Buoyant densities of dissolved and crystalline lactate dehydrogenase from pig heart and pig muscle

Abstract

Buoyant densities of the H4 and M4 isoenzymes of lactate dehydrogenase from pig, in both dissolved and crystalline cross-linked states, have been determined at neutral pH in concentrated CsCl solutions by equilibrium centrifugations in an ultracentrifuge. In both states, buoyant densities of the M4-LDH were lower than those of the H4-LDH, presumably due to the binding of counterions and hydration water by ten additional charged amino acid residues present on the M4-LDH. The cross-linked crystals had lower buoyant densities than enzymes in dissolved state consistent with the lower density of the cross-linking reagent. The variations in buoyant densities observed in CsCl solutions are in agreement with a previously described model for the water-and ion-binding to proteins which assigns almost all of the bound water to the charged amino acid side chain residues. In Cs2SO4 solutions the buoyant densities of the H4-LDH in both physical states were less than in CsCl, probably because strongly hydrated sulphate ions bind to the charged amino acid residues.