Mechanisms of protein modification during model anti-viral heat-treatment bioprocessing of beta-lactoglobulin variant A in the presence of sucrose.

To ensure the safety of plasma and recombinant therapeutic proteins, heat treatment is routinely applied to these biopharmaceuticals as a means of virus inactivation. However, to maintain protein integrity during heat treatment it is necessary to use high concentrations of thermostabilizing excipients, such as sucrose, in order to prevent protein damage. In this study we describe the covalent modifications inferred to a model protein, beta-lactoglobulin A, that occur during typical and extended anti-viral heat treatments. The chemical derivation and mechanisms by which these modifications arise are addressed. Heat treatment initiated hydrolysis of sucrose to glucose and fructose, which in turn were degraded to glyoxal. Glyoxal and the free reducing sugars reacted with free amino groups in beta-lactoglobulin A to yield Maillard glycation adducts and advanced glycation end products (AGEs). The major mechanism for AGE formation was via degradation of glucose-derived Schiff-base adducts. Heat treatment and glycation of beta-lactoglobulin A resulted in thiol-disulphide interchange reactions leading to protein oligomerization. A small population of beta-lactoglobulin A non-disulphide-linked dimers were also observed with increasingly harsh heat treatments. These findings have implications for (i) improvements in the safety and efficacy of heat-treated protein biopharmaceuticals and (ii) our understanding of the mechanisms of protein glycation and AGE adduct formation.