Kinetic properties and regulation of biliverdin reductase

In kinetic studies of the dual nucleotide enzyme biliverdin reductase, product inhibition patterns obtained with bilirubin as the inhibitor, using either the NADPH-linked reaction at pH 8.7 or the NADH-linked reaction at pH 7.0, are consistent with a random order of substrate addition and product release at either pH. With NAD(P) as the product inhibitor, the inhibition patterns suggest the existence of an enzyme-NAD(P)-biliverdin abortive complex. In the absence of added products, substrate inhibition observed at high biliverdin concentrations is also consistent with such a complex. Direct evidence for the existence of enzyme-NADP-biliverdin abortive complexes has been obtained at both pH 7.0 and 8.7 using the fluorescent NADP analog nicotinamide-1-N-6-ethenoadenine dinucleotide phosphate. Etheno-NADP fluorescence is enhanced in enzyme:etheno-NADP complexes but quenched in enzyme:biliverdin:etheno-NADP complexes. The effects of iron hematoporphyrin and cobalt protoporphyrin on the activity of biliverdin reductase have been examined. Iron-hematoporphyrin is a competitive inhibitor with respect to coenzyme at either pH. Comparison of Km values for biliverdin and Ki values for bilirubin or iron-hematoporphyrin indicate that while the binding of biliverdin is relatively pH independent, bilirubin binds more tightly at pH 7.0 than pH 8.7. Conversely, Fe-hematoporphyrin binds considerably tighter at pH 8.7 than pH 7.0. With cobalt-protoporphyrin, however, activation is observed, suggesting the existence of a regulatory site on biliverdin reductase for porphyrin derivatives. Resonance energy transfer measurements, from the fluorescent coenzyme analog etheno-NADP to biliverdin, iron-hematoporphyrin, or cobalt-protoporphyrin indicate that while the biliverdin site is located approximately 15 A from the etheno moiety of the coenzyme, both porphyrin derivatives are located 23.5 A away. This suggest the existence of a separate porphyrin binding site on biliverdin reductase which, depending upon the nature of the porphyrin binding, can produce activation or inhibition of the enzyme.