The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase is a key process in the degradation of heme in the body. This study demonstrates that the conversion of heme to bilirubin occurs primarily in microsomes, requiring NADPH and molecular oxygen, and is inhibited by carbon monoxide. The process involves the oxidative cleavage of ferriprotoporphyrin IX to form bilirubin, a reaction that is catalyzed by an enzymatic mechanism. The study also shows that the activity of this enzyme is highest in the spleen, although the liver is the main organ responsible for heme turnover in the intact animal. The enzyme system is capable of converting heme to bilirubin, and the reaction is consistent with the concept of a mixed-function oxidation. The findings suggest that microsomal heme oxygenase plays a major role in the degradation of heme in the body, contributing to the formation of bilirubin. The study also highlights the importance of microsomal fractions in the conversion of heme to bilirubin, and the role of various subcellular fractions in this process. The results indicate that the enzyme system is essential for the physiological degradation of heme and the formation of bilirubin in the body.The enzymatic conversion of heme to bilirubin by microsomal heme oxygenase is a key process in the degradation of heme in the body. This study demonstrates that the conversion of heme to bilirubin occurs primarily in microsomes, requiring NADPH and molecular oxygen, and is inhibited by carbon monoxide. The process involves the oxidative cleavage of ferriprotoporphyrin IX to form bilirubin, a reaction that is catalyzed by an enzymatic mechanism. The study also shows that the activity of this enzyme is highest in the spleen, although the liver is the main organ responsible for heme turnover in the intact animal. The enzyme system is capable of converting heme to bilirubin, and the reaction is consistent with the concept of a mixed-function oxidation. The findings suggest that microsomal heme oxygenase plays a major role in the degradation of heme in the body, contributing to the formation of bilirubin. The study also highlights the importance of microsomal fractions in the conversion of heme to bilirubin, and the role of various subcellular fractions in this process. The results indicate that the enzyme system is essential for the physiological degradation of heme and the formation of bilirubin in the body.