GLYCEROPHOSPHOCHOLINE AND BETAINE COUNTERACT THE EFFECT OF UREA ON PYRUVATE-KINASE

Renal medullary cells contain large quantities of organic osmolytes wh en the levels of salt and urea in renal medullary interstitial fluid a re high. Two of these osmolytes, betaine and glycerophosphocholine (GP C), are methylamines. Methylamines generally counteract the perturbing effects of urea on enzymes and other macromolecules. Betaine was prev iously shown to counteract the effect of urea on enzymes in vitro and to protect renal cells in tissue culture from harmful effects of high urea. Nevertheless, renal medullary cells in vivo and in tissue cultur e specifically accumulate GPC rather than betaine, in response to high urea. In the present studies we tested directly whether GPC counterac ts the effect of urea on the K-m of pyruvate kinase (PK) for ADP and c ompared the effectiveness in the regard of GPC to that of betaine. We find the urea increases the K-m (as previously observed), that betaine and GPC decrease it, and that the increase caused by urea is countera cted by betaine or by GPC. The effects of GPC are slightly less than t hose of betaine. In addition, other renal medullary organic osmolytes (namely sorbitol, inositol and taurine) were already known to be compa tible osmolytes whose accumulation protects renal medullary cells from hypertonicity because they have little effect on enzyme function. In agreement with this generalization we find that high sorbitol or inosi tol has little or no effect on PK activity, but surprisingly that taur ine reduces V-max and greatly elevates K-m. In conclusion, the main fi nding is direct evidence that GPC is a counteracting osmolyte, which e xplains its accumulation in response to high urea. However, we do not find that GPC is a more effective counteracting osmolyte than betaine, which leaves unexplained the preference of renal cells for GPC over b etaine for counteracting the perturbing effect of urea.