Cysteine homocysteine disulfide

Tyrosine Cysteine-homocysteine disulfide 3-Methoxytyrosine Erythromycin glucoheptonate... 

The biochemical phenotype of homocystinuria is characterized by increased plasma concentrations of methionine, free homocysteine and cysteine-homocysteine disulfide, together with low cystine (Figure 55-6, C). Determination of total homocysteine after treatment of the sample with... 

The diagnosis of homocystinuria is based on the recognition of the clinical phenotype in conjunction with the identification of an elevated total plasma homocysteine and elevated plasma methionine concentrations (via quantitative plasma amino acid analysis). Low cystine and low cystathionine are also seen (Box 14.3). In addition, increased urinary excretion of homocysteine as well as cysteine-homocysteine disulfide can be identified on urine amino acid analysis. Confirmation of the diagnosis can be done via enzyme assay, typically performed on cultured skin fibroblasts, lymphocytes, or liver tissue, or via molecular studies. 

The fraction of intracellular homocysteine that does not undergo transsulfuration or remethylation is secreted into the extracellular space and ultimately finds its way into the blood. One major source of blood homocysteine is the liver, but some homocysteine is secreted into the blood by endothelial cells, circulating blood cells, and other tissues. Only about 2% of homocysteine in blood remains in its reduced, thiol form. The remainder circulates as a variety of different oxidation adducts, which include the disulfide, homocystine, as well as homocysteine-cysteine mixed disulfide and several protein-bound disulfides.About 70% of total homocysteine in blood is bound to the protein albumin through a disulfide linkage.When blood homocysteine is measured in the clinical laboratory, a reducing agent is added to the sample... 

In plasma, homocysteine is present as both free (< 1 %) and oxidized forms (>99%). The oxidized forms include protein (primarily albumin)-bound homocysteine mixed disulfide (80-90%), homocysteine-cysteine mixed disulfide (5-10%), and homocystine (5-10%). Several studies have shown the relationship between homocysteine and altered endothelial cell function leading to thrombosis. Thus, hyperhomocysteinemia appears to be an independent risk factor for occlusive vascular disease. Five to ten percent of the general population have mild hyperhomocysteinemia. 

A similar separation of 50 physiological amino adds can be obtained with the highly efficient 3 pm cation exchanger from Hitachi (Tokyo, Japan) in about 2 h. The Hitachi column is characterized by an extremely low back pressure of 370-800 psi over the course of the run. In addition to key amino adds related to certain metabolic errors, less common amino acids such as homocysteine, homocitrulline, //o-isoleucine, argininosuccinic acid, cysteine-homocysteine mixed disulfides, homocystine, and argininosuccinic acid anhydride can also be separated under similar chromatographic conditions. 

Homocysteine is an intermediate sulfur amino acid. Normal fasting homocysteine levels in plasma are between 5 and 15 pmol/1. Hyperhomocysteinemia are considered to be moderate between 16 and 30 pmol/l, intermediate (31 lOOpmol/1) and severe > 100 pmol/1. In plasma, 70-80% of homocysteine is bound to plasma proteins and 20-30% is free homocysteine circulating as homocysteine disulfide (homocystine) or mixed disulfides (cysteine-homocysteine). Folate deficiency leads to the decrease of homocysteine... 

Guan, X., B. Hoffman, C. Dwivedi et al. 2003. A simultaneous liquid chroma-tography/mass spectrometric assay of glutathione, cysteine, homocysteine and their disulfides in biological samples./. Pharm. Biomed. Anal. 31 251-261. 

There are numerous abnormalities of cysteine metabolism. Cystine, lysine, arginine, and ornithine are excreted in cystine-lysinuria (cystinuria), a defect in renal reabsorption. Apart from cystine calculi, cystinuria is benign. The mixed disulfide of L-cysteine and L-homocysteine (Figure 30-9) excreted by cystinuric patients is more soluble than cystine and reduces formation of cystine calculi. Several metabolic defects result in vitamin Bg-responsive or -unresponsive ho-mocystinurias. Defective carrier-mediated transport of cystine results in cystinosis (cystine storage disease) with deposition of cystine crystals in tissues and early mortality from acute renal failure. Despite... 

A. Jacobi, D. Seebach, How to Stabilize or Break P-Peptidic Helices by Disulfide Bridges Synthesis and CD Investigation of P-Peptides with Cysteine and Homocysteine Side Chains Helv. Chim. Acta 1999, 82, 1150- 1172. 

Elevated concentrations of plasma homocysteine (HCY) are related to an increased risk of cardiovascular disease, which exists in numerous forms in plasma, with the main form existing as a disulfide with itself, cysteine, or albumin. Therefore, the first step in the measurement involves treatment with a reducing agent, in this case dithiothreitol (DTT), to obtain HCY in its free form (Eq. 16.34). Some amino acids (e.g., L-cysteine and L-methionine) are present in human plasma at higher molar concentrations than HCY and may interfere with this assay. To avoid this possible interference, the highly selective enzymatic conversion of HCY to S-adenosyl-L-homocysteine (SAH), as shown in Eq. 16.34, is used. Both reactions (reduction and conjugation) are accomplished in 30 min at 34 °C. 

Regulation of liver D-fructose 1,6-diphosphatase could involve modification of the sulfhydryl group of the enzyme.397 Cystamine (2,2 -dithiobisethylamine) or homocysteine undergoes a disulfide exchange-reaction with two reactive cysteine residues. This exchange leads to a four-fold increase in catalytic activity.405 Other sulfhydryl reagents, such as l-fluoro-2,4-dinitrobenzene (FDNB), p-mercuri-benzoate (PMB), and 2-iodoacetamide, also activate liver D-fructose... 

Cystathionine fisynthase (EC 4.2.1.22). Failure to form cystathionine from homocysteine and serine. Elevated homocysteine and methionine in serum. Urine contains homocystine and homocysteine-cysteine disulfide. Cystathionine virtually absent from brain, where it is normally present in significant quantities. Mental retardation. Lens detachment. Skeletal abnormalities (tall stature, arachnodactyly). Arterial and venous thrombosis. 

Kleinman and Richie used the in-series approach with gold amalgam electrodes to measure 12 thiols and disulfides, including GSH, GSSG, cysteine, cystine, homocysteine, homocystine and several mixed disulfides in rat tissues. The first (upstream) electrode was maintained at —1.0 V and the second (downstream)... 

Determination of total plasma homocysteine and cysteine is preferable due to instability of free thiols in the presence of plasma proteins, which bind 70% and -30% of homocysteine and cysteine, respectively. The total amount of one of these compounds is the sum of the free thiol plus the residues bound via disulfide bonds to thiols, peptides, or proteins. 

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