Heme degradation is the body’s method of dismantling the iron-containing molecule at the core of hemoglobin, and it runs almost entirely on autopilot, processing the contents of roughly two million aging red blood cells every second. The pathway converts heme through a chain of enzymatic reactions into bilirubin, the yellow pigment responsible for the color of bruises and jaundice, while salvaging iron for reuse and releasing carbon monoxide as a signaling molecule. Each step hands off its product to the next enzyme or transport system, moving from macrophages in the spleen and liver, through the bloodstream, into liver cells, down through bile, and ultimately out through the gut.
Red Blood Cell Recycling Starts the Process
A red blood cell circulates for about 120 days before it becomes too damaged or rigid to squeeze through the narrowest capillaries. When a red blood cell reaches the end of its life, changes on its surface, including the flipping of certain membrane molecules to the outer leaflet, flag it for removal.1PubMed. A physiological model to study iron recycling in macrophages Specialized immune cells called macrophages, concentrated in the spleen’s red pulp and among the liver’s Kupffer cells, recognize these signals and swallow the aged cell whole in a process called erythrophagocytosis.2PubMed Central. The Multiple Facets of Iron Recycling
The scale of this recycling operation is staggering. Each red blood cell contains around 270 billion hemoglobin molecules, and the body clears roughly two million senescent red blood cells per second, making erythrophagocytosis the dominant source of iron turnover in the body.3PubMed Central. Macrophages and iron trafficking at the birth and death of red cells Once inside the macrophage, hemoglobin is broken apart. The protein chains (globin) get digested into amino acids. The heme groups, the flat ring-shaped molecules that actually hold iron, are handed off to the enzyme that performs the pathway’s signature reaction.
Heme Oxygenase Cracks the Ring Open
The enzyme heme oxygenase sits in the membrane of the endoplasmic reticulum inside the macrophage and does one thing extremely well: it cleaves the heme ring. Using oxygen and electrons, it splits one of the four bridge carbons in the porphyrin ring, and in doing so releases three products at once: biliverdin (a green pigment), free iron, and a single molecule of carbon monoxide.4PubMed Central. Heme oxygenase-1/carbon monoxide: from metabolism to molecular therapy This reaction is the only known biological source of carbon monoxide in the human body.5PubMed. The heme oxygenase-carbon monoxide system: regulation and role in stress response and organ failure
Two forms of the enzyme exist. HO-1 is the inducible version, ramped up by stress signals like oxidative damage, heat shock, and inflammation. HO-2 is the constitutive version, always active at a steady level, particularly abundant in the brain and testes. Both catalyze the same ring-opening reaction. The inducible form matters most in clinical contexts because its expression surges during tissue injury, acting as part of the body’s stress-response toolkit.6PubMed Central. The Heme Oxygenase/Biliverdin Reductase System and Its Genetic Variants in Physiology and Diseases
What Carbon Monoxide Does After Release
The idea that the body deliberately produces carbon monoxide surprises most people, given CO’s reputation as a toxic gas. But the tiny amounts generated by heme oxygenase act as a signaling molecule rather than a poison. CO relaxes blood vessel walls, influences smooth muscle growth, and discourages platelets from clumping together. These vascular effects work through a mechanism shared with nitric oxide: CO binds to the iron center of an enzyme called guanylate cyclase, which then produces a second messenger that tells smooth muscle cells to relax.7PubMed Central. Heme oxygenase/carbon monoxide signaling pathways: regulation and functional significance
Beyond vascular regulation, CO has anti-inflammatory and cell-protective effects that appear to work through a separate set of signaling cascades involving stress-responsive kinases. Animal studies have shown that CO confers protection in models of oxidative lung injury, which is part of why HO-1 induction during inflammation is considered a defense mechanism rather than mere waste disposal.7PubMed Central. Heme oxygenase/carbon monoxide signaling pathways: regulation and functional significance
Iron Gets Recycled, Not Wasted
The iron freed by heme oxygenase faces two possible fates inside the macrophage. Some of it is loaded into ferritin, the cell’s iron-storage protein, where it sits until needed later. The rest is exported back into the bloodstream through ferroportin, a channel in the macrophage’s membrane that is the only known iron exporter in mammals.8PubMed. Erythrophagocytosis and recycling of heme iron in normal and pathological conditions; regulation by hepcidin Once outside the cell, iron is picked up by transferrin, the blood’s iron-carrying protein, and delivered to the bone marrow, where it gets loaded into new hemoglobin molecules for freshly minted red blood cells.
The hormone hepcidin acts as a master regulator of this export step. When hepcidin levels rise, as they do during infection or iron overload, it binds to ferroportin and triggers its destruction, trapping iron inside the macrophage. Diseases that disrupt ferroportin function, such as type 4 hemochromatosis, illustrate how essential this export channel is for keeping iron in circulation.8PubMed. Erythrophagocytosis and recycling of heme iron in normal and pathological conditions; regulation by hepcidin
Biliverdin Becomes Bilirubin
Biliverdin, the green pigment left after the ring opens, is rapidly converted to bilirubin by an enzyme called biliverdin reductase.9PubMed Central. Biliverdin reductase: a major physiologic cytoprotectant This reaction adds hydrogen atoms across one of biliverdin’s double bonds, shifting its color from green to yellow-orange. The color change is visible in a healing bruise: the initial dark red of trapped blood shifts to green (biliverdin), then to yellow (bilirubin) over days as macrophages in the tissue work through the pathway locally.
