N-15 enrichment of ammonium, glutamine-amide and urea, measured via mass isotopomer analysis of hexamethylenetetramine

Ammonium is an important intermediate of protein metabolism and is a key co mponent of acid-base balance, Investigations of the metabolism of NH4+ in v ivo using isotopic techniques are difficult because of the low concentratio n of NH4+ in biological fluids and because of frequent artifactual Isotopic dilution of the enrichment of NH4+ during the assay. A nem gas chromatogra phic mass spectrometric method was designed to monitor the N-15 enrichment and concentration of NH4+ in vivo. These are both calculated from the mass isotopomer distribution of hexamethylenetetramine (HMT) formed by reacting NH4+ with formaldehyde. The enrichment of NH4+ is amplified four times sinc e the HMT molecule contains four atoms of nitrogen derived from NH4+ This a llows the measurement of low N-15 enrichment of NH4+, down to 0.1%. N-15 en richment of urea and of the amide N of L-glutamine are measured by enzymati c release of NH4+ and conversion of the latter to HMT. These new techniques facilitate in vivo investigations of the metabolism of NH4+ and related co mpounds. Copyright (C) 1999 John Whey & Sons, Ltd.