Carbodiimide-dependent inactivation of dihydrofolate reductase

Dihydrofolate reductase from amethopterin-resistant Lactobacillus casei was inactivated by a water soluble carbodiimide, 1-ethyl-3-(3-dimethyl-aminopropyl)-carbodiimide HCl. The rapid inactivation observed at pH 5.0–6.0, coupled with lack of recovery of activity from inactivated samples incubated with NH 2OH was consistent with modification of enzymic carboxyl groups. Significant protection against inactivation was provided by 7,8-dihydrofolate and NADPH. Analysis of the reaction order suggests that the carbodiimide-dependent inactivation may result from modification of a single essential carboxyl group.