Partial characterization and subunit analysis of major phosphoproteins of egg yolk in the fish, Oryzias latipes

Purification of two lipovitellins and development of immunoassays for two forms of their precursors (vitellogenins) in medaka (Oryzias latipes)

Two distinct yolk proteins (YP1 and YP2) were purified from the ovary of medaka, and specific antisera against YPs were generated to characterize YPs and reveal their relation to two vitellogenins (Vg1 and Vg2). The molecular masses of purified YP1 and YP2 on gel filtration were 270 and 380 kDa, respectively. YPs were confirmed to be lipoproteins by staining with Sudan black. Amino acid compositions of YP1 and YP2 were similar to those of Vg1 and Vg2, respectively. In double immunodiffusion using anti-Vg1, a precipitin line of YP1 formed a spur against the Vg1 line. YP2 and Vg2 were reacted with anti-Vg2, and a precipitin line of YP2 formed a fuse against the Vg2 line. These biochemical and immunological analyses of purified YPs revealed that YP1 is lipovitellin 1 (Lv1) derived from Vg1 and YP2 is lipovitellin 2 (Lv2) derived from Vg2. Using specific antibodies against Lvs and Vgs, specific, high sensitivity chemiluminescent immunoassays (CLIAs) for two Vgs were developed to reveal basal Vg levels and response to exogenous estradiol-17beta (E2). The measurable range of both CLIAs was from 0.975 to 1000 ng/ml. The cross-reactivity to the alternative Vg in each CLIA was extremely low (<or=0.57%). When immature fish were immersed in water containing E2 for 1 h, both Vgs were induced by 0.5 microg/L of E2 at 24 h after treatment. Vg1 increased in a concentration-dependant manner up to 100 microg/L E2, while Vg2 reached a plateau at 10 microg/L of E2. The ratio of Vg1:Vg2 in E2-treated fish changed in a concentration-dependent manner from 1.5:1 to 8.5:1. The results from E2-treatment suggest that differential regulation may control the expression of medaka Vgs.

An in vitro vitellogenin bioassay for oestrogenic substances in the medaka (Oryzias latipes)

We developed an in vitro bioassay for oestrogenic compounds based on vitellogenin as a biomarker. To quantify the induction of vitellogenin in medaka (Oryzias latipes) we established a sandwich enzyme-linked immunosorbent assay (ELISA). This ELISA was carried out with two different monoclonal antibodies specific for the yolk precursor protein vitellogenin and the high molecular weight yolk proteins (lipovitellin) of medaka. Purified lipovitellin of medaka was used as a standard for the ELISA and could be quantified down to a concentration of 9 ng/ml. The calibration curve was linear up to at least 600 ng/ml and the intra-assay coefficient of variance was 3.2%. The application of the sandwich ELISA was tested using blood samples of males and females as well as primary hepatocyte cultures of medaka. Vitellogenin synthesis in cultured liver cells of males was induced by 1 and 100 nM of the xenoestrogen 17alpha-ethynylestradiol (EE(2)). The first production of vitellogenin was detected 6 days after hormone application. In contrast, isolated liver cells of sexually mature females, which were treated in the same manner, showed vitellogenin expression from the beginning of cultivation and its synthesis increased and remained high at 100 nM EE(2). However, the induction of vitellogenin synthesis in male hepatocytes in vitro could be maintained for at least one month and this indicated viable and differentiated liver cells. The hepatocyte cultures of male medaka in combination with the sandwich ELISA were shown to be a suitable tool to detect and quantify oestrogenic activity of chemicals.