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Previous studies have indicated that the glucose-6-phosphate dehydrogenase (G6PD) contained within mouse ova is synthesized before ovulation and is under the genetic control of the X-chromosome (Epstein, Wegienka & Smith, 1969; Epstein, 1969). As such, it is different from the autosomally controlled hexose-6-phosphate dehydrogenase (H6PD) or glucose dehydrogenase found in liver microsomes but not in erythrocytes (Beutler & Morrison, 1967). To establish the applicability of these genetic results with ova to other tissues of the mouse, G6PD from ova has been characterized and compared with erythrocyte G6PD. Ova and pre-implantation embryos were obtained from Swiss albino mice (Mus musculus) by superovulation. For microspectrophotometric assay of enzyme activity, they were lysed by freezing and thawing in 0-05 M-tris buffer, pH 7-5, containing 0-05% bovine serum albumin (BSA) (Epstein et al., 1969). For polyacrylamide disc gel electrophoretic studies, the ova and embryos were lysed in upper gel buffer containing 10" 5m-NADP and 0-1 % BSA (Kirkman & Hanna, 1968). Erythrocytes obtained from the same animals were lysed by freezing and thawing in the same solutions. Hexose-6-phosphate dehydrogenase was prepared from isolated liver microsomes by extraction with 0-5 % sodium cholate (Beutler & Morrison, 1967). G6PD and H6PD activities in erythrocytes and liver were determined by a micromodification of the procedure of Glock & McLean (1953) in which correction is made for 6-phosphogluconate dehydro¬ genase activity (Epstein et al., 1969). No such correction is required for egg G6PD, since the latter enzyme is not measurable in these cells. Enzyme activity was measured as a function of substrate concentration, and the results are shown in Text-fig. 1. When erythrocyte and egg G6PD were adjusted to the same initial activity with 0-45 mM-substrate, both enzymes gave the identical line in the plot of S/V versus S, where S represents substrate concentration and V represents reaction velocity. The Michaelis constant, Km, for the enzymes from both sources is calculated to be 42-3 µ , a value not too dissimilar from the Km of 49 ± 4 µ reported for human G6PD-B (Dern, McCurdy & Yoshida, 1969). On polyacrylamide disc gel electrophoresis (Plate 1), G6PD from ova migrates toward the anode as a single major band. Fifty ova were used for each sample. Erythrocyte G6PD migrates as a rapidly moving major band and one

THE CHARACTERIZATION OF GLUCOSE-6-PHOSPHATE DEHYDROGENASE IN MOUSE OVA