INVOLVEMENT OF L-TYROSINE AND PHENOL OXIDASE IN THE TANNING OF AEDES-AEGYPTI EGGS

L-Tyrosine was found to be the initial precursor involved in the tanning of the chorion of Aedes aegypti eggs. This amino acid became detectable at about 24 h post-blood feeding and significant amounts of this compound accumulated in the ovaries during subsequent egg development. However, no o-diphenolic compounds were synthesized during the development of eggs within the mosquito ovaries. Oviposition of eggs to the outside environment initiated eggshell tanning due to activation of phenol oxidase, which then catalyzed the production of L-DOPA from L-tyrosine and further oxidized L-DOPA and dopamine to their respective o-quinones. Hardening and blackening of the chorion was completed in about 4 h. DOPA decarboxylase (DDC) also seemed to be involved in chorion tanning because a high percentage Of L-DOPA, produced by the action of phenol oxidase following its activation, was decarboxylated to dopamine in in vitro assays. In addition, the application of the DDC inhibitor, m-hydroxybenzylhydrazine, resulted in an increased time required for the complete tanning of the chorion.