Development of novel formulations for mucosal delivery of protein ...

BSA foams by diffusing into the air-water interface in solution with subsequent
reduction in surface tension. Partial unfolding and association occurs resulting in an
intermolecular cohesive film. Improvement in foam expansion and stability is achieved by
BSA interaction with clupeine (Poole et al., 1984). Gelation of the protein follows a two step
process of initial unfolding or dissociation and a subsequent aggregation step resulting in gel
formation under suitable conditions. The heating of BSA results in the formation of soluble
aggregates, through non-covalent and disulphide bonds, which leads to the formation of a gel.
Albumin binds reversibly with a wide range of ligands serving as the principal carrier for
insoluble fatty acids in plasma circulation. Oxidation of approximately 30 % of free
sulphydryl Cys-34 in circulating plasma is by glutathione and cysteine (Peters, 1985).
1.2.2.2 Insulin (INS)
Insulin (INS) is a polypeptide hormone which regulates carbohydrate metabolism and
produced in the Islets of Langerhans in the pancreas. As a primary effector in carbohydrate
homeostasis, INS also partakes in fat and protein metabolism. Clinically, INS is used in some
forms of diabetes mellitus. While Type 1 diabetes mellitus patients depend on injected
exogenous INS for survival, Type 2 diabetes patients occasionally require INS administration
in addition to adequate medication to control blood glucose level.
Chemical structure: The chemical structure of INS varies slightly between species however,
porcine INS is similar in composition to human INS. The empirical formula of the protein is
C 254 H 377 N 65 O 75 S 6 with a molecular weight of approximately 6,000. INS is synthesised from a
single chain precursor, proinsulin. Proteolysis removes 4 basic amino acids and the remaining
connector or C peptide to convert human proinsulin to INS which consists of two chains of A
(acidic, consisting of 21 amino acids with glycine at the amino terminal residue) and B (basic,
consisting of 30 amino acids with phenylalanine at the amino terminus) amino acids linked
together by two disulphide bonds (Figure 1.2 (i) and (ii)). INS may exist as a monomer
(biologically active), dimer or hexamer. The hexamer has two molecules of coordinated Zn 2+
and is stored as granules in the beta cell. Porcine INS differs from that of humans only by the
substitution of threonine by alanine residue in the B chain at the carboxyl terminus. Bovine
INS, however, differs by three amino acid residues, making it more antigenic.