The article by Tenhunen, Marver, and Schmid investigates the enzymatic conversion of heme to bilirubin in microsomes. They found that heme, when incubated with liver microsomes, spleen microsomes, or kidney microsomes, forms bilirubin in the presence of molecular oxygen and NADPH. The reaction is inhibited by carbon monoxide, suggesting it involves a mixed-function oxidation. The activity of the enzyme is highest in the spleen, which is consistent with the idea that red blood cell hemoglobin degradation primarily occurs in the spleen, with the liver playing a secondary role. The study provides evidence for a previously undescribed enzyme system in microsomes capable of degrading heme to bilirubin, which may play a major role in heme turnover in the intact animal.The article by Tenhunen, Marver, and Schmid investigates the enzymatic conversion of heme to bilirubin in microsomes. They found that heme, when incubated with liver microsomes, spleen microsomes, or kidney microsomes, forms bilirubin in the presence of molecular oxygen and NADPH. The reaction is inhibited by carbon monoxide, suggesting it involves a mixed-function oxidation. The activity of the enzyme is highest in the spleen, which is consistent with the idea that red blood cell hemoglobin degradation primarily occurs in the spleen, with the liver playing a secondary role. The study provides evidence for a previously undescribed enzyme system in microsomes capable of degrading heme to bilirubin, which may play a major role in heme turnover in the intact animal.