Vitellogenin (Vtg) is a serum lipophosphoglycoprotein that serves as the major precursor to the egg-yolk protein of oviparous vertebrates. It is synthesized in the liver, under the receptor-mediated regulation of 17β-estradiol (E2). 1 Teleost Vtg appears to circulate in the plasma as a dimer with an apparent molecular mass ranging from approximately 300 to 600 kDa. 2 Two distinct forms of Vtg have been identified in tilapias, 3 Japanese common goby, 4 barfin flounder 5 and haddock. 6 Normally, Vtg can only be detected in sexually active female fish. However, when male and juvenile fish are exposed to estrogen or estrogen mimic chemicals, 7,8 Vtg production can be induced. Therefore, the presence of Vtg in the blood of male and juvenile fish has been used as an indicator of the influence of xenobiotic estrogens. 7 In order to be effectively utilized as a biomarker, Vtg in the plasma has to be accurately measured.A number of methods have been developed for Vtg fish plasma analysis.
9-11Enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay (RIA) are the most commonly used approaches.
12,13These antibody-based methods have considerable advantages of sensitivity and selectivity, however, at the same time they suffer from problems related to antibody specificity when facing multiple fish species. An antibody made against Vtg from one species is limited in its application as a probe for another, 14 and the preparation of an antibody usually needs several months. Although cross-reaction was also found in some species, it is not possible to make an accurate quantification of low levels of Vtg in other species because of the affinity difference.1 Moreover, Vtg is very susceptible to proteolytic degradation. During the long process incubation period required in immunoassay methods, 9 the degradation of Vtg is inevitable, thus changing the overall reactivity of the antibody towards Vtg, even with the presence of antiproteolytic agents. For these reasons, to develop a new rapid and accurate determination method for Vtg is necessary and important.High performance liquid chromatography (HPLC) has been widely applied in the determination of proteins. [15][16][17] Vtg is a type of high molecular weight protein and can be precipitated by organic solvents. Even with large porous and short chain reverse phase columns, such as C4, it is difficult to separate Vtg from other proteins. On the other hand, ion exchange chromatography is widely used to purify Vtg. An anion exchange POROS HQ column has been adopted to analyze Vtg combined with UV detection at the wavelength of 280 nm, 16 and the detection limit of Vtg was reported as 2 μg per assay. Recently a two-step chromatographic method, combining anion exchange membrane purification with high performance size exclusion chromatography, was reported for the quantitative determination of Vtg in loach and sea catfish at the wavelength of 210 nm, with the detection limit of 20 μg ml -1 Vtg. 15 Although many peptides and proteins can absorb UV light and emit fluorescence due to th...