Yolk formation and degradation during oocyte maturation in seabream Sparusaurata: Involvement of two lysosomal proteinases

Oocyte growth within the follicle is preponderantly due to the accumulation of hepatically derived yolk protein (vitellogenin, VTG) by receptor-mediat ed endocytosis; once in the oocyte, VTG is partially processed and stored i n yolk globules. In some pelagic egg-laying marine teleosts, additional cle avages of yolk proteins followed by a pronounced water uptake occur concomi tantly with final oocyte maturation. The aim of this study was to establish the lysosomal enzymes involved in these two proteolytic processes that cha racterize oocyte maturation of seabream Spares aurata. The enzymatic activities of several cathepsins were assessed in the various classes of oocytes. Changes in cathepsin B, D, and L activity were found d epending on the oocyte maturation stage; cathepsin B and D were found to be at maximum level in early-vitellogenesis oocytes, and cathepsin L in mid-v itellogenesis ones. Cathepsin D and L were purified from seabream ovary, an d their roles in VTG and lipovitellin (LV) proteolysis, respectively, were analyzed. Here we demonstrate directly that one of the catalysts for the in traoocytic processing of VTG in yolk proteins is cathepsin D; however, we c annot exclude also a role of cathepsin B in the same process. On the other hand, cathepsin L is responsible for the second proteolytic cleavage of the LV components. We postulate that the acquisition of buoyancy by eggs throu gh the hydration process may be regulated by enzymatic activation at the ap propriate time of oocyte maturation, this process probably being the key ev ent in the reproduction of this marine pelagic egg spawner.