Diethyl pyrocarbonate inactivation of human placental aldehyde reductase II

Diethyl pyrocarbonate inactivated aldehyde reductase II (l-gulonate: NADP⁺ 6-oxidoreductase, EC 1.1.1.19) from human placenta. A concentration of 0.5-1.0 mM diethyl pyrocarbonate caused 40-65% loss of activity. The inactivation of the enzyme by diethyl pyrocarbonate was reversed by hydroxylamine and was accompanied by a large change in the absorbance of the protein at 242 nm, but not at 278 nm, indicating that only the histidine residues were modified. NADPH, but not glucoronate afforded significant protection to the enzyme from inactivation by diethyl pyrocarbonate. With 0.2-1.0 mM diethyl pyrocarbonate, 4-5 histidine residues were modified with a pseudo-first-order rate process. A double log plot of the fraction of the unmodified residues indicates that only one functional histidine residue is essential for the catalytic activity of aldehyde reductase II.