Pteridine Biosynthesis in the Butterfly Colias eurytheme

Abstract

Quantitative data are presented on the distribution of pteridines, primarily xanthopterin, leucopterin, erythropterin, and sepiapterin, in adults and developing pupae of the butterfly Colias eurytheme. The pteridine biosynthetic pathway in the developing wings of Colias has been studied by the use of radioactive precursors injected into whole pupae and incubated in Ringer's solution with excised, developing wings. By comparison of isotope incorporation, by pupae and isolated wings, from uniformly labeled adenosine- and guanosine-14C and guanosine-8-14C, it was concluded that Colias forms the pteridine ring from guanosine (or a phosphorylated derivative) with loss of the guanosine carbon atom 8. Pteridine interconversions were examined by studying the kinetics of isotope incorporation by the pteridines from uniformly labeled guanosine-14C immediately after its injection into whole pupae, and by studying the incorporation patterns obtained upon incubation of isolated wings with xanthopterin-14C, leucopterin-14C, and erythropterin-14C. The pteridine pathway is bipartite, one branch leading to position 6 side chain-bearing pteridines, the other leading to pteridines not alkylated at position 6. Within the latter branch, xanthopterin serves as precursor of leucopterin and erythropterin. The origins of position 6 side chain of sepiapterin and position 7 side chain of erythropterin were studied by incubation of isolated wings with radioactive pyruvate, malate, lactate, serine, threonine, and β-hydroxybutyrate. The results showed no incorporation into the sepiapterin side chain from any of these, but strong incorporation into the erythropterin side chain from pyruvate, malate, and lactate. In addition, sepiapterin made by Colias from uniformly labeled guanosine-14C was oxidized to 2-amino-4-hydroxypteridine-6-carboxylic acid, and the specific radioactivities of the parent compound and oxidation product were compared. The observed loss of specific activity agrees quantitatively with that expected if the sepiapterin side chain arises by modification of substitution on the initial pteridine position 6 side chain derived from guanosine ribose. The predictions of the alternative hypothesis of cleavage and replacement of the initial side chain are not compatible with the values observed. The present findings are discussed in relation to other recent work on pteridine biosynthesis.