Effect of lactate dehydrogenase isoenzymes on the coupled enzymatic assay for alanine aminotransferase activity.

We show an example of the importance of specifying the form of isoenzyme and source of indicator enzymes to be used in coupled enzymatic assays. When we compared H-4 (pig heart) and M-4 (rabbit muscle) isoenzymes of lactate dehydrogenase for their suitability as indicator enzymes in the assay for alanine aminotransferase activity, we found that about fourfold as much M-4 as H-4 was required in terms of lactate dehydrogenase activity to reflect accurately equivalent amounts of alanine aminotransferase activity. Moreover, the substrate specificities of the two isoenzymes differed quantitatively.