Measurement and clinic applications of homocysteine and methylated arginines
Weaving, Gary Ronald
Homocysteine is an amino acid formed by the metabolism of methionine. Increased plasma homocysteine concentrations are associated with cardiovascular disease, and it has been suggested that homocysteine lowering therapy may reduce cardiovascular risk. Plasma homocysteine measurements are frequently requested by clinicians investigating patients with vascular disease. A mechanism for homocysteine causing vascular disease has not yet been proven, but one possibility is that an elevated plasma homocysteine concentration may lead to the accumulation of asymmetric dimethylarginine (ADMA), a naturally occurring amino acid that inhibits nitric oxide synthase, resulting in impaired nitric oxide production, and therefore vascular dysfuntion. The aim of this project was to develop analytical methods suitable for the measurement of homocysteine and related metabolites in a routine clinical laboratory, and two methods have been established; i) for homocysteine, cysteine and methionine and ii) for asymmetric dimethylarginine, symmetric dimethylarginine (SDMA, a sterioisomer of ADMA), monomethylarginine (MMA) and arginine. A novel feature of the method for ADMA is that the use of unique daughter ions allows the determination of both ADMA and SDMA without the need to separate the isomers chromatographically. In addition, the synthesis and application of isotopically labelled SDMA, for use as an internal standard, is described for the first time. When the methods were applied to the analysis of routine clinical samples no association was detected between plasma total homocysteine and plasma ADMA concentrations. Measurements were also performed on samples from patients enrolled in a clinical trial investigating the progression of vascular dysfuntion, as measured by carotid-femoral pulse wave velocity (CF-PWV), in chronic kidney disease. Again no association could be found between plasma total homocysteine and plasma ADMA concentrations. In addition plasma total homocysteine was not a determinant of CF-PWV. These findings do not support the hypothesis that hyperhomocysteinaemia causes vascular disease by increasing ADMA concentrations.