| Abstract: | Automated assays of lactate dehydrogenase (LD) in serum are based on measuring the rate of NADH produced in a reverse LD reaction using lactate and NAD. The observed nonlinearity of LD reaction used in earlier assays performed in phosphate buffers has generally been attributed to the formation of a ternary complex of NAD, pyruvate, and phosphate. This is not satisfactory to explain the course of assay reaction carried out in organic buffers. Investigation of the possible causes of nonlinearity during the course of the reverse LD reaction during LD assays performed in Tris or other organic buffers indicated that inhibition of LD activity by pyruvate may be chiefly responsible for the observed effects, especially in serum exhibiting abnormally high LD enzyme activity. Most of the LD activity in serum was inhibited by 5 mMoles/L pyruvate. By contrast, the LD isoenzyme activities were inhibited partially at 0.5 mMole/L pyruvate, LD1 being the most and LD4 the least susceptible. In assays of serum samples with abnormally high LD and PYR concentration using LD reagent containing Tris buffer, pH 9.3, the inclusion of a bacterial pyruvate oxidase (PO) enabled the removal of pyruvate accumulating in situ, making it possible to assay LD activity in the absence of inhibitory concentration of pyruvate. The inclusion of 10 U/L of PO in our routine LD reagent was sufficient to overcome pyruvate inhibition, thus permitting the assay of serum exhibiting high LD activity, hence the extension of the upper limits of linearity of LD assay without compromising assay performance. |
