EXPRESSION OF GLYCOLYTIC ISOZYMES IN RAT THYMOCYTES DURING CELL-CYCLEPROGRESSION

The time courses of activities of aldolase, glyceraldehyde-3-phosphate dehydrogenase, hexokinase and pyruvate kinase were determined in stim ulated rat thymocytes at 24 h intervals during a period of 72 h of cul ture. In parallel the mRNA levels of these enzymes were analysed by No rthern blotting with specific probes. Both the enzyme activities and t he corresponding mRNA levels reached their maxima 48 h after stimulati on coinciding with the S-phase of the cell cycle. The isozyme types of aldolase and hexokinase in resting and in mitogen-stimulated rat thym ocytes were identified by Northern blot hybridisation using isozyme-sp ecific probes. In these cells the aldolase A is expressed, whereas typ e B and C could not be detected. The transcription of the aldolase A g ene can be regulated by two different promoters. Depending on the alte rnative usage of the promoters the aldolase A-specific mRNA either con tains the non-translated exons M1 or AH1. In rat thymocytes the promot er proximal to the exon AH1 is used while the expression of mRNA I, th e type characteristic for muscle tissue, was not observed. In contrast to aldolase two isozyme types of hexokinase were detected. Hexokinase I as well as hexokinase II were present in thymocytes whereas hexokin ase III was not detectable. A shift in the isozyme pattern was not obs erved during the cell cycle progression.