Transport of amino acids and ammonium in mycelium of Agaricus bisporus

Mycelium of Agaricus bisporus took up methylamine (MA), glutamate, glutamin e and arginine by high-affinity transport systems following Michaelis-Mente n kinetics. The activities of these systems were influenced by the nitrogen source used for mycelial growth. Moreover, MA, glutamate and glutamine upt akes were derepressed by nitrogen starvation, whereas arginine uptake was r epressed. The two ammonium-specific transport systems with different affini ties and capacities were inhibited by NH4+, with a K-i of 3.7 mu M for the high-velocity system. The K-m values for glutamate, glutamine and arginine transport were 124, 151 and 32 mu M, respectively. Inhibition of arginine u ptake by lysine and histidine showed that they are competitive inhibitors. MA, glutamate and glutamine uptake was inversely proportional to the intrac ellular NH4+ concentration. Moreover, increase of the intracellular NH4+ le vel caused by PPT (DL-phosphinotricin) resulted in an immediate cessation o f MA, glutamine and glutamate uptake. It seems that the intracellular NH4concentration regulates its own influx by feedback-inhibition of the uptake system and probably also its efflux which becomes apparent when mycelium i s grown on protein. Addition of extracellular NH4+ did not inhibit glutamin e uptake, suggesting that NH,I and glutamine are equally preferred nitrogen sources. The physiological importance of these uptake systems for the util ization of nitrogen compounds by A. bisporus is discussed. (C) 1999 Elsevie r Science B.V. All rights reserved.