Chemical evidence for covalent linkages of a semisynthetic glycoconjugate vaccine forHaemophilus influenzae type b disease

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Abstract
We have defined the nature of the covalent linkages in aHaemophilus influenzae type b oligosaccharide-CRM197 conjugate vaccine, designated HbOC. The conjugate was acid hydrolyzed to release a novel amino-acid derivative,Nε-(2-hydroxyethyl)lysine (OHEt-Lys), identifiable with an amino-acid analyzer. This amino-acid derivative was formed by reduction of Schiff bases formed betweenH. influenzae type b oligosaccharides (HbO) and the lysyl ε-amino groups of CRM197 (a non-toxic, cross-reactive variant of diphtheria toxin), followed by acid hydrolysis of HbOC. Quantification of OHEt-Lys per CRM197 molecule allowed the determination of a covalency ratio, a useful parameter for evaluating the stoichiometry and consistency of HbOC preparations. Covalent association between HbO and CRM197 was also demonstrated by the coincidence of immunoreactivity of gelelectrophoresed HbOC on a Western blot probed with anti-CRM197 and anti-saccharide antisera.