Effect of Nucleic Acid Derivatives in the Reversal of Aminopterin Inhibition in the Chick Embryo. ∗

Abstract

Summary

Several purine bases and related nucleosides were tested for their effectiveness in potentiating the reversal by thymidine of aminopterin inhibition of the chick embryo. Of the free purine bases, hypoxan-thine was most active; adenine and guanine were also active, while xanthine was inactive. Of the nucleosides tested, only inosine was active. Inosine was more active than hypo-xanthine; together with thymidine it completely overcame the effects of the low level of aminopterin employed. The results are interpreted to mean that aminopterin prevents embryonic growth by preventing synthesis of compounds essential for formation of nucleic acids; by supplying these substances preformed, growth occurs even though amino-pterin is present. It follows that thymidine, and the active purine bases and derivatives listed above, are utilizable for nucleic acid synthesis by the chick embryo.