Activation of human biliverdin-IXα reductase by urea: Generation of kinetically distinct forms during the unfolding pathway

Activation of enzymes by low concentrations of denaturants has been reported for a limited number of enzymes including lipocalin-type prostaglandin D synthase (L-PGDS) and adenylate kinase. During unfolding studies on human biliverdin-IXα reductase it was discovered that the enzyme is activated at low concentrations of urea. Under standard assay conditions the native enzyme displays pronounced substrate inhibition with biliverdin as variable substrate; however in the presence of 3 M urea, the substrate inhibition is abolished and the enzyme exhibits Michaelian kinetics. When the initial rate kinetics with NADPH as variable substrate are conducted in 3 M urea, the Vmax is increased 11-fold to 1.8 μmol/min/mg and the apparent Km for biliverdin increases from 1 to 3 μM. We report the existence of two kinetically distinct folded intermediates between the native and unfolded forms. When the period of incubation with urea was varied prior to measuring enzyme activity, the apparent Vmax was shown to decay to half that seen at zero time with a half life of 5.8 minutes, while the apparent Km for NADPH remains constant at approximately 5 μM. With NADH as cofactor the half life of the activated (A) form was 2.9 minutes, and this form decays in 3 M urea to a less active (LA) form. The apparent Km for NADH increases from 0.33 mM to 2 mM for the A and LA forms. These kinetically distinct species are reminiscent of the activity-enhanced and inactive forms of L-PGDS observed in the presence of urea and guanidine hydrochloride.