Protein synthesis and phosphorylation by elongating 13-15-day-old cattle blastocysts

Most early embryonic loss in cattle occurs during the period of blastocyst formation and elongation. In this study de novo protein synthesis and phosphorylation by 13-15-day-old cattle blastocysts was determined. Blastocysts were classified by dimension into spherical, ovoid and elongated stages and cultured under oil for 4 h in drops of KSOM medium containing either 35S-methionine or 32P-orthophosphate. Protein content increased with shape from spherical to ovoid to elongated. There were differences (P<0.05) within and among days in the percentage of radiolabel incorporated. Incorporation of 35S-methionine increased with embryo protein content. However, incorporation of 35S-methionine into TCA-insoluble material, expressed per microg of protein, decreased with shape (P<0.001) with more than a 10-fold difference between spherical and elongated 13-day-old blastocysts. Incorporation of 32P-orthophosphate also tended to decrease with shape, with a 3-fold difference between spherical and elongated Day 13 blastocysts. It was concluded that, from Day 13, protein synthesis and phosphorylation, when expressed per unit of protein, decreased from a much higher level, which in the present study was around the time the blastocysts began to elongate.