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Hcy-thiolactone was discovered by serendipity in 1934 as a by-product of an early assay for the quantification of methionine in proteins [187]. The assay involves boiling with hydriodic acid (128 C, 3 h). This causes demethylation of methionine with the formation of methyl iodide, whose quantitative recovery (99.5 AE 2.0 %) by absorption in alcoholic solution of silver nitrate was the basis of the methionine assay. The residue from methionine that remains in the acid has been identified by elemental analysis as Hcy-thiolactone. The recovery of Hcy-thiolactone from methionine digestion is also quantitative (97.0 AE 2.1 %) [187].Recent studies have found that during hydriodic acid digestion, L-methionine is racemized to D,L-Hcy-thiolactone (Reaction 3.1 and Fig. 3.1 (Fig. 3.1) [188]. In contrast to hydriodic acid, the digestion with hydrochloric acid is ineffective in demethylation of methionine [3]. For example, during incubation with 6 N HCl at 135 or 120 C, the rate of conversion of methionine to Hcy-thiolactone is 1,000-and 10,000-fold slower than that observed with hydriodic acid [79]. Concentrated sulfuric acid (18 N) also causes demethylation of methionine at 125-135 C. However, due to oxidative properties of sulfuric acid, homocystine (in 42.5 % yield) is obtained in this case [3]. C, 50 % of ring closure condensation occurs in 3 h, 15 min, or <5 min, respectively [68]. Because Hcy-thiolactone, in contrast to Hcy, absorbs ultraviolet light ( Fig. 3.2), the conversion to Hcy-thiolactone has been exploited in the first spectrophotometric assay for Hcy [189]. The acid-dependent conversion to Hcythiolactone followed by its quantification by high-performance liquid chromatography (HPLC) is now used as a convenient procedure for the determination of Hcy in biological samples [64, 79,[93][94][95] 190]. Physicochemical Properties UV SpectrumDifferences in physicochemical properties of Hcy-thiolactone and Hcy are highlighted in Table 3.1. Similar to other thioesters, Hcy-thiolactone absorbs ultraviolet light with a maximum at 240 nm and ε ¼ 5,000 M À1 cm À1 in water ( Fig. 3.2) [68]. The ability to absorb at 240 nm is also a characteristic property of a deprotonated sulfhydryl group [191, 192]. Monitoring absorbance at 240 nm facilitates studies of nucleophilic [73] and electrophilic [84] reactions involving Hcy-thiolactone as well as quantification of Hcy-thiolactone in biological samples [64, 67, 190, 193, 194]. 3 Homocysteine-Thiolactone
Hcy-thiolactone was discovered by serendipity in 1934 as a by-product of an early assay for the quantification of methionine in proteins [187]. The assay involves boiling with hydriodic acid (128 C, 3 h). This causes demethylation of methionine with the formation of methyl iodide, whose quantitative recovery (99.5 AE 2.0 %) by absorption in alcoholic solution of silver nitrate was the basis of the methionine assay. The residue from methionine that remains in the acid has been identified by elemental analysis as Hcy-thiolactone. The recovery of Hcy-thiolactone from methionine digestion is also quantitative (97.0 AE 2.1 %) [187].Recent studies have found that during hydriodic acid digestion, L-methionine is racemized to D,L-Hcy-thiolactone (Reaction 3.1 and Fig. 3.1 (Fig. 3.1) [188]. In contrast to hydriodic acid, the digestion with hydrochloric acid is ineffective in demethylation of methionine [3]. For example, during incubation with 6 N HCl at 135 or 120 C, the rate of conversion of methionine to Hcy-thiolactone is 1,000-and 10,000-fold slower than that observed with hydriodic acid [79]. Concentrated sulfuric acid (18 N) also causes demethylation of methionine at 125-135 C. However, due to oxidative properties of sulfuric acid, homocystine (in 42.5 % yield) is obtained in this case [3]. C, 50 % of ring closure condensation occurs in 3 h, 15 min, or <5 min, respectively [68]. Because Hcy-thiolactone, in contrast to Hcy, absorbs ultraviolet light ( Fig. 3.2), the conversion to Hcy-thiolactone has been exploited in the first spectrophotometric assay for Hcy [189]. The acid-dependent conversion to Hcythiolactone followed by its quantification by high-performance liquid chromatography (HPLC) is now used as a convenient procedure for the determination of Hcy in biological samples [64, 79,[93][94][95] 190]. Physicochemical Properties UV SpectrumDifferences in physicochemical properties of Hcy-thiolactone and Hcy are highlighted in Table 3.1. Similar to other thioesters, Hcy-thiolactone absorbs ultraviolet light with a maximum at 240 nm and ε ¼ 5,000 M À1 cm À1 in water ( Fig. 3.2) [68]. The ability to absorb at 240 nm is also a characteristic property of a deprotonated sulfhydryl group [191, 192]. Monitoring absorbance at 240 nm facilitates studies of nucleophilic [73] and electrophilic [84] reactions involving Hcy-thiolactone as well as quantification of Hcy-thiolactone in biological samples [64, 67, 190, 193, 194]. 3 Homocysteine-Thiolactone
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