The activity of lactate dehydrogenase can be measured by monitoring the following reaction:
Pyruvate + NADH → Lactate + NAD+
The molar extinction coefficient of NADH at 340 nm is 6220 M^–1.cm^–1.
NAD+ does not absorb at this wavelength. In an assay, 25 µL of a sample of enzyme (containing 5 µg protein per mL) was added to a mixture of pyruvate and NADH to give a total volume of 3 mL in a cuvette of 1 cm pathlength. The rate of decrease in absorbance at 340 nm was 0.14 min^–1. The specific activity of the enzyme will be _________ µmol.min^–1.mg^–1.