Effect of proline on lactate dehydrogenase activity: testing the generality and scope of the compatibility paradigm

The k(cat) and K(m) kinetic parameters of the labile enzyme rabbit muscle lactic dehydrogenase were determined as a function of the concentration of proline, a solute (osmolyte) accumulated in the cells of many organisms to protect them against environmental stresses. Proline is believed to protect against the stress(es) without altering the functional activity of cellular macromolecules, a property defining it as a "compatible osmolyte." In the range of 0-2 M proline, K(cat) and K(m) values for both substrates are essentially unchanged, but between 2 M and 4 M proline, k(cat) decreases by a factor of 3 to 4, whereas K(m) values are only modestly changed, if at all. These results are consistent with the proposal that compatible osmolytes do not affect functional activity, that the property of compatibility expressed by such osmolytes is generic without regard to the evolutionary history of the protein, and that the organic osmolyte concentration range over which compatibility is exhibited is extensive. In short, the results are in full accord with the principal hypothesis of "compatible osmolytes" in detail and scope.