Clinical Applications of Enzymes Clinical examples and case studies.

Hepatitis A 36-year old man was admitted to a hospital following episodes of nausea, vomiting, and general malaise. His urine was darker than usual. Upon examination it was discovered that his liver was enlarged and tender to palpation. Liver function tests were abnormal; plasma ALT was 1500 IU/L (Alanine aminotransferase 6.0 – 21 U/L); AST was 400 IU/L (Aspartate aminotransferase 7.0 – 20 U/L). During the next 24 hours the man developed jaundice, and his plasma total bilirubin was 9.0 mg/dL (0.2 – 1 mg/dL). A diagnosis of hepatitis was made.

Biochemical Questions: 1. What reactions are catalyzed by AST and ALT? What is the coenzyme? 2. What conditions are important to maintain in performing the enzyme assays? 3. Which other enzymes might have been elevated in the plasma? 4. How does “total” bilirubin relate to “direct” and “indirect” bilirubin?

Case discussion: Hepatitis is an inflammation of the liver. 1. Transaminases (amino acids metabolism) 1. Catalyze the transfer of α-amino groups from amino acid to a α-keto acid through the intermediary coenzyme pyridoxal phosphate (derived from the B 6 vitamin, pyridoxine) 2. Amino acids enter into the Krebs cycle for oxidation to CO 2 and H 2 O 3. Amino acid 1 + keto acid 2 ↔ amino acid 2 + keto acid 1 (pyridoxal phosphate ↔ pyridoxamine phosphate) slide 11 ALT and AST

2. Temperature and pH 1. Excessive shaking and elevated temp should be avoided 2. Anticoagulants shouldn't inhibit the assays 3. Hemolysis should be avoided in order not to release enzymes of the blood cells

3. Other enzymes that could be elevated: 1. A number of proteins may leak from cell into plasma, such as other transaminases and LD can be elevated in liver diseases 2. AST and ALT high levels occurs before jaundice is noted 3. There is poor correlation of enzyme activity with severity of the disease

4. Bilirubin 1. Biliribin is derived from the breaking down of the hemoglobin of aging red blood cells 2. Bilirubin is insoluble in water; to be excreted it is converted to a water-soluble bilirubin diglucuronide in the liver 3. The first formed “indirect” bilirubin is bound to albumin and rapidly transported in plasma to the liver; albumin is not taken into the liver cells 4. “Direct” bilirubin (water soluble) is secreted into the bile canaliculus, together with other bile constituents and is collected in the gallbladder 5. Some of the bile pigments are excreted in the feces and some water soluble bilirubin (not free bilirubin) is excreted by the kidney into the urine

Bilirubin continue: In hepatitis, the formation of water soluble bilirubin is less efficient The secretion of “direct” bilirubin (water soluble bilirubin) into the bile canaliculi is impaired The result is a backing-up of the bilirubin excretion with a build-up of total bilirubin in the blood. The initial increase is expressed as a darkened urine, followed by jaundice