Activity of isocitrate dehydrogenase from three filamentous fungi in relation to osmotic and solute effects

Crude extracts of NADP+-specific isocitrate dehydrogenase (EC 1.1.1.42) were prepared from three filamentous fungi with different tolerances to water stress. There was no difference in the activity of this enzyme extracted from Phytophthora cinnamomi which had been grown on media of osmotic potential of 0 to -2MPa. Glycerol, proline and glucose caused little or no inhibition of the activity of the enzyme from P. cinnamomi, Penicillium chrysogenum and Chrysosporium fastidium over the range 0 to -10MPa. Potassium chloride and sucrose were both inhibitory, proportional to the concentration present in the assay buffer and hence inversely proportional to the osmotic potential. Both proline and glycerol were able to relieve the inhibition of the enzyme from P. cinnamomi due to KCl and sucrose, but glycerol did not have this effect on inhibition due to KCl of the other two enzymes. Glycerol was required for maximal activity of isocitrate dehydrogenase from P. chrysogenum and C. fastidium even in the absence of other solutes. The results are discussed in relation to the growth characteristics of the three species.