Old RBCs are destroyed in "tissue macrophage system." In the breakdown of hemoglobin, bilirubin is formed from

Options

A. Globin part
B. Porphyrin
C. Mainly from globin and a part from heme
D. Iron part

Answer

To solve the question regarding the formation of bilirubin from hemoglobin during the breakdown of old red blood cells (RBCs), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Breakdown of RBCs**:
- Old RBCs are destroyed primarily by macrophages in the tissue macrophage system, particularly in the liver (via Kupffer cells), spleen, and bone marrow.

2. **Release of Hemoglobin**:
- When RBCs are phagocytized, hemoglobin is released. Hemoglobin consists of two main components: the globin (protein part) and heme (iron-containing part).

3. **Iron Recycling**:
- The iron from the heme portion of hemoglobin is released by macrophages and is either transported back into the bloodstream or stored in the liver as ferritin for future use in the production of new RBCs.

4. **Conversion of Heme to Bilirubin**:
- The heme portion undergoes a series of enzymatic reactions facilitated by macrophages. The heme is broken down into a series of intermediates, ultimately leading to the formation of bilirubin.

5. **Formation of Bilirubin**:
- The specific part of hemoglobin that is converted into bilirubin is the **porphyrin** portion of the heme group. This conversion occurs through a series of steps involving the breakdown of the porphyrin ring.

6. **Bilirubin Transport**:
- Once formed, bilirubin is released into the bloodstream, where it is transported to the liver. From the liver, it is secreted into bile and eventually excreted from the body.

### Conclusion:
The correct answer to the question is that bilirubin is formed from the **porphyrin** part of hemoglobin.

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