Protein metabolism in preimplanted mouse ova

Intracellular specific radioactivity of an amino acid (leucine) was measured in preimplantation mouse ova. The measured specific activity along with leucine incorporation rates allow calculation of the protein synthetic rate in mouse ova. Unfertilized ova, fertilized ova, two-cell ova, and blastocysts convert amino acid to protein at the rate of 8.60, 7.36, 6.92, and 56.68 × 10 −13 moles of amino acid per hour per ovum, respectively. The specific activity measurements also allowed the calculation of intracellular leucine pool size. For the unfertilized, fertilized, two-cell, and blastocyst stage, the endogenous leucine pool was 11.9, 15.9, 6.4, and 62.4 × 10 −14 moles of leucine, respectively. Most of the increase in pool size in the blastocyst probably results from the increase in total volume of cells. Protein degradation measurements indicated a marked difference between protein turnover in the two-cell and blastocyst stage. Approximately 10% of the protein at the two-cell stage turns over with a half-life of 18.3 hr and 35% of the protein at the blastocyst stage turns over with a half-life of 11.2 hr. The large remaining fraction of protein turns over much more slowly.