Degradation of vitellin during embryonic and larval development in the Pacific oysterCrassotrea gigas

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Abstract
The kinetic profiles of vitellin degradation, protease activity and free amino acids in the embryo and larvae of the Pacific oyster, Crassostrea gigas, have been investigated during the period from the unfertilized egg through the 48 h straight-hinge larva. Immunoblotting using anti-vitellin showed that two major bands (179 and 110 kD) and several faint bands detected in the unfertilized egg become weak at the trochophore stage and disappear 48 h postfertilization. In gel filtration the main peak of the intact molecule of vitellin, estimated to be 530 kD, tended to become low from the blastula stage onwards. The relative vitellin content determined by enzyme-linked immunosorbent assay showed the same decreasing pattern as in gel filtration. The increase in the protease activity during larval development agreed well with the timing of the vitellin degradation and this protease directly degraded larval vitellin protein. The total free amino acids drastically increased at the same time as the increase in protease activity and were reduced at the 24 h straight-hinge larva stage. The results obtained here suggest that oyster vitellin is degraded during larval development and free amino acids, generated by hydrolysis of vitellin protein, may play a role in embryonic and larval development.