Microsomal Heme Oxygenase

TLDR

Microsomal heme oxygenase catalyzes the rate‑limiting oxidation of heme to biliverdin, which is subsequently reduced to bilirubin by soluble NADPH‑dependent biliverdin reductase. This study aims to characterize the previously undescribed microsomal heme oxygenase enzyme. The enzyme converts heme to bilirubin (predominantly the IXα isomer), is localized to the microsomal fraction, requires NADPH and O₂, produces equimolar CO, is inhibited by CO and various chemicals, shows mixed‑function oxygenase activity with a Km of ~5 µM for protohemin IX, and its kinetics and distribution underscore its major role in hemoprotein degradation to bile pigment.

Abstract

Abstract This study characterizes microsomal heme oxygenase, a previously undescribed enzyme which catalyzes the oxidation of heme at the α-methene bridge to form biliverdin. This step is then coupled with soluble NADPH-dependent biliverdin reductase to form bilirubin; microsomal heme oxygenase is rate-limiting in this pathway. By all analytical criteria, the product of this reaction is bilirubin. Most, if not all, of the bilirubin is of the IX α configuration, which is the sole isomeric form of bilirubin occurring physiologically. Heme oxygenase is localized specifically to the microsomal fraction, has an absolute and stoichiometric requirement for NADPH and molecular oxygen, generates carbon monoxide in amounts equimolar to bilirubin, and is inhibited by carbon monoxide. These and other data suggest that this enzyme is a mixed function oxygenase. The enzyme is most active with protohemin IX or methemalbumin; substrates with less activity are methemoglobin, the α and β chains of hemoglobin, deuterohemin IX, coprohemin I, and the hemoglobin-haptoglobin complex, in this order. Oxyhemoglobin, carboxyhemoglobin, myoglobin, and free porphyrins are not acted upon by the enzyme. The apparent Km for protohemin IX is 5.0 µm, and for the other substrates ranges from 4.5 to 5.1 µm. Sodium dodecyl sulfate, lipase, phospholipase, trypsin, potassium cyanide, sodium azide, and p-hydroxymercuribenzoate inhibit the enzyme. The kinetics and tissue distribution of this enzyme suggest that it is of major importance in the physiological degradation of hemoglobin and other hemoproteins to bile pigment.