Add time:08/14/2019         Source:sciencedirect.com

The enzymatic oxidations of [α-14C]hematohemin IX and 2,4-diacetyl[α-14C]-deuterohemin IX were carried out by using a microsomal heme oxygenase system from rat liver in combination with the biliverdin reductase from the same origin. In every case the bilirubins formed were devoid of radioactivity, indicating the α-selective oxidation of the three hemins. Hematohemin IX was oxidized at the highest rate, followed by deuterohemin IX and 2,4-diacetyldeuterohemin. When the three hemins were preincubated with microsomal heme oxygenase in the absence of NADPH, and the latter was added after the preincubation period, it was found that the enzymatic oxidation of the hemins was inhibited. Therefore, for the maximal rate of oxidation both hemin and NADPH must be present simultaneously. In the presence of hemin IX (the natural substrate), the enzymatic oxidation of the synthetic hemins was inhibited. The oxidation of 2,4-diacetyldeuterohemin IX was the most inhibited, while the oxidation of hematohemin IX was affected to a much lesser degree. These results are in agreement with the higher affinity (Km=150 μM) of hematohemin IX for the enzyme, as compared to 2,4-diacetyldeuterohemin IX (Km=660 μM) and deuterohemin IX (ifKm=330 μM)

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