The predominant isoenzyme of LDH in cardiac muscle is

Lactate dehydrogenase (LDH) is a tetrameric enzyme consisting of two monomer types: H (for the hea) and M (for muscle) that combine to yield five LDH isozymes: HHHH (I1), HHHM (I2), HHMM (I3), HMMM (I4), and MMMM (I5). The relative propoions of each subunit in the cells of a paicular organ are determined by tissue-specific patterns in the expression of the H and M genes. Isozyme I1 predominates in hea tissue and isozyme I5 in the liver. Thus, when LDH levels rise in blood plasma, the identity of the injured tissue can be inferred from its characteristic pattern of LDH isozymes. In the clinical laboratory, individual isozymes can be separated by electrophoresis and detected using a coupled assay (Figure 7-12). While historicallyof impoance, the assay of LDH has been superseded as a marker for MI by proteins that appear in plasma more rapidly than LDH.Ref: Harper&;s Biochemistry; 30th edition; Chapter 7; Enzymes: Mechanism of Action