<prompt> The predominant isoenzyme of LDH in cardiac muscle isLDH 1LDH 2LDH 3LDH 5  <response>: 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: HHH I1, HHHM I2, HHMM I3, HMMM I4, and MMM 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 While historicallyof impoance, the assay of LDH has been superseded as a marker for MI by proteins that appear in plasma more rapidly than LDH.Ref: Harper;s Biochemistry; 30th edition; Chapter 7; Enzymes: Mechanism of Action 