[Observation of non-equilibrium phase transitions in a closed enzyme system. Intermittent regulation of the activity of lactate dehydrogenase and similar enzymes].

Kinetic models of closed enzymic systems which allow the existence of multi-steady-states were considered. In the basis of these models lies the assumption about the coexistence in solution of different substrate forms and their non-enzymic transitions one to another. If the forms of substrate have different affinity to the enzyme and, moreover, one of the forms inhibits the enzyme, then the enzymic activity in certain conditions may change discretely. The transition of the system incidentally from one steady state to another is similar to the first order phase transition. It is shown that for the enzymic reaction catalyzed by lactate dehydrogenase, the conditions may exist for the non-equilibrium phase transition. It is shown that the existence of the metastability region in the system causes the unusual shifts of the discrete transition from one experiment to another. A model of an allosteric enzyme that describes the discrete behaviour similar to the phase transition is also considered.