Effects of phosphate on the dissociation and enzymic stability of rabbit muscle lactate dehydrogenase

Frontal gel chromatography and sedimentation studies have been used to establish that rabbit muscle lactate dehydrogenase dissociates essentially completely into dimeric species (molecular weight ~70,000) in 0.2 I acetate-chloride (pH 5.0). Tetramer-dimer conversion, which occurred within a minute of adjusting the pH of an enzyme solution from 7 to 5, was reversed by restoring the solution to neutral pH, provided that exposure to the acidic environment was restricted to less than 4 hr at 4°. However, despite this re-formation of tetramer, some irreversible changes in enzyme structure were indicated by the recovery of only 70% of the original activity after exposure of the enzyme to pH 5 for 4 hr. Inclusion of phosphate (>30 mM) in the acetate-chloride solutions of rabbit muscle lactate dehydrogenase prevented any detectable dissociation into dimer; this increased stability of tetramer is allied with essentially complete retention of enzymic activity. Similar stabilizing effects were observed with lactate dehydrogenase solutions in which the phosphate was replaced by either reduced or oxidized nicotinamide adenine dinucleotide. Kinetic studies at pH 7 have established a pronounced activating effect of phosphate on the enzymic conversion of lactate to pyruvate. Under the same conditions the lactate dehydrogenase was shown to be tetrameric in the presence and absence of phosphate. Thus, although phosphate shares with the pyridine nucleotides the ability to stabilize the quaternary structure of rabbit muscle lactate dehydrogenase, the specific nucleotidebinding sites are not involved in these phosphate interactions unless phosphate and coenzyme can be accommodated simultaneously on the same sites