L-arginine increases plasma homocysteine in apoE-/-/iNOS-/- double knockout mice.
Previous studies have shown that L-arginine (L-Arg) administration to apoE-/-/iNOS-/- double knockout mice (dKO) on a Western diet paradoxically results in an increase in atherosclerotic lesion size. We hypothesized that the potential beneficial effects of L-Arg could be offset, in part, by the byproducts of L-Arg catabolism, especially the atherogenic risk factor, homocysteine. In the kidney, L-Arg is converted to L-ornithine and guanidinoacetate (GAA) by L-arginine-glycine amidinotransferase. The efficient transmethylation of GAA by an S-adenosyl-methionine (SAM)-dependent methyltransferase in liver yields creatine and S-adenosylhomocysteine (SAH), which is readily hydrolyzed to homocysteine and adenosine. We, therefore, measured total plasma homocysteine in the dKO mice and control mice. We found that L-Arg supplementation caused a 37% increase in total plasma homocysteine (tHcy) levels in dKO mice compared to controls not treated with L-Arg (5.2+/-2.2 vs 3.8+/-1.5 microM Hcy, p<0.04). In a liver cell line, HepG2, addition of 10 and 50 microM GAA in the presence of 50 microM L-methionine (L-Met) increased tHcy production by approximately 1.47 (p<0.0001) and 2.3-fold (p<0.0001), respectively. In the presence of additional 100 microM L-Met, baseline homocysteine production was elevated by 20% (p<0.005), and 10 and 50 microM GAA augmented homocysteine production by an additional 1.88- (p<0.0001) and 3.4-fold (p<0.001), respectively, compared with 50 microM L-Met. These data suggest that increased concentrations of a methyl acceptor, such as L-Arg-derived GAA, drives SAM-dependent-methylation and consequent homocysteine formation. Furthermore, L-Met levels can also influence homocysteine production likely by regulating the synthesis of the methyl donor SAM. Epidemiological studies have suggested that homocysteine is a graded risk factor. In animal models, modestelevations of homocysteine can cause endothelial dysfunction and augment atherosclerosis. Our data suggest that L-arginine supplementation may contribute to vascular injury and atherogenesis under some circumstances by elevating homocysteine levels.
Handy
D E
DE
Whitaker Cardiovascular Institute and Evans Department of Medicine, Boston University School of Medicine, Boston, MA 02118, USA.
Scolaro
J
J
Chen
J
J
Huang
P
P
Loscalzo
J
J
D000818:Animals; D001057:Apolipoproteins E / Q000235:genetics*; D001120:Arginine / Q000097:blood*; D001161:Arteriosclerosis; D002460:Cell Line; D019587:Dietary Supplements; D004305:Dose-Response Relationship, Drug; D005998:Glycine / Q000031:analogs & derivatives* / Q000378:metabolism; D006710:Homocysteine / Q000097:blood* / Q000737:chemistry / Q000235:genetics; D008715:Methionine / Q000737:chemistry; D008745:Methylation; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D019001:Nitric Oxide Synthase / Q000235:genetics*; D052247:Nitric Oxide Synthase Type II; D009952:Ornithine / Q000378:metabolism; D012307:Risk Factors; D013997:Time Factors
D001057:Apolipoproteins E; D006710:Homocysteine; D001120:Arginine; D008715:Methionine; D009952:Ornithine; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496317:Nos2 protein, mouse; C004946:glycocyamine; D005998:Glycine