In normal serum, which LD isoenzyme predominates?

• A. LD2
• B. LD1
• C. LD3
• D. LD4

Answer: (A)

Lactate dehydrogenase (LDH) in serum exists as five isoenzymes formed from H and M subunits, and their relative amounts reflect tissue sources and release into blood. In healthy individuals, the isoenzyme pattern is dominated by LD-2, the form with three heart-type subunits and one muscle-type subunit, making it the major circulating LDH. This predominance arises because the tissues contributing most LDH to serum—such as certain blood cells and lung/reticuloendothelial sources—predominantly release the LD-2 composition. In contrast, LD-1 (the H4 form) is less abundant in normal serum; it becomes relatively higher when pathological processes like myocardial infarction or hemolysis occur, leading to a characteristic shift where LD-1 rises compared with LD-2. Therefore, LD-2 is the best answer for normal serum.