Figure 1. One-cell embryos that do not express the slc7a2 product (CAT2 knockout embryos) have reduced system b⁺₁ activity relative to one-cell embryos from genetically similar control mice. CAT2 knockout mice (and embryos) do not express a functional CAT2 protein (slc7a2 product). System b⁺₁ activity was measured as defined previously (Van Winkle, 2001). The mean (±S.E.) activities shown were each calculated from 6–13 determinations obtained in 3–6 independent experiments. The total system b⁺₁ activity in one-cell embryos from CAT2 knockout mice was significantly lower than the activity in embryos from otherwise genetically similar control mice as determined in a group comparison t-test (p<0.01). The reduction in activity is attributed to the lack of functional CAT2 expression, whereas the activity remaining in one-cell embryos from CAT2 knockout mice is attributed to CAT1 (slc7a1 product) and, possibly, CAT3 (slc7a3 product) expression (Table 1). CAT1, CAT2 and probably CAT3 normally are expressed throughout preimplantation development. The lack of reduction in system b⁺₁ activity in CAT2 knockout two-cell conceptuses is attributed to increased expression of CAT1 or CAT3 upon activation of the embryo genome at the late one-cell stage to compensate for the missing CAT2 activity. See text for further discussion.

Figure 2. Changes in the aspartate (Asp), glutamate (Glu) and glutamine (Gln) content of pre-implantation mouse embryos during development in vivo. Stages of development at various times after fertilization are: 35 h=2-cell, 59 h=4- to 8-cell, 83 h=earlier blastocyst and 107 h=later blastocyst. (Data from Tables 2 and 5 of Van Winkle and Dickinson, 1995).