Novel Method for the Isolation of Proteins and Small Target Molecules from Biological and Aqueous Media by Salt-Assisted Phase Transformation of Their PEGylated Recognition Counterparts

An efficient and simple method for the application of PEGylated affinity ligands in precipitative isolation of protein target molecules (TMs) from a biological fluid such as blood serum or small target molecules from an aqueous medium is presented for the first time. This approach is based on the high binding specificity of PEGylated recognition molecules (PEG-RMs) to their TMs and the unique physicochemical properties of PEG that result in their salt-assisted phase transformation. Addition of PEG-RM to blood serum results in the formation of an RM-specific macromolecular complex (PEG-RM + TM → PEG-RM.TM) that undergoes facile salt-assisted phase transformation to a separable semisolid with ammonium sulfate. PEG-RM.TM is then dissociated into its components by pH reduction or an increase of ionic strength (PEG-RM.TM → PEG-RM + TM). PEG-RM is salted out to afford pure TM in solution. The same phenomenon is observed when RM or TM are small molecules. The general applicability of the method was validated by PEGylation of two proteins (protein A, sheep antihuman IgG) and a small molecule (salicylic acid) used as model RMs for the isolation of Igs, IgG, and serum albumin from blood serum. The isolated protein TMs were shown to be pure and aggregate-free by gel electrophoresis and dynamic light scattering (DLS). IgG isolated by this method was further characterized by peptide mass fingerprinting. PEGylated protein A was used to demonstrate the recyclability and scale-up potential of PEG-RM. IgG isolated by this method from blood serum of a hepatitis C-vaccinated individual was tested for its binding to sheep antihuman IgG by UV spectroscopy, and its bioactivity was ascertained by comparison of its enzyme-linked immunosorbent assay (ELISA) result to that of a blood sample from the same individual. Reciprocity of RM and TM was ascertained using PEGylated salicylic acid to obtain pure serum albumin, and PEGylated serum albumin was utilized for near-exclusive isolation of one drug from an aqueous equimolar mixture of three drugs (salicylic acid, 91%; capecitabine, 6%; and deferiprone, 3%). Advantages of this approach, including target specificity and general applicability and celerity, over other affinity methods for the isolation of proteins are discussed at a molecular level.

Conformational flexibility of a model protein upon immobilization on self-assembled monolayers

The present study reports on the retention of conformational flexibility of a model allosteric protein upon immobilization on self-assembled monolayers (SAMs) on gold. Organothiolated SAMs of different compositions were utilized for adsorptive and covalent attachment of bovine liver glutamate dehydrogenase (GDH), a well-characterized allosteric enzyme. Sensitive fluorimetric assays were developed to determine immobilization capacity, specific activity, and allosteric properties of the immobilized preparations as well as the potential for repeated use and continuous catalytic transformations. The allosteric response of the free and immobilized forms towards ADP, L-leucine and high concentrations of NAD(+), some of the well-known activators for this enzyme, were determined and compared. The enzyme immobilized by adsorption or chemical binding responded similarly to the activators with a greater degree of activation, as compared to the free form. Also loss of activity involving the two immobilization procedures were similar, suggesting that residues essential for catalytic activity or allosteric properties of GDH remained unchanged in the course of chemical modification. A recently established method was used to predict GDH orientation upon immobilization, which was found to explain some of the experimental results presented. The general significance of these observations in connection with retention of native properties of protein structures upon immobilization on SAMs is discussed.

Interaction of hexokinase with the outer mitochondrial membrane and a hydrophobic matrix

The major portion of rat brain hexokinase (HK type 1) is bound to the outer membrane of mitochondria and glucose-6-phosphate (G6P) can release the bound enzyme. In an attempt to look at the 'hydrophobic' component of binding, interaction of the enzyme with a purely hydrophobic matrix, palmityl-substituted Sepharose-4B (Sepharose-lipid) was investigated. Hexokinase readily bound to this matrix with retention of its catalytic activity. Glucose-6-phosphate which has a releasing effect on the mitochondrially bound enzyme, enhanced binding of the enzyme on the hydrophobic matrix. Chymotrypsin treatment of hexokinase which causes loss of binding to mitochondria, also results in loss of adsorption to the hydrophobic matrix, thus demonstrating that the 'hydrophobic tail' present at its N-terminal end is essential for binding in both cases. Data presented provide some new information relevant to understanding how hexokinase interacts with its natural binding matrix, the mitochondrion.

Glucoamylase absorption and desorption process

This paper reports the study of glucoamylase absorption and desorption processes on spherical particles constituting acrylic supports. The kinetic (reaction order, half-life of the reaction, reaction rate constant), and thermodynamic parameters (activation energy, pre-exponential factor) of the glucoamylase immobilization reaction dynamics inside the acrylic supports have been studied. Hindrance of the penetrants (protein molecules), which defeats the polymer's cohesive forces and passes through the diffusional barriers towards the sites available to receive it, denotes the influence of the diffusion phenomena, and might be expressed by the values of the effective diffusion coefficient and of the energetic parameters, as well. The desorption dynamics of glucoamylase in specific buffer concentration gradients and temperature are dependent on the eluent volume and on the protein eluate.

Reversible association of ox liver glutamate dehydrogenase with the inner mitochondrial membrane

A comparative study on the catalytic and allosteric properties of particulate and soluble forms of ox liver glutamate dehydrogenase has been carried out. The response of the bound enzyme to release by various effectors was investigated. The particulate enzyme was found to have catalytic activities similar to the free enzyme in contrast to its behaviour when bound to pure anionic phospholipids. Possible reasons for such outstanding differences are discussed.