Probing AGXT2 enzyme activity in mouse tissue by applying stable isotope‐labeled asymmetric dimethyl arginine as substrate

Asymmetric dimethylarginine (ADMA) is a metabolite of the amino acid l ‐arginine. It competitively inhibits the enzymatic production of the cell‐signaling substance nitric oxide. Therefore, increased levels of ADMA are associated with a range of cardiovascular and other diseases. ADMA is biologically eliminated by direct renal excretion and hydrolysis by the enzyme DDAH. Recently, a further elimination pathway via the transamination by the enzyme AGXT2 to α‐keto‐δ‐(N^G,N^G‐dimethylguanidino)valeric acid (DMGV) has come into the focus of biological research. In this work, we describe an assay for the AGXT2 activity in mouse liver and kidney tissue. It is based on the transformation of isotope‐labeled ADMA‐d₆ to DMGV‐d₆. The quantification of the DMGV‐d₆ produced by this reaction in tissue homogenate samples was accomplished by chromatographic separation on a porous graphitic carbon column and tandem mass spectrometric detection. DMGV‐d₆ with the deuterium labels in different molecular positions was used as internal standard. The overall production rates of DMGV‐d₆ in mice were 195.37 pmol/min/mg total protein in liver and 85.21 pmol/min/mg total protein in kidney tissue, with coefficients of variation of 6.31% and 11.25%, respectively. This method can be applied as a tool for the characterization of the ADMA elimination by the AGXT2 pathway. Copyright © 2012 John Wiley & Sons, Ltd.