The equilibrium constant at 38 degrees and I 0.25 of the triose phosphate isomerase reaction was found to be 22.0 and that of the aldolase reaction, 0.99x10(-4)m. The [dihydroxyacetone phosphate]/[glyceraldehyde phosphate] ratio was found to be 9.3 in rat liver. The causes of the apparent deviation of the triose phosphate isomerase system from equilibrium in vivo have been investigated. 2. The equilibria of the triose phosphate isomerase and aldolase reactions were studied with relatively large concentrations of crystalline enzymes and small concentrations of substrates, approximating to those found in rat liver and muscle. There was significant binding of fructose diphosphate by aldolase under these conditions. There was no evidence that binding of glyceraldehyde phosphate by either enzyme affected the equilibria. 3. The deviation from equilibrium of the triose phosphate isomerase system in rat liver can be accounted for by the low activity of the enzyme, in relation to the flux, at low physiological concentrations of glyceraldehyde phosphate (about 3mum). It has been calculated that a flux of 1.8mumoles/min./g. wet weight of liver would be expected to cause the measured degree of disequilibrium found in vivo. 4. The conclusion that the triose phosphate isomerase is not at equilibrium is in accordance with the situation postulated by Rose, Kellermeyer, Stjernholm & Wood (1962) on the basis of isotope-distribution data. 5. The triose phosphate isomerase system is closer to equilibrium in resting muscle probably because of a very low flux and a high enzyme concentration. 6. The aldolase system deviated from equilibrium in rat liver by a factor of about 10 and by a much greater factor in resting muscle. 7. The measurement of total dihydroxyacetone phosphate and glyceraldehyde phosphate content indicates the concentrations of the free metabolites in the tissue. This may not hold for fructose diphosphate, a significant proportion of which may be bound to aldolase.
