Effects of glycose on NADP and NAD glutamate dehydrogenase activities and their relation to ammonium and pH levels in cultures of Bipolaris maydis race T

Abstract

NADP-glutamate dehydrogenase (NADP-GDH) and NAD-glutamate dehydrogenase (NAD-GDH) activities from Bipolaris maydis race T (ATCC 36180) were determined by measuring the change in absorbance at 340 nm of either reduced NADP or NAD in a reaction mixture of NH4C1, α-ketoglutarate and a cell free extract of the fungus. NADP-GDH activity was high at 48 h, but low at 72 and 96 h when the fungus was incubated on a reciprocal shaker at 28 °C in a mineral salts medium containing 2 g/l glucose and 4 g/l Lasparagine. In contrast, in these cultures NAD-GDH activity was low at 48 h, but high at 72 and 96 h. At 72 and 96 h glucose was not detected in the culture medium. In addition, levels of ammonium and pH increased from 0.0 μmoles/ml and pH 5.8 at 48 h to 10.6 μmoles/ml and pH 7.2 at 72 h, and to 23.0 μmoles/ml and pH 8.4 at 96 h. Fungal mycelia were transferred after 48 h of incubation on media containing 2 g/l glucose and 4 g/l L-asparagine to fresh media containing 0, 2 or 5 g/l glucose with and without 4 g/l L-asparagine. Twenty-four h after transfer to fresh media containing 5 g/l glucose with L-asparagine or 2 or 5 g/l glucose without L-asparagine, NADP-GDH activity was high and NAD-GDH activity was low. Glucose was detected in the culture medium, ammonium was not detected and the pH remained unchanged or decreased. In contrast, 24 h after transfer to fresh media with 0 or 2 g/l glucose with L-asparagine and on media lacking glucose or L-asparagine, NADP-GDH activity was low and NAD-GDH activity was high. Glucose was not detected in the culture medium, ammonium levels were high and the pH increased. Thus, accumulation of ammonium and pH increases accompanying depletion of glucose in a L-asparagine medium could be related to a change in the capacity of B. maydis race T to assimilate and produce ammonium via pathways involving glutamate dehydrogenases.