Protein degradation in the mouse blastocyst.

The degradation characteristics of 56 individual newly synthesized proteins of the Day 4 mouse blastocyst have been examined employing double isotope labeling of proteins for half-life measurement and two-dimensional electrophoresis for separation of proteins. The half-lives ranged from 1 to approximately 30 h with a mean of 12.4 h. Several proteins appeared to have half-lives greater than 30 h but decay times were insufficient to provide precise information for these proteins. The results suggest there is a tendency for proteins with acidic isoelectric points to be degraded more rapidly than basic proteins, and for high molecular weight proteins to be degraded more rapidly than low molecular weight proteins. Although the regressions of these two parameters on half-life were not significant, the direction and magnitude of the trends were similar to those previously described for liver proteins. Two specific proteins, tubulin and actin, were tentatively identified, and their half-lives determined. Tubulin had a half-life of 9.0 h. The half-lives of the provisionally identified gamma, beta, and alpha forms of actin were 2.2, 8.7, and 5.4 h respectively.