Hydrogen sulfide, the third gaseous signaling molecule with cardiovascular properties, is decreased in hemodialysis patients.

Hydrogen sulfide, H(2)S, is the third endogenous gas with cardiovascular properties, after nitric oxide and carbon monoxide. H(2)S is a potent vasorelaxant, and its deficiency is implicated in the pathogenesis of hypertension and atherosclerosis. Cystathionine beta-synthase, cystathionine gamma-lyase, and 3-mercaptopyruvate sulfurtransferase catalyze H(2)S formation. Chronic kidney disease is characterized by high prevalence of hyperhomocysteinemia, hypertension, and high cardiovascular mortality, especially in hemodialysis patients. H(2)S levels are decreased in hemodialysis patients through transcriptional deregulation of genes encoding for the H(2)S-producing enzymes. Potential implications relate to the pathogenesis of the manifestations of the uremic syndrome, such as hypertension and atherosclerosis.

Hyperhomocysteinemia in uremia--a red flag in a disrupted circuit.

Hyperhomocysteinemia is an independent cardiovascular risk factor, according to most observational studies and to studies using the Mendelian randomization approach, utilizing the common polymorphism C677T of methylene tetrahydrofolate reductase. In contrast, the most recent secondary preventive intervention studies, in the general population and in chronic kidney disease (CKD) and uremia, which are all negative (with the possible notable exception of stroke), point to other directions. However, all trials use folic acid in various dosages as a means to reduce homocysteine levels, with the addition of vitamins B6 and B12. It is possible that folic acid has negative effects, which offset the benefits; alternatively, homocysteine could be an innocent by-stander, or a surrogate of the real culprit. The latter possibility leads us to the search for potential candidates. First, the accumulation of homocysteine in blood leads to an intracellular increase of S-adenosylhomocysteine (AdoHcy), a powerful competitive methyltransferase inhibitor, which by itself is considered a predictor of cardiovascular events. DNA methyltransferases are among the principal targets of hyperhomocysteinemia, as studies in several cell culture and animal models, as well as in humans, show. In CKD and in uremia, hyperhomocysteinemia and high intracellular AdoHcy are present and are associated with abnormal allelic expression of genes regulated through methylation, such as imprinted genes, and pseudoautosomal genes, thus pointing to epigenetic dysregulation. These alterations are susceptible to reversal upon homocysteine-lowering therapy obtained through folate administration. Second, it has to be kept in mind that homocysteine is mainly protein-bound, and its effects could be linked therefore to protein homocysteinylation. In this respect, increased protein homocysteinylation has been found in uremia, leading to alterations in protein function.

Hydrogen sulphide-generating pathways in haemodialysis patients: a study on relevant metabolites and transcriptional regulation of genes encoding for key enzymes.

BACKGROUND: Hydrogen sulphide, H(2)S, is the third endogenous gas with putative cardiovascular properties, after nitric oxide and carbon monoxide. H(2)S is a vasorelaxant, while H(2)S deficiency is implicated in the pathogenesis of hypertension and atherosclerosis. Cystathionine beta-synthase (CBS), cystathionine gamma-lyase (CSE) and 3-mercaptopyruvate sulphurtransferase (MPS) catalyze H(2)S formation, with different relative efficiencies. Chronic kidney disease (CKD) is characterized by elevation of both plasma homocysteine and cysteine, which are substrates of these enzymes, and by a high prevalence of hypertension and cardiovascular mortality, particularly in the haemodialysis stage. It is possible that the H(2)S-generating pathways are altered as well in this patient population. METHODS: Plasma H(2)S levels were measured with a common spectrophotometric method. This method detects various forms of H(2)S, protein-bound and non-protein-bound. Blood sulphaemoglobin, a marker of chronic exposure to H(2)S, was also measured, as well as related sulphur amino acids, vitamins and transcriptional levels of relevant genes, in haemodialysis patients and compared to healthy controls. RESULTS: Applying the above-mentioned methodology, H(2)S levels were found to be decreased in patients. Sulphaemoglobin levels were significantly lower as well. Plasma homocysteine and cysteine were significantly higher; vitamin B(6), a cofactor in H(2)S biosynthesis, was not different. H(2)S correlated negatively with cysteine levels. CSE expression was significantly downregulated in haemodialysis patients. CONCLUSIONS: Transcriptional deregulation of genes encoding for H(2)S-producing enzymes is present in uraemia. Although the specificity of the method employed for H(2)S detection is low, the finding that H(2)S is decreased is complemented by the lower sulphhaemoglobin levels. Potential implications of this study relate to the pathogenesis of the uraemic syndrome manifestations, such as hypertension and atherosclerosis.

Is homocysteine toxic in uremia?

High levels of homocysteine have been implicated as a cardiovascular risk factor in the general population and in patients with chronic renal failure, and particularly patients on hemodialysis. To classify a risk factor as causally related to a certain disease, both strong epidemiologic data and sound basic-science studies establishing a mechanism are needed. Among the latter, the hypomethylation of proteins and DNA, and protein homocysteinylation, have been investigated in uremia, providing for an array of toxic effects in this disease.

Plasma protein homocysteinylation in uremia.

Protein homocysteinylation is proposed as one of the mechanisms of homocysteine toxicity. It occurs through various means, such as the post-biosynthetic acylation of free amino groups (protein-N-homocysteinylation, mediated by homocysteine thiolactone) and the formation of a covalent -S-S- bond found primarily with cysteine residues (protein-S-homocysteinylation). Both protein modifications are a cause of protein functional derangements. Hemodialysis patients in the majority of cases are hyperhomocysteinemic, if not malnourished. Protein-N-homocysteinylation and protein-S-homocysteinylation are significantly increased in hemodialysis patients compared to controls. Oral folate treatment normalizes protein-N-homocysteinylation levels, while protein-S-homocysteinylation is significantly reduced. Albumin binding experiments after in vitro homocysteinylation show that homocysteinylated albumin is significantly altered at the diazepam, but not at the warfarin and salicilic acid binding sites.