Non-enzymic nature of the pyridine haemochrome-cleaving activity of mammalian tissue extracts ('haem alpha-methenyl oxygenase')

Detection of biliverdin reductase activity

The conversion of biliverdin to the bile pigment bilirubin is catalyzed by biliverdin reductase, a cytosolic enzyme with two pH optima with different cofactors. The enzyme is assayed at pH 8.7 with NADPH as a cofactor and at 6.75 with NADH. The production of bilirubin is detected as described in this unit with a spectrophotometric assay. The enzyme can be qualitatively assayed by immunoblotting and changes in reductase isoform expression can be detected by two-dimensional electrophoresis.

Purification and properties of cow splenic biliverdin reductase

Biliverdin reductase was purified from cow spleen. The specific activity of the final enzyme preparation was 24.01 u/mg, representing 686-fold purification as measured with NADPH. The yield was 3 grams of enzyme per 100 grams of cow spleen. The purified enzyme was a monomeric protein with an apparent molecular weight of about 34,000 and an isoelectric point of about 6.2. The biliverdin reductase was specific for biliverdin and reduced IX alpha faster than the biliverdin isomers IX beta, IXr, or IX delta. The purified enzyme could utilize both NADH and NADPH, but the kinetic properties of the NADH-dependent and the NADPH-dependent enzyme activities were different: the time course of the NADPH-dependent reaction displayed a sigmoidal curve, whereas that of the NADH-dependent reaction did not. Km for biliverdin IX alpha was 4 x 10(-4) mM in the NADPH system, while it was 1.5 x 10(-3) mM in the NADH system. Both enzyme activities were inhibited by excess biliverdin, but the inhibition of the NADPH-dependent enzyme activity was more pronounced. The pH optimum was 7.0 with NADH, and 6.8 with NADPH.

Methene bridge carbon atom elimination in oxidative heme degradation catalyzed by heme oxygenase and NADPH-cytochrome P-450 reductase

Physiological heme degradation is mediated by the heme oxygenase system consisting of heme oxygenase and NADPH-cytochrome P-450 reductase. Biliverdin IX alpha is formed by elimination of one methene bridge carbon atom as CO. Purified NADPH-cytochrome P-450 reductase alone will also degrade heme but biliverdin is a minor product (15%). The enzymatic mechanisms of heme degradation in the presence and absence of heme oxygenase were compared by analyzing the recovery of 14CO from the degradation of [14C]heme. 14CO recovery from purified NADPH-cytochrome P-450 reductase-catalyzed degradation of [14C]methemalbumin was 15% of the predicted value for one molecule of CO liberated per mole of heme degraded. 14CO2 and [14C]formic acid were formed in amounts (18 and 98%, respectively), suggesting oxidative cleavage of more than one methene bridge per heme degraded, similar to heme degradation by hydrogen peroxide. The reaction was strongly inhibited by catalase, but superoxide dismutase had no effect. [14C]Heme degradation by the reconstituted heme oxygenase system yielded 33% 14CO. Near-stoichiometric recovery of 14CO was achieved after addition of catalase to eliminate side reactions. Near-quantitative recovery of 14CO was also achieved using spleen microsomal preparations. Heme degradation by purified NADPH-cytochrome P-450 reductase appeared to be mediated by hydrogen peroxide. The major products were not bile pigments, and only small amounts of CO were formed. The presence of heme oxygenase, and possibly an intact membrane structure, were essential for efficient heme degradation to bile pigments, possibly by protecting the heme from indiscriminate attack by active oxygen species.

Metabolism of biliverdin. Biliary excretion of bile pigments after intravenous injection of biliverdin isomers

14C-labelled biliverdins IX alpha, beta, gamma and delta have been prepared in vitro from haemoglobin obtained from duck erythrocytes incubated with 5-amino[4-14C]-laevulinic acid. When injected intravenously into rats with biliary fistulae, about 60% of the label was recovered in the bile in 24 h after the alpha isomer was given, while approximately 10% was recovered with injection of the beta isomer. The gamma and delta isomers gave intermediate values. In each experiment, most of the recovered isotope was found in association with conjugated bile pigment. Thus, the metabolic pathway for bile pigment excretion in the rat handles the IX alpha isomer preferentially but is not specific for it.

Haemoglobin catabolism: the role of ferrihaems in studies of the degradation pathway

The importance of ferrihaem aggregation in studies of haemoglobin catabolism is assessed in the light of recent work. Experimental evidence is put forward suggesting that monomeric ferrihaems are degraded much more readily than dimeric species. This may offer an alternative explanation for the apparent ;apoprotein catalysis' recently observed.