The egg-shell of Drosophila melanogaster III. Covalent crosslinking of the chorion proteins involves endogenous hydrogen peroxide.

Utilizing two cytochemical methods, namely, diaminobenzidine for the assay of peroxidases and cerium(III) chloride for the localization of hydrogen peroxide it was found that the enzyme exists in two out of the five egg-shell layers: the innermost choronic layer and the endochorion. In addition, hydrogen peroxide which acts as a substrate for the enzyme in vitro enabling the formation of covalent bonding between the egg-shell proteins, was found to be produced at the follicle cell plasma membrane during the last stage of oogenesis. It is concluded that hydrogen peroxide is an endogenous, programmed product of the follicle cells, responsible for the action of peroxidase in order to oxidize the tyrosyl residues producing di-tyrosine and tri-tyrosine bonds between the chorion polypeptides.