Modifications of Substituted Seryl and Threonyl Residues in Phospopeptides and a Polysialoglycoprotein by β-Elimination and Nucleophile Additions1

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Abstract
The β-elimination and nucleophile addition reactions of the substituted serine and threonine residues were studied using several synthesized fluorescence-labeled phosphopeptides and a salmon egg polysialoglycoprotein (PSGP). The reagents used were 1 MCH3SH-0.43 M NaOH, 1 M NaBH-0.1 M NaOH, 1 M CH3NH2-0.1 M NaOH, and 1 M Na2SO3 -0.1 M NaOH. The β-elimination reaction of a phosphoserine peptide, Gly-Ser(PO4)-Glu-AEAP, was about 20 times faster than that of the corresponding phosphothreonine peptide. The carboxyl-side amino acid of the phosphoamino acids in peptides greatly affected the β-elimination rate. The β-elimination reaction rates of O-glycosyl serine and threonine in the polysialoglycoprotein were similar and were about a half of that of the phosphoserinepeptide. The rate of addition of the three nucleophiles and hydrogen to α-aminoacrylic acid (β-elimination product of substituted serine) in the peptide decreased in the order of CH3SH, Na2SO3, CH3NH2, and H2(NaBH4), and the addition to α-aminocrotonic acid (β-elimination product of substituted threonine) in the order of Na2SO3, CH3NH2, CH3SH, and H2. These results indicated that sulfite is the most recommended nucleophile because of its high addition rate. If sulfite addition is carried out in the presence of NaBH4, sugar chains can be released as alditols. converting the sugar-attaching amino acids to β-sulfoamino acids.

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