Overview Of Porphyrins
When acetate is associated with propionate, it is a uroporphyrin. When methyl (which is decarboxylated acetyl) is associated with propionate, it is a coproporphyrin.
- Symmetrical arrangement of substrates on the heme is called type I
- Asymmetrical arrangement on the heme is called type III
- Type III is more common in nature
- Excretion of Porphyrin is called "porphyrias"
Overview Of Heme Catabolism
Heme oxidase opens the methylene bridge in the form of CO to form biliverdin. Biliverdin reductase reduces the methylene bridge to methyl bridges to form bilirubin, which is yellow and insoluble. The bilirubin then forms bilirubin glucoronide, which is conjugated and soluble by glucoronic acid.
Clinical Importance
In the newborn, because of immature liver enzymes, bilirubin cannot conjugate with glucoronic acid. Bilirubin is not soluble in aqueous solution but is soluble in fat.
- If bilirubin concentration is less than 10mg/dL the baby is safe.
- If the bilirubin concentration is greater than 10mg/dL but less than 15mg/dL, the baby is exposed to phototherapy.
- If the bilirubin concentration is greater than 15mg/dL, the condition is critical, and the baby can develop kernicterus.
- Excess bilirubin in the blood greater than 20μg/L or 2mg/dL will result in the deposition of bilirubin in the tissues of the palm, sole of feet, sclera of eyes, etc., appearing 5 days after birth.
- Assay for total conjugated bilirubin concentration.
Jaundice
Jaundice occurs when the bilirubin concentration is greater than 20μg/L or 2mg/dL. Sufferers have yellow palms, soles of feet, and sclera of the eyes. Jaundice is classified into three types:
- Pre-hepatic jaundice
- Hepatic jaundice
- Post-hepatic jaundice
Pre-Hepatic Jaundice
Caused by anything that will cause hemolysis of blood, e.g., sickle cell anemia, acute malaria, glucose-6-phosphate dehydrogenase deficiency, acute bacterial infection with the release of toxins that lyse RBCs and cause severe hemolysis, antigen-antibody reactions (incompatibility of blood).
Hepatic Jaundice
Caused by any insult to the liver, e.g., viral hepatitis, alcohol hepatitis, heavy metal poisoning of the liver, carcinoma (cancer) of the liver, fatty liver, hepatitis B or C.
Post-Hepatic Jaundice
Saturated bile can crystallize to form gallstones that block the gallbladder. Also known as obstructive jaundice – could be complete or partial carcinoma of the head of the pancreas.
Differentiating Jaundice: Test The Urine
Haemolytic (Pre-Hepatic) Jaundice
- Urobilinogen will be very high
- Bilirubin will be absent
- Stercobilinogen will be very high
Hepatic Jaundice
- Urobilinogen present but not very high
- Bilirubin will be present
- Stercobilinogen will be decreased
Obstructive (Post-Hepatic) Jaundice
- Urobilinogen will be absent
- Bilirubin will be present
- Stercobilinogen may either be absent or found in trace amounts.
- Automatic oxidation converts stercobilinogen to stercobilin.
- In obstructive jaundice, the stool will be fatty and white since bilirubin is not getting to the GIT.
- Alkaline phosphate will be very high.
- Unconjugated bilirubin is more dangerous to the brain since it is fat-soluble, so it can cross the blood-brain barrier to be deposited in the basal ganglia to cause kernicterus.
- Conjugated bilirubin cannot cross the blood-brain barrier.
iZrEl profisy4CHRIST
…till I breathe my last
