Mechanism of high NaCl‐ and urea‐induced inhibition of GDPD5, an enzyme whose phosphodiesterase activity breaks down the osmoprotective organic osmolyte glycerophosphocholine

We previously found that high NaCl‐ and urea‐induced increase of glycerophosphocholine (GPC) involves a post translational modification that inhibits GPC phosphodiesterase (GPC‐PDE) activity of GDPD5, and we tentatively identified the modification as dephosphorylation of GDPD5 at threonine 587. The present studies were aimed at extending those observations. We now find that the activity of recombinant GDPD5, immunoprecipitated from HEK293 cells, is greatly reduced by mutating T587 to an alanine, which cannot be phosphorylated. GDPD5‐T587 is a consensus phosphorylation site for CDK1 kinase, previously found to be inhibited by high NaCl. We now find that CDK1/5 inhibitor or siRNA knockdown of CDK1 also reduces HEK293 GPC‐PDE activity, confirming that CDK1 may be involved. High NaCl or urea increases reactive oxygen species (ROS). ROS apparently are also involved since the oxidant, H 2 O 2 , decreases GPC‐PDE activity and the antioxidant, NAC, prevents NaCl‐ and urea‐induced inhibition of the GPC‐PDE activity. The relationship between the effects of ROS and CDK1 remains to be established.