Developmental patterns of three X chromosomal enzyme activities in the brains of female and male mice. Lack of sex-dependent reaction to a fractionated prenatal x-ray dose.

The developmental profiles of the activities of the X chromosome-linked enzymes alpha-galactosidase, hypoxanthine guanine phosphoribosyltransferase, and glucose-6-phosphate dehydrogenase were studied in the brains of mice. X chromosome inactivation, which takes place at an early stage of development, led to identical enzyme activities in females and males between the 15th day of gestation and the 64th day after birth. The enzyme activities were also studied after an X-ray dose of 3 times 1.05 Gy, delivered on gestational days 11, 12 and 13. Treatment with this dose, but not with 3 times 0.95 Gy or 1.15 Gy, respectively, was followed by the death of predominantly female offspring within 2 days after birth. The assumption that the reason for this might be a reactivation of inactive female X chromosomal genes in brain cells was tested in the present experiment. There were marked radiation-induced alterations of the activities of the above-mentioned enzymes. However, sex differences of these enzyme activities were not found after the irradiation treatment. A participation of X chromosome reactivation in the elevated mortality of female offspring is therefore unlikely.