Most of the heme degraded in humans comes from hemoglobin. Approximately 6-8 grams of hemoglobin is degraded daily which is equivalent to approximately 300 milligrams of heme per day. Heme is not recycled so it must be degraded and excreted. The iron, however, is conserved. There are two steps to heme degradation;
1. cleavage of the heme ring by a microsomal heme oxygenase producing biliverdin
2. biliverdin is reduced to bilirubin.
Bilirubin can then be conjugated with glucuronic acid and excreted.