Other enzymatic cardiac markers (Myoglobin and LDH)

MYOGLOBIN:

It is a oxygen binding and storing protein. Its low molecular weight and cytoplasmic location accounts for its early release following muscle (heart or skeletal) injury. It cannot be used in differential diagnosis of muscle injury or cardiac injury.

Measured in serum by RIA, latex agglutination, and immunoassay based on monoclonal antibodies

LACTATE DEHYDROGENASE (EC 1.1.1.27) ISOENZYMES:

It is a cytoplasmic enzyme. This enzyme has mol weight of 134,000 and has four peptides chain of two types: M for muscle (or A) and H for heart (or B) each under separate genetic control as like M and B of CK. LD is inhibited by both pyruvate and lactate in excess although the effect of pyruvate is greater. Oxidation of –SH in enzyme and its inactivation is prevented by cysteine or glutathione.

Highest activities are found in skeletal muscle, liver, heart, kidney, and red blood cells. 

Isoenzymes                 % of LDH activity (Source)

q  LDH-1 (H₄)                 Mostly heart 60%, RBC 40%, kidney 28%

q  LDH-2 (H₃M)              Mostly kidney, myocardium, RBC, 30-34%

q  LDH-3 (H₂M₂)            Mostly spleen, lung, kidney, RBC

q  LDH-4 (HM₃)              Mostly Spleen, Lung, RBC, Kidney

 q  LDH-5 (M₄)                   Mostly liver 94%, skeletal muscle 76% 

A different sixth LD isoenzyme LD-X (also called LD_c) composed of four X (or C) subunits is present in post pubertal human testes. A seventh LD, called LD-6 has been identified in the sera of severely ill patients.

LDH catalyzes the reversible conversion of pyruvate to lactate. 

(Source: Tietz Clinical Chemistry, 4th Edition)

Analytical Measurement:

    

Enzymatic measurement: 

(Source: Tietz Clinical Chemistry, 4th Edition)

 

 

The decrease in absorbance at 340 nm followed. At pH 8.8 to 9.8 the reversible reaction is favored.

Reference range : 125-220 IU/L 

Isoenzyme analysis –

It can be done by electrophoresis or immunoinhibition assay where antibodies inhibit other isoenzymes except LD1. Formation of NBT formazan is the detection technique after electrophoretic separation. 

(Source: Harper's Illustrated biochemistry, 28th Edition)