Hypoxanthine phosphoribosyltransferase activity in bovine embryos during the early embryonic development

The activity of hypoxanthine phosphoribosyltransferase (HPRT) was determined in the bovine embryo during early embryonic development. Microassay, using [(3)H] hypoxanthine, was improved to measure enzyme activity in the embryonic extract. This activity depended on the reaction time and the concentration of phosphorybosyl pyrophosphate (PRPP) in a reaction. mixture. Maximum activity was obtained at 4 hours of reaction time and at a concentration of 1 mM PRPP, but was much lower than the activity recorded in the mouse embryo. During early embryonic development, HPRT activity rapidly increased beyond the 8-cell stage. When distributions and activities of HPRT, adenine phosphorybosyltransferase (APRT), and the ratio of HPRT: APRT were examined in individual blastocysts, HPRT activity was broadly distributed, but it did not clearly show the bimodal distribution expected. Six of demi-embryos with high or low HPRT:APRT ratios were transferred to recipient cows from which 2 calves were obtained. Both offspring were of the sex predicted by the HPRT: APRT ratio. These results indicate that HPRT activity of bovine preimplantation embryos can be microassayed using radiolabeled hypoxanthine, and this assay could provide an alternative method for embryo sexing.