Effects of glycine betaine and glycerophosphocholine on thermal stability of ribonuclease

Urea in renal medullas is sufficiently high to perturb macromolecules, yet the cells survive and function. The counteracting osmolytes hypothesis holds that methylamines, such as glycine betaine (betaine) and glycerophosphocholine (GPC) in renal medullas, stabilize macromolecules and oppose the effects of urea. Although betaine counteracts effects of urea on macromolecules in vitro and protects renal cells from urea in tissue culture, renal cells accumulate GPC rather than betaine in response to high urea both in vivo and in tissue culture. A proposed explanation is that GPC counteracts urea more effectively than betaine. However, we previously found GPC slightly less effective than betaine in counteracting inhibition of pyruvate kinase activity by urea. To test another macromolecule, we now compare GPC and betaine in counteracting reduction of the thermal stability of RNase A by urea. We find that urea decreases the thermal transition temperature and that betaine and GPC increase it, counteracting urea approximately equally. Therefore, the preference for GPC in response to high urea presumably has some other basis, such as a lower metabolic cost of GPC accumulation.