EFFECT OF NITROGEN AND CARBON-SOURCES ON GROWTH AND ACTIVITIES OF NADAND NADP DEPENDENT ISOCITRATE DEHYDROGENASES OF LACCARIA BICOLOR

The ectomycorrhizal Laccaria bicolor, cultivated axenically in modifie d Pachlewski's medium, can use a broad spectrum of nitrogen and carbon sources. The fungus exhibited greater growth in the presence of ammon ium or nitrate than with ammonium nitrate. With the latter salt, the d rop in pH of the culture medium indicated that ammonium was taken up p referentially to nitrate. L. bicolor grew more poorly on amino acids s uch as glycine, alanine, aspartate and glutamate, the latter three bei ng 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 dehydrog enase (IDH), a key enzyme linking carbon and nitrogen metabolism, is p resent in L. bicolor as NAD-and NADP-dependent proteins. Both enzymes were stimulated in response to nitrogen starvation and appeared to ope rate in close association with the glutamine synthetase and NADP-depen dent glutamate dehydrogenase of the fungus, indicating that 2-oxogluta rate produced by IDHs is probably utilized in the assimilation of inor ganic nitrogen. The highest specific activities of NAD and NADP-IDH co incided in most cases with the rapid growth periods of the fungus, alt hough in slow growing conditions obtained by addition of glycine or so rbitol to the culture media, only NADP-IDH activity increased in conju nction with growth. In addition, activity of the NADP-IDH was constant ly 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.