A radioimmunoassay method for 1-beta-D-arabinofuranosyluracil using antibodies directed against 1-beta-D-arabinofuranosylcytosine.

Above pH 7.0 1-beta-D-arabinofuranosyluracil (ara-U) shows marked pH-dependent cross-reactivity with antibodies directed towards 1-beta-D-arabinofuranosylcytosine. Since this peculiar phenomenon has not been observed with other nucleosides and nucleotides thus far tested, it is probably the result of base-catalyzed tautomerism of ara-U to its enolic form which renders it more structurally similar to 1-beta-D-arabinofuranosylcytosine. By performing the radioimmunoassay at both pH 6.2 and 8.6 we could determine 1-beta-D-arabinofuranosylcytosine and ara-U simultaneously. This method for ara-U assay is simple, fairly reliable, and applicable to blood level studies.