Ornithine decarboxylase activity from an extremely thermophilic bacterium, Clostridium thermohydrosulfuricum. Effect of GTP analogues on enzyme activity.

The activity of ornithine decarboxylase has been detected for the first time in extracts of a thermophilic bacterium, Clostridium thermohydrosulfuricum. The temperature optimum of the thermoresistant ornithine decarboxylase was 55 degrees C and the pH optimum was 7.5. It required pyridoxal phosphate and a thiol (dithiothreitol) for activity. The activity of the enzyme was closely connected to the growth of the thermophilic bacteria, since the activity was highest during the logarithmic growth. The enzyme was not inhibited (in contrast to the enzyme from Escherichia coli) by putrescine, spermidine or other naturally occurring polyamines. When the effect of GTP and a number of GTP analogues was tested on the activity of the enzyme, it was observed that GTP or dGTP was necessary for the full activity. The modification of either the purine base or 5'-phosphate chain of GTP leads to a stimulation smaller than that caused by GTP. Modification of the 3'-carbon of the ribose part of GTP (magic spot I and II of Cashel and Gallant, Nature 221 (1969) 838-841) caused a distinct inhibition of the enzyme activity, indicating that ornithine decarboxylase contains at least two domains for binding of GTP. The enzyme was inhibited irreversibly by high concentrations (50 mM) of difluoromethylornithine. Extracts of the bacterium contained also arginine decarboxylase, but its activity was always very much lower than that of ornithine decarboxylase. The activity of arginine decarboxylase was inhibited irreversibly by difluoromethylarginine, which is an irreversible suicide inhibitor of bacterial arginine decarboxylase (Kallio, A., McCann, P.P. and Bey, P. (1981) Biochemistry 20, 3163-3166).