Glucose metabolism during the final stage of human oocyte maturation: genetic expression of hexokinase, glucose phosphate isomerase and phosphofructokinase

The low involvement of glucose metabolism in early preimplantation embryos has suggested the presence of metabolic blocks in the glycolytic pathway. Genetic expression of hexokinase (HK) glucose-6-phosphate isomerase (GPI) or phosphofructokinase (PFK) were qualitatively analysed using the reverse transcription nested polymerase chain reaction (RT-nested PCR) in individual human immature oocytes at the germinal vesicle (GV) stage and in single non-fertilised human metaphase II (MII) oocytes, after NF or ICSI failures. Transcripts encoding for HK, GPI and PFK were detectable in the majority of the oocytes analysed, and with different expression patterns. GPI transcripts were consistently present and appear to be constitutively expressed during GV and MIT stages. In contrast, low quantities of transcripts encoding for HK and PFK were observed in all oocytes analysed. Our data revealed that the metabolic block previously reported at GPI or PFK levels underwent post-transcriptional regulation. The expression profiles of HK and PFK transcripts in GV and MII reflect a low level of transcription or active translation of maternal transcripts during oocyte maturation. Nevertheless, enzymatic activities of HK and PFK have previously been determined in human oocytes. This suggests that HK and PFK may be accumulated in protein (enzyme) form instead of maternal mRNA during human oocyte maturation. The expression pathway of glycolytic metabolism reflects the presence of different mechanisms involved in gene expression/ regulation at the transcriptional and translational level and their accumulation during human oocyte maturation.