Measurement of urinary total ¹³C and ¹³C urea by isotope ratio mass spectrometry after administration of lactose [¹³C]-Ureide

Abstract

A method of measuring total ¹³C excreted in urine after oral administration of lactose [¹³C]-ureide was developed using isotope ratio mass spectrometry. Furthermore, a method to measure ¹³C urea excreted in the urine was developed. Each urine sample collected over a 24 hour period, after administration of the tracer dose, was analysed for both total ¹³C and ¹³C urea. Combustion of the dried urine samples allowed measurement of the total ¹³C content. Treatment of urine samples with urease (EC 3.5.1.5) and analysis by isotope ratio mass spectrometry of the CO₂ evolved allowed measurement of ¹³C urea in the urine sample. The total ¹³C and ¹³C urea contents of each urine sample, obtained throughout the protocol, were compared to total ¹³C and ¹³C urea contents of a urine sample taken before the test. This allowed calculation of the fraction of tracer incorporated into urea and the fraction of tracer excreted in total. Analyses showed that approximately 15% of the dose administered, in terms of ¹³C, was recovered in the urine over the sampling period. Further analysis for urinary ¹³C urea showed that less than 1% of the label was incorporated into urea excreted over the sampling period.