Biological samples were prepared from fresh pig liver homogenized in potassium phosphate buffer (0.1 M) (1:5 v/w). The samples were centrifuged at 1000 × g for 10 min and the supernatants were collected. To simulate the conditions of a genuine enzyme kinetics study, potassium phosphate were added together with a volume of nicotineamide adenine dinucleotide phosphate/nicotineamide adenine dinucleotide (NADPH/NADH) mix to a final concentration of 0.1 M and 1 mM, respectively (concentration of the stock solutions were 2 M and 20 mM, respectively). The enzymatic reactions were terminated by adding an equal volume of acetonitrile (ACN) to the sample mixture, also resulting in partial protein precipitation. Samples were centrifuged and the supernatant was diluted 10 times with water before analysis. 10 μL of analytes and internal standards were added to 200 μL of the sample before the final dilution step to give concentrations according to what is stated in following sections.