Effect of nitrogen and carbon sources on growth and activities of NAD and NADP dependent isocitrate dehydrogenases of Laccaria bicolor

The ectomycorrhizal Laccaria bicolor, cultivated axenically in modified Pachlewski's medium, can use a broad spectrum of nitrogen and carbon sources. The fungus exhibited greater growth in the presence of ammonium or nitrate than with ammonium nitrate. With the latter salt, the drop in pH of the culture medium indicated that ammonium was taken up preferentially to nitrate. L. bicolor grew more poorly on amino acids such as glycine, alanine, aspartate and glutamate, the latter three being very poor carbon sources. By contrast, glycine was used as nitrogen and carbon sources. Glucose and maltose in mixture, with a 20:5 ratio, were the most effective carbohydrates for promoting growth, followed by starch, dextrins, maltose, glucose and sorbitol. L. bicolor failed to grow in the presence of sucrose and galactose. Isocitrate dehydrogenase (IDH), a key enzyme linking carbon and nitrogen metabolism, is present in L. bicolor as NAD-and NADP-dependent proteins. Both enzymes were stimulated in response to nitrogen starvation and appeared to operate in close association with the glutamine synthetase and NADP-dependent glutamate dehydrogenase of the fungus, indicating that 2-oxoglutarate produced by IDHs is probably utilized in the assimilation of inorganic nitrogen. The highest specific activities of NAD and NADP-IDH coincided in most cases with the rapid growth periods of the fungus, although in slow growing conditions obtained by addition of glycine or sorbitol to the culture media, only NADP-IDH activity increased in conjunction with growth. In addition, activity of the NADP-IDH was constantly higher than that of NAD-IDH. These results are indicative that the NADP-dependent enzyme plays a substantial biosynthetic role, possibly by an additional production of reduced nucleotides.