Immunochemical detection of precursor proteins of yolk and vitelline envelope, and their annual changes in the blood of Diodon holocanthus

Abstract

Two distinct groups of female‐specific proteins, vitellogenin (VTG) and vitelline envelope proteins (VEP), were detected in the blood of the porcupine fish Diodon holocanthus, and annual changes in concentration were measured immunochemically. Using antisera against yolk proteins (ab.a‐E) and VEP (ab. a‐VEP), VTG and VEP could be detected in the blood of maturing female fish and oestradiol‐17β (E2)‐treated fish. Neither protein was detected in the blood of male fish. Immunohistochemistry showed that yolk globules and the vitelline envelope enclosing developing oocytes stained with ab.a‐E. The vitelline envelope was stained specifically with ab.a‐VEP. Hepatocytes from the E2‐treated fish had immunoreactivity with both antisera. Thus, VTG and VEP appear to be synthesized in the liver by direct stimulation of E2, released into the circulation, and incorporated into respective target sites. VTG and VEP in female serum maintained high levels from April until June, suggesting that yolk accumulation, as well as vitelline envelope formation, are occurring actively during these months. Unlike VTG, small amounts of VEP were detected between December and March, suggesting that vitelline envelope formation precedes yolk accumulation and that a slightly different hormonal regulation exists in the synthesis of both proteins in the liver during the early phase of oogenesis.