Bilirubin has long been called a physiological antioxidant, and at low concentrations it does scavenge reactive oxygen species. Biliverdin reductase itself has been described as a major cytoprotectant.10PubMed Central. Biliverdin reductase: new features of an old enzyme and its potential therapeutic significance An influential early hypothesis proposed that bilirubin and biliverdin cycle back and forth in a redox amplification loop, with tiny amounts of bilirubin providing outsized antioxidant protection. Subsequent work has cast doubt on the magnitude of that effect: experiments showed that the oxidation of bilirubin in cell membranes does not reliably regenerate biliverdin, and knocking down biliverdin reductase in cells did not make them more vulnerable to oxidative damage.11PubMed Central. Limited role for the bilirubin-biliverdin redox amplification cycle in the cellular antioxidant protection by biliverdin reductase The antioxidant capacity of bilirubin is real but probably more modest than early enthusiasm suggested.
Albumin Ferries Bilirubin to the Liver
Unconjugated bilirubin is barely soluble in water, so it cannot float freely through the bloodstream. Instead, it hitches a ride on albumin, the most abundant protein in blood plasma. The binding is remarkably tight: less than 0.01% of total bilirubin circulates in its unbound form under normal conditions.12PubMed Central. Binding and transport functions of human serum albumin and its clinical implications in liver disease – Section: HSA binding with bilirubin This binding matters for safety. Unbound bilirubin is lipophilic enough to cross cell membranes, including the blood-brain barrier, which is why dangerously high levels of unconjugated bilirubin can injure brain tissue. Albumin binding prevents that by keeping bilirubin sequestered and shuttling it toward the liver for processing.13PubMed Central. Albumin binding of unconjugated [3H]bilirubin and its uptake by rat liver basolateral plasma membrane vesicles
Liver Uptake and Conjugation
When the albumin-bilirubin complex reaches the liver’s sinusoidal blood vessels, bilirubin must cross the hepatocyte membrane. This is not a passive process. Liver cells use a transporter called OATP1B1 (also known as OATP2) to pull bilirubin inside. OATP1B1 is liver-specific and has higher affinity for bilirubin than other members of the same transporter family.14PubMed. Inhibition of human organic anion transporting polypeptide OATP 1B1 as a mechanism of drug-induced hyperbilirubinemia Studies in transfected cells confirmed that OATP1B1 mediates high-affinity bilirubin uptake even when bilirubin is bound to albumin, consistent with its role as the long-sought bilirubin uptake transporter on the hepatocyte surface.15Journal of Biological Chemistry. Hepatic Uptake of Bilirubin and Its Conjugates by the Human Organic Anion Transporter SLC21A6
Once inside the hepatocyte, bilirubin is conjugated, meaning sugar molecules (glucuronic acid) are attached to it. The enzyme responsible is UGT1A1, which tags bilirubin with one or two glucuronic acid groups, producing mono- and diglucuronides.16PubMed Central. Bilirubin glucuronidation revisited: proper assay conditions to estimate enzyme kinetics with recombinant UGT1A1 Conjugation transforms bilirubin from a lipid-soluble molecule that would accumulate in cell membranes into a water-soluble one that can be excreted. The conjugated bilirubin is then actively pumped into the bile canaliculi, the tiny channels that drain into the bile duct system and ultimately into the intestine.
Gut Bacteria Finish the Job
Conjugated bilirubin arrives in the intestine dissolved in bile. Bacterial enzymes in the gut first strip off the glucuronic acid groups, regenerating unconjugated bilirubin. From there, the bilirubin can go one of two directions. Some of it is reabsorbed through the intestinal wall and sent back to the liver via the portal vein, a recycling loop called the enterohepatic circulation. The rest is further reduced by gut microbes into a group of colorless compounds called urobilinoids.17PubMed Central. Bilirubin in the Liver-Gut Signaling Axis
The initial reduction product is urobilinogen. A specific gut microbial enzyme, dubbed bilirubin reductase (BilR), was recently identified as the catalyst for this conversion, found in several common intestinal species.18PubMed Central. Discovery of the gut microbial enzyme responsible for bilirubin reduction to urobilinogen Most urobilinogen is further oxidized in the colon to stercobilin, the brown pigment that gives feces its characteristic color. A small fraction of urobilinogen is absorbed from the gut into the bloodstream, filtered by the kidneys, and excreted in urine, where spontaneous oxidation converts it to urobilin, the pigment responsible for urine’s yellow tint.
When the Pathway Backs Up
Jaundice, the yellowing of skin and eyes, is the visible sign that bilirubin is accumulating faster than the body can clear it. The causes map neatly onto the pathway’s steps. Pre-hepatic jaundice happens when red blood cells are destroyed too quickly, as in hemolytic anemias, flooding the system with more heme than the liver can conjugate. Hepatic jaundice occurs when liver cells themselves are damaged or genetically impaired, slowing conjugation or uptake. Post-hepatic (obstructive) jaundice results from a physical blockage of bile flow, such as a gallstone lodged in the bile duct, which traps conjugated bilirubin and forces it back into the blood.
Genetic disorders that target specific enzymes in the pathway illustrate how precisely each step matters. Gilbert syndrome, which affects somewhere around 5 to 10 percent of the population, involves reduced UGT1A1 activity. People with Gilbert syndrome have mildly elevated unconjugated bilirubin, which fluctuates with fasting, stress, or illness, but the condition is generally harmless. On the severe end of the same spectrum sits Crigler-Najjar syndrome, where UGT1A1 is absent or nearly so. Without the ability to conjugate bilirubin at all, affected infants develop dangerously high unconjugated bilirubin levels from birth.19PubMed. Hyperbilirubinemia syndromes (Gilbert-Meulengracht, Crigler-Najjar, Dubin-Johnson, and Rotor syndrome)
Drug interactions can also jam the pathway. Several medications inhibit OATP1B1, the liver’s bilirubin uptake transporter, effectively slowing bilirubin clearance and raising blood levels.14PubMed. Inhibition of human organic anion transporting polypeptide OATP 1B1 as a mechanism of drug-induced hyperbilirubinemia This is one of the mechanisms behind drug-induced jaundice, where bilirubin rises not because the liver is injured but because its uptake machinery is temporarily blocked.
Bilirubin Toxicity in Newborns
Newborns are uniquely vulnerable to bilirubin accumulation. Their livers are immature, with UGT1A1 activity that takes days to weeks to ramp up fully. Their gut flora is sparse, meaning less intestinal conversion to urobilinoids and more reabsorption of unconjugated bilirubin back into the blood. The result is that most healthy newborns develop visible jaundice in the first few days of life, which typically resolves on its own.
When unconjugated bilirubin climbs too high, however, the albumin-binding capacity of the blood is overwhelmed, and free bilirubin crosses the blood-brain barrier. The resulting damage, called kernicterus, preferentially strikes specific brain regions: the basal ganglia, the cochlear nuclei involved in hearing, and the oculomotor nuclei that control eye movement.20PubMed. Kernicterus and the molecular mechanisms of bilirubin-induced CNS injury in newborns The injury reflects bilirubin’s affinity for neuronal membranes and its ability to disrupt mitochondrial function within those cells. Modern MRI can reveal the characteristic bilateral lesions in the globus pallidus that mark bilirubin-induced brain injury.21PubMed. Bilirubin toxicity in the developing nervous system
The standard treatment is phototherapy, in which the infant is placed under blue-spectrum lights. Phototherapy does not destroy bilirubin. Instead, it reshapes it. The light energy converts the tightly folded bilirubin molecule into structural isomers that are more water-soluble, allowing the body to excrete them without the conjugation step that the immature liver cannot yet perform efficiently.22PubMed Central. Bilirubin Photoisomers in Neonatal Jaundice In severe cases, exchange transfusion physically removes bilirubin-laden blood and replaces it with donor blood, buying time for the liver to catch up.
Heme Degradation Beyond Red Blood Cells
Red blood cell recycling accounts for the bulk of heme turnover, but heme is also a working component in many other proteins throughout the body. Enzymes involved in detoxifying drugs in the liver, proteins in the mitochondrial respiratory chain, and catalase (which breaks down hydrogen peroxide) all use heme as a functional group. When these proteins are retired, their heme is degraded by the same heme oxygenase system. HO-1 induction in non-blood-cell contexts, such as in inflamed lung tissue, is considered cytoprotective, both through removal of free heme (which is toxic on its own) and through the anti-inflammatory effects of the CO and bile pigments generated as byproducts.23PubMed Central. Significance of Heme and Heme Degradation in the Pathogenesis of Acute Lung and Inflammatory Disorders
How Bacteria Break Down Heme Differently
Humans are not the only organisms that degrade heme, and not every species does it the same way. At least three distinct families of heme-degrading enzymes are now recognized across life, differing in their structures, their use of oxygen, and the end products they generate.24PubMed Central. Making and breaking heme The mammalian heme oxygenases represent the canonical pathway, always producing biliverdin as the tetrapyrrole product. But pathogenic bacteria such as Staphylococcus aureus and Mycobacterium tuberculosis use structurally distinct enzymes with a different protein fold that distorts the heme ring before cleaving it, yielding degradation products other than biliverdin.25PubMed Central. Heme utilization by pathogenic bacteria: not all pathways lead to biliverdin
For these bacteria, heme degradation is primarily about scavenging iron from the host rather than detoxifying a waste product. Iron is scarce in infected tissues because the host actively sequesters it as a defense strategy, so cracking open stolen heme becomes a survival tactic. The structural differences between bacterial and mammalian heme-degrading enzymes have attracted interest as potential drug targets: an antibiotic that blocks the bacterial enzyme without affecting human heme oxygenase could starve pathogens of iron without disrupting the host’s own heme recycling.