The predominant isozyme of LDH in cardiac muscle is:
**Core Concept**
The lactate dehydrogenase (LDH) enzyme is a tetrameric complex consisting of two types of subunits: H (heart) and M (muscle). The isozyme composition and distribution vary among different tissues, and this question focuses on the specific isozyme found in cardiac muscle.
**Why the Correct Answer is Right**
In cardiac muscle, the predominant isozyme of LDH is the H4 isozyme, which is composed of four H subunits. This isozyme has a higher affinity for NAD+, allowing it to efficiently catalyze the conversion of pyruvate to lactate during anaerobic glycolysis. The H4 isozyme is also more heat-stable than the M4 isozyme, making it a useful marker for cardiac damage.
**Why Each Wrong Option is Incorrect**
* **Option A:** The M4 isozyme is predominantly found in skeletal muscle, not cardiac muscle.
* **Option B:** The H2M2 isozyme is found in the liver and other tissues, but it is not the predominant isozyme in cardiac muscle.
* **Option D:** The LDH isozyme composition can vary among different tissues, but the H4 isozyme is specifically associated with cardiac muscle.
**Clinical Pearl / High-Yield Fact**
LDH isozyme analysis can be used to diagnose cardiac damage or myocardial infarction, as elevated levels of the H4 isozyme are often seen in these conditions. It's essential to note that LDH isozyme analysis has largely been replaced by more specific and sensitive cardiac markers, such as troponins.
**Correct Answer: C. H4**