Effect of Salts on the Freezing Denaturation of Lactate Dehydrogenase

In this work the effect of added salts in protein solutions on the denaturation of proteins during a freeze–thawing cycle was studied experimentally using lactate dehydrogenase (LDH) as the model proteins. We tested the addition of nine different compounds on their individual effect on LDH during freezing and thawing including inorganic salts, MgSO4, Na2SO4, MgCl2, CaCl2, NaCl, KCl, Mg(NO3)2, Na(NO3) and one organic salt CH3COONa. The effect of freezing and thawing rate on the recovery of LDH activity was experimentally studied using a cryostage set-up. For all the tested solutions, it was observed that slow cooling caused less activity loss of LDH than rapid cooling, confirming our recent results reported elsewhere. Salt type greatly affects the LDH activity recovery, with MgSO4, Na2SO4 and CH3COONa offering improved protection to LDH, and MgCl2, CaCl2 and Mg(NO3)2 causing more damage. The protecting effect of each individual salt roughly follows its position in the so-called Hofmeister Series. The results can guide the selection of protein solution make-up for processes involving freezing preparation, and freezing–thawing protocols.