STABILITY STUDIES ON PIG-HEART MITOCHONDRIAL MALATE-DEHYDROGENASE - THE EFFECT OF SALTS AND AMINO-ACIDS

The effect of different salts and amino acids on the thermal stability and quaternary conformation of pig heart mitochondrial malate dehydro genase (phm-MDH) in solution has been determined. The effectiveness of salts of anions in the stabilisation of phm-MDH followed the order: C itrate > SO42- greater than or equal to Tartrate > Phosphate > F-, CH3 COO-> Cl-> Br-. Anions above and including Cl- in this series were inc reasingly effective in stabilising phm-MDH with a rise in salt concent ration from 0.05-2 M, whilst Br- was destabilising under similar condi tions. The effect of potassium salts of acetate, chloride and bromide at a concentration of 1 M on the quaternary conformation of phm-MDH co rrelated also with the relative order of anion stabilisation above, wi th the anions higher in the series increasingly promoting the formatio n of the dimeric conformation of the enzyme. The cations of the corres ponding salts had a relatively neutral (Cs+, K+, Na+, (CH3N+, NH4+) to a destabilising ((CH3N+, NH4+, Li+) effect on phm-MDH. Potassium ferr ocyanide and potassium ferricyanide conferred complex, concentration d ependent effects on the stability of phm-MDH, unlike the salts describ ed above. Salts of amino acids were effective in the stabilisation of phm-MDH against temperature induced changes, following the order: NaGl utamatec similar or equal to NaAspartate > NaGlycinate > lysine . HCl > arginine . HCl. The magnitudes and trends of the effects of these sa lts and amino acids on the stability and quaternary structure of phm-M DH were observed to correlate well with considerations based on the Ho fmeister series of anions and solvophobic concepts as they apply to th e influence of co-solvents at intermediate to higher concentrations. O ther, more specific effects were also evident in the stabilisation and destabilisation of phm-MDH by low concentrations of the salts, as not ed most particularly in the presence of potassium ferrocyanide and pot assium ferricyanide.