Influence of Monovalent Cations on the Activity of l ‐Serine ( l ‐Threonine) Dehydratase from Rat Liver

The effect of monovalent cations on the enzymatic properties of highly purified l ‐serine ( l ‐threonine) dehydratase from rat liver was studied. If the enzyme, freed from NH 4 + or K + , was diluted or treated with heat it lost much more activity on l ‐threonine than on l ‐serine. NH 4 + or K + stimulate the enzyme activity on threonine in a much higher degree than on serine, and this degree of activation was especially high at low enzyme concentrations. The activity ratio of the enzyme on l ‐threonine to l ‐serine varied from 0.3 to 1.6 depending on the conditions applied. The apparent dissociation constant for the coenzyme, pyridoxal 5′‐phosphate, was increased by diluting the enzyme and decreased markedly by the addition of NH 4 + or K + . From the saturation curves for pyridoxal 5′‐phosphate it was suggested that the coenzyme binds to the apoenzyme less tightly with l ‐threonine as substrate than with l ‐serine, and the apparent binding constants decrease with increasing the substrate concentration. One possible explanation for the change in value of the activity ratio is that serine and threonine affect differently the binding of pyridoxal 5′‐phosphate, thus altering differently the balance between the amount of active holoenzyme and inactive apoenzyme in the reaction systems. Since the binding constant for the coenzyme is larger with serine than with threonine as substrate, the treatments which promote the dissociation of pyridoxal 5′‐phosphate (diluting and heating the enzyme) lower the activity on threonine more than on serine, whereas those which promote the binding of the coenzyme (concentrating the enzyme and addition of NH 4 + or K + ) enhance the activity on threonine more than on serine.