IFCC METHODS FOR THE MEASUREMENT OF CATALYTIC CONCENTRATION OF ENZYMES .8. IFCC METHOD FOR LACTATE-DEHYDROGENASE (L-LACTATE-NAD(+) OXIDOREDUCTASE, EC-1.1.1.27)

Human lactate dehydrogenase is a tetramer made up of two types of subunits, either H (heart) or M (muscle). Combination of these subunits gives rise to the five isoenzymes of lactate dehydrogenase which are found in mammalian tissues. The relative proportions of the individual isoenzymes found in serum of patients is related to the severity of the lesion in the ol-gan ol tissue from which they originate and the half-life of the individual tissue-specific enzymes. Thus, one cannot predict the relative proportions of the different isoenzymes in any one patient sample. Lactate dehydrogenase catalyses the reversible oxidation of lactate to pyruvate and either reaction call be measured readily. However, in this method, the lactate to pyruvate reaction has been selected because of the following reasons; the time-course of the reaction is more linear, the reaction results in an increase in absorbance and optimisation of substrates is possible (see appendix A). The principles applied in the selection of the conditions of measurement are those slated In previous publications by the IFCC's Committee on Enzymes [1]. Human serum and tissue extracts have been used as the soul-ces of enzymes. The final concentration of substrates and the pH have been selected on the basis of experiments and empirical optimization techniques and have been confirmed by calculation from rate equations. The catalytic and physical properties of the isoenzymes differ, but because of the importance of the heart specific isoenzyme (LDI) in the assessment of coronary heart disease and as a tumour marker, this method has been optimized for this isoenzyme. However, the method is also suitable, although less optimally, for the determination isoenzymes of lactate dehydrogenase which may be present in serum.