Thiolation of proteins with homocysteine thiolactone: preparation of immunoglobulin G heavily labelled with methylmercury

Conundrum of dehydroascorbic acid and homocysteine thiolactone reaction products: Structural characterization and effect on peptide and protein N-homocysteinylation

An excessive blood level of homocysteine (HcySH) is associated with numerous cardiovascular and neurodegenerative disease conditions. It has been suggested that direct S-homocysteinylation, of proteins by HcySH, or N-homosteinylation by homocysteine thiolactone (HTL) could play a causative role in these maladies. In contrast, ascorbic acid (AA) plays a significant role in oxidative stress prevention. AA is oxidized to dehydroascorbic acid (DHA) and if not rapidly reduced back to AA may degrade to reactive carbonyl products. In the present work, DHA is shown to react with HTL to produce a spiro bicyclic ring containing a six-membered thiazinane-carboxylic acid moiety. This reaction product is likely formed by initial imine condensation and subsequent hemiaminal product followed by HTL ring opening and intramolecular nucleophilic attack of the resulting thiol anion to form the spiro product. The reaction product was determined to have an accurate mass of 291.0414 and a molecular composition C10H13NO7S containing five double bond equivalents. We structurally characterized the reaction product using a combination of accurate mass tandem mass spectrometry, 1D and 2D-nuclear magnetic resonance. We also demonstrated that formation of the reaction product prevented peptide and protein N-homocysteinylation by HTL using a model peptide and α-lactalbumin. Furthermore, the reaction product is formed in Jurkat cells when exposed to HTL and DHA.

Elevated plasma homocysteine leads to alterations in fibrin clot structure and stability: implications for the mechanism of thrombosis in hyperhomocysteinemia

Elevated plasma homocysteine is associated with an increased risk of atherosclerosis and thrombosis. However, the mechanisms by which homocysteine might cause these events are not understood. We hypothesized that hyperhomocysteinemia might lead to modification of fibrinogen in vivo, thereby causing altered fibrin clot structure. New Zealand White rabbits were injected intraperitoneally (i.p.) every 12 h through an indwelling catheter with homocysteine or buffer for 8 weeks. This treatment raised the plasma homocysteine levels to about 30 micro mol L(-1) compared with 13.5 micro mol L(-1) in control rabbits by the end of the treatment period. The fibrinogen levels were 3.2 +/- 0.6 in homocysteine-treated and 2.5 +/- 1.1 mg mL(-1) in control rabbits. The reptilase time was prolonged to 363 +/- 88 for plasma from homocysteine-treated rabbits compared with 194 +/- 48 s for controls (P < 0.01). The thrombin clotting time (TCT) for the homocysteine-treated rabbits was significantly shorter, 7.5 +/- 1.7 compared with 28.6 +/- 18 s for the controls (P < 0.05). The calcium dependence of the thrombin clotting time was also different in homocysteinemic and control plasmas. Clots from plasma or fibrinogen of homocysteinemic rabbits were composed of thinner fibers than control clots. The clots formed from purified fibrinogen from homocysteine-treated rabbits were lyzed more slowly by plasmin than comparable clots from control fibrinogen. Congenital dysfibrinogenemias have been described that are associated with fibrin clots composed of thin, tightly packed fibers that are abnormally resistant to fibrinolysis, and recurrent thrombosis. Our results suggest that elevated plasma homocysteine leads to a similar acquired dysfibrinogenemia. The formation of clots that are abnormally resistant to fibrinolysis could directly contribute to the increased risk of thrombosis in hyperhomocysteinemia.

Influence of hydrolysis on plasma homocysteine determination in healthy subjects and patients with myocardial infarction

After acid hydrolysis, mean plasma homocysteine concentrations, measured as homocysteine disulphides, of about 1000 and 40 mumol/l have recently been reported in 26 survivors of myocardial infarction and 26 matched control subjects, respectively. This finding contrasts sharply with those more than 50 times lower total homocysteine concentrations found by other research groups in non-hydrolysed plasma from survivors of myocardial infarction. Using the same hydrolysis conditions, we could not detect any homocysteine disulphides in plasma hydrolysates from 9 survivors of myocardial infarction and 10 healthy subjects, who had mean total homocysteine concentrations in non-hydrolysed plasma of 16.9 +/- 6.5 and 15.8 +/- 10.3 mumol/l, respectively. The chromatograms contained several peaks, probably representing peptides, which disappeared with more complete hydrolysis and which might have been misinterpreted as homocysteine disulphides in the reported study. Only after reduction of disulphides and by using a sulphydryl-selective extraction procedure were we able to determine mean homocysteine concentrations in hydrolysed plasma to be 26.2 +/- 7.9 mumol/l in the survivors of myocardial infarction and 24.5 +/- 12.2 mumol/l in the healthy reference subjects. Thus, we could not confirm that survivors of myocardial infarction have homocysteine concentrations that are many times higher than found in healthy subjects.

Homocysteine thiolactone disposal by human arterial endothelial cells and serum in vitro

Previous work with cultured mammalian cells and perfused laboratory animals suggested to us that hydrolysis of homocysteine thiolactone was catalyzed in these systems. We confirmed this finding by measuring the sulfhydryl-releasing activity of cultured endothelial cells from human umbilical arteries in homocysteine thiolactone solution, pH 7.4, 37 degrees C. The reaction was vigorous and stereospecific and showed saturation kinetics (Km values for L- and D,L-homocysteine thiolactone were 3.9 and 8.2 mmol/l, respectively, and Vmax values were 10.75 and 10.1 mumol/min/10(9) cells, respectively). L-Homocysteine thiolactone was quantitatively converted to homocysteine, as measured by amino acid analysis. Human serum also accelerated the elimination of homocysteine thiolactone, although in this process, the majority of the newly formed sulfhydryl-containing product was precipitable by sulfosalicylic acid, indicating likely homocysteinylation of serum proteins. However, approximately 38% of the sulfhydryl-containing product was not precipitated, and because thiolactone elimination stereospecifically favored the L-enantiomer, a possible subsidiary role for serum-catalyzed hydrolysis of the thiolactone was suggested. No homocysteine thiolactone could be found in serum samples from six patients with acute myocardial infarction, three patients with cystathionine beta-synthase deficiency, and six normal subjects. Thus, humans have active vascular systems for elimination of homocysteine thiolactone, a process that could be responsible for an absence of the compound in serum.