Quantification of three lidocaine metabolites and their conjugates

A method has been developed to quantify three lidocaine metabolites, N-ethylglycyl-2,6-xylidide (MEGX), glycyl-2,6-xylidide (GX), and 4-hydroxy-2,6-xylidine (4-OH-XY), and their conjugates in pooled human urine using enzymic hydrolysis. The commonly used enzymes, pure beta-glucuronidase, sulfatase, and a mixture of the two, were tested for their efficiencies in hydrolyzing the conjugates. Initially, it was found that 4-OH-XY was highly unstable after it was released from conjugates by beta-glucuronidase and the enzyme mixture. This problem was corrected by purging the sample with nitrogen prior to incubation. It has been determined that 4-OH-XY is present in human urine exclusively as its glucuronide. The percentage of MEGX in free and in conjugated forms (glucuronide, sulfate, and others) are 44.9 +/- 6.8, 16.6 +/- 4.5, 6.6 +/- 1.8, and 31.9 +/- 4.4, respectively. GX was present mostly in the free form (90.6 +/- 10.5%).