Metabolism of methylarginines by human vasculature; implications for the regulation of nitric oxide synthesis

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Abstract
The metabolism of methylarginines by human cultured endothelial cells and human saphenous vein was studied in vitro. The human endothelial cell line (SGHEC‐7), primary cultures of human umbilical vein endothelial cells (HUVEC) and human saphenous vein were incubated with [14C]‐monomethyl‐l‐arginine ([14C]‐l‐NMMA) and the cytosolic extract analysed by high performance liquid chromatogaphy (h.p.l.c.) with on‐line radioisotope detection. SGHEC‐7, HUVEC and human saphenous vein metabolized [14C]‐l‐NMMA to a compound which co‐eluted with [14C]‐citrulline. A second metabolite which co‐eluted with [14C]‐arginine was evident on the radiochromatograms of HUVEC cytosol and saphenous vein extracts. The intracellular levels of [14C]‐l‐NMMA and [14C]‐citrulline in SGHEC‐7 cells incubated with [14C]‐l‐NMMA (0.5 μCi ml−1: 8.9 μm) for 1 h were 113 ± 22 and 67.6 ± 6.2 pmol mg−1 cell protein respectively (n = 7). Co‐incubation with NGNGdimethyl‐l‐arginine (ADMA; 100 μm) but not NGN′Gdimethyl‐l‐arginine (SDMA; 100 μm) reduced the intracellular level of [14C]‐citrulline to 26.3 ± 3.7 pmol mg−1 cell protein (P < 0.01; n = 3) without reducing the intracellular level of [14C]‐l‐NMMA. The intracellular levels of [14C]‐citrulline in SGHEC‐7 cells incubated with [14C]‐l‐NMMA for 1 h were reduced following co‐incubation with NGnitro‐l‐arginine methylester (l‐NAME; 1 mm), NGnitro‐l‐arginine (l‐NOARG; 1 mm) and l‐canavanine (1 mm) to 47.1 ± 6.2, 24.7 ± 3.6 and 12.5 ± 2.8% of control levels (P < 0.001; n = 9). ADMA (1 mm; n = 3) reduced intracellular [14C]‐citrulline levels to 4 ± 4% of control (P < 0.01) but SDMA (1 mm; n = 3) had no effect. The accumulation of endogenously synthesized ADMA in the culture supernatant of SGHEC‐7 cells was increased by co‐incubation with l‐NMMA (1 mm) from 1.98 ± 0.08 to 2.74 ± 0.36 nmol mg−1 cell protein, an increase of 40%. These results demonstrate that human vasculature possesses an enzyme which has similar properties to dimethylarginase; human endothelial cells and human saphenous vein metabolize l‐NMMA to citrulline via a process inhibited by ADMA but not SDMA. The increase in endothelium‐derived ADMA following co‐incubation with l‐NMMA is consistent with competition between ADMA and l‐NMMA for dimethylarginase. Inhibition of this enzyme might increase the intracellular concentration of ADMA, an endogenously produced compound that inhibits nitric oxide synthesis.

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