Two MethodsComparedfor MeasuringLD-liTotaJLD Activityin Serum

We presentevidencefor the utilityof an improvedassay for the activity of lactate dehydrogenase (EC 1.1.1.27) isoenzymes1 and 2 in serum, involvinginhibitionof the H-subunit of LD by pyruvate at pH 7.1. Resultscorrelatewell with the LD-1/total LD ratio as evaluated by immunologicalassay and, as an indexto infarct, the method is superiorto either the change in CK-MB activityor to the LD-1 activity or to a combinationof these tests, as is the percentage of LD-1 to total LD activity. Moreover, the percentage inhibition ofLD activity by pyruvate may have an advantageoverother methods of isoenzyme fractionation because of its smaller populationCV for patientswith acute myocardialinfarction than is true of other methods. We also demonstrate how, usinga lineardiscriminantanalysis,we comparedthismethod with alternativemethods.We determinedthat evaluation ofCK-MB isoenzymecontributesno informationinadditionto that obtainedfrom the LD-1 isoenzyme.