seemingly have a complex nucleotide metabolism involving conversion of stored diguanosine 5’, 5/“-(PI, P4tetraphosphate) into ATP. When nucleosides uniformly labeled with 14C are administered to the organism, its ATP shows an unusual labeling pattern. When [UJ4C]adenosine is used as a precursor, the purine moiety is primarily incorporated into ATP, whereas when brine shrimp are exposed to [UJ4C]guanosine, the ribose component is preferentially incorporated into ATP. This selective labeling of the purine and ribose moieties of ATP occurs at early time intervals after exposure to the labeled nucleoside, whereas AMP and ADP have [14C]purine to [14C]ribose ratios similar to that of the original nucleoside administered. Thus there appears to be a route from adenosine and guanosine to ATP that does not proceed through AMP and ADP. A pathway is proposed involving the direct participation of diguanosine 5’,Sn’-(PI, P4tetraphosphate) as an intermediate. a consequence of these observations, we have focused our attention on the mechanism by which brine shrimp convert guaninecontaining precursors t,o ATP. We observed that when nauplii of A. salina are given uniformly labeled guanosine or adcnosine for short periods of time, the ratio of [14C]purine to [14C]ribose in ATP shows an extensive disproportionality, and the selective utilization of the individual components of the precursor is dictated by the nitrogenous base. Further, this unusual labeling pattern of ATP in nauplii occurs despite the fact that AMP and ADP have purine to ribose ratios similar to that of the nucleoside administered